18 Arachnida

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Jonathan A. Coddington

Gonzalo Giribet

Mark S. Harvey

Lorenzo Prendini

David E. Walter

296

Although the earliest arachnids were apparently marine,

arachnid diversity has been dominated by terrestrial forms

from at least the Devonian. Even though arachnid fossils are

scarce (perhaps only 100 pre-Cenozoic taxa), representatives

of all major arachnid clades are known or cladistically implied

from the Devonian or earlier, suggesting very early origins

(Selden and Dunlop 1998). The more recent great

radiation of insects, in contrast, seems to be Permian

(Kukalovб-Peck 1991, Labandeira 1999). Taxonomically,

arachnids today are composed of approximately 640 families,

9000 genera, and 93,000 described species (table 18.1),

but untold hundreds of thousands of new mites and spiders,

and several thousand species in the remaining orders, are still

undescribed. Arachnida include 11 classically recognized

recent clades, ranked as “orders,” although some acarologists

regard Acari as a subclass with three superorders. Acari (ticks

and mites) are by far the most diverse, with Araneae (spiders)

second, and the remaining orders much less diverse. Discounting

secondarily freshwater and marine mites, and a few

semiaquatic spiders and one palpigrade, all extant arachnid

taxa are terrestrial. Arachnids evidently arose in the marine

habitat (Dunlop and Selden 1998, Selden and Dunlop 1998,

Dunlop and Webster 1999), invaded land independently of

other terrestrial arthropod groups such as myriapods, crustaceans,

and hexapods (Labandeira 1999), and solved the

problems of terrestrialization (skeleton, respiration, nitrogenous

waste, locomotion, reproduction, etc.) in different

ways.

Arachnids and Chelicerata

The monophyly of extant Euchelicerata—the arachnids and

their marine sister group, the horseshoe crabs or merostomes—

is consistently indicated by both morphology and

molecular data (Snodgrass 1938, Wheeler 1998, Zrzavэ et al.

1998, Giribet and Ribera 2000, Giribet et al. 2001, Shultz

2001). However, their relationship to the “sea spiders”

(Pycnogonida), an enigmatic and morphologically highly

specialized group of marine predators, remains controversial.

Pycnogonids are variously seen as sister to euchelicerates

(Weygoldt and Paulus 1979, Weygoldt 1998, Giribet and

Ribera 2000, Shultz and Regier 2000, Regier and Shultz 2001,

Waloszek and Dunlop 2002) or as sister to euchelicerates and

all remaining arthropods (Zrzavэ et al. 1998, Giribet et al.

2001).

Phylogeny of Arachnida

Arachnid monophyly is supported by at least 11 synapomorphies,

among which extraintestinal digestion (although

some mites and all members of Opiliones are particulate feeders),

slit sense sensilla (absent in palpigrades), a single medial

genital opening, and an anteroventrally directed mouth

are particularly convincing (Weygoldt and Paulus 1979,

Shultz 1990, 2001). If fossils are considered, arachnid monophyly

is less certain mainly because of the character conflict

Arachnida 297

created by marine scorpions and eurypterids. Paleontologists

consider some fossil scorpions to have been marine (Jeram

1998, Dunlop 1998, Dunlop and Webster 1999, Dunlop and

Selden 1998), which, if true, implies either that terrestrial

scorpions invaded land independently, or that they returned

to the seas secondarily. If the former, the similar arachnid

innovations for terrestrial life may be convergent rather than

homologous (Jeram 1998, Dunlop and Selden 1998, Dunlop

and Webster 1999). Some paleontologists have argued that

scorpions are derived merostomes (Dunlop 1999, Dunlop

and Selden 1998, Jeram 1998, Dunlop and Braddy 2001),

but the paucity of informative characters and the poor or

incomplete preservation of the (very) few fossils that exist

make conclusions ambiguous and tentative. Paleontologists

now recognize three extinct arachnid orders: the clearly tetrapulmonate

Trigonotarbida (50 species, including Anthracomarta;

Dunlop 1996b), Haptopoda (one species), and

Phalangiotarbida (26 species), the latter two orders of uncertain

affinities (Selden and Dunlop 1998, Dunlop 1996b,

1999). The paleontological arguments tend to emphasize a

few characters (e.g., absence of respiratory structures on the

genital somite and subdivision of the abdomen into a proximal

broader section and a distal tail) while discounting contrary

evidence, especially that not preserved in fossils.

Cladistic analyses based on morphological data for extant taxa

place scorpions deep inside the recent arachnid clade, possibly

related to Opiliones, pseudoscorpions, and solifuges

(Shultz 1990, 2000, Wheeler and Hayashi 1998, Giribet et al.

2002), but this clade becomes ambiguously resolved when

fossil scorpions and eurypterids are coded, possibly because

of the large amount of conflicting character states, because

of the aquatic habitat and missing data imposed by the fossils

(Giribet et al. 2002). The extinct eurypterids are also

chelicerates and are apparently closer to arachnids than to

xiphosurans (Weygoldt and Paulus 1979). Molecular data

sometimes place scorpions as true arachnids (Wheeler et al.

1993, Giribet et al. 2001, 2002) but can nest horseshoe crabs

within “true” arachnids as well (Wheeler 1998, Wheeler and

Hayashi 1998, Edgecombe et al. 2000, Giribet et al. 2002).

The phylogeny of Arachnida itself is contentious, but not

as contentious as a perusal of the recent literature might suggest.

Specialists may disagree on analytical methodology and

interpretation of fossil morphology but largely agree that more

data are needed before incongruence should be taken seriously.

Classical morphological analysis more or less strongly suggests

various clades: Acaromorpha (= ricinuleids–mites), Haplocnemata

(= pseudoscorpions–solifuges), Camarostomata (= whip

scorpions–schizomids), and Tetrapulmonata (four-lunged

arachnids: Araneae, Uropygi, Schizomida, Amblypygi). Besides

the controversy over scorpions mentioned above, the positions

of Palpigradi, Opiliones, Ricinulei, and Acari are unsettled

(Weygoldt and Paulus 1979, Weygoldt 1998, Shultz 1990,

1998, Wheeler et al. 1998, Giribet et al. 2002). Weygoldt and

Paulus’s early analysis was the first explicit phylogenetic treatment

of arachnid relationships, selecting characters that they

considered to be of phylogenetic importance while dismissing

contradictory evidence as convergence or secondary loss

without regard to parsimony. Later authors analyzed morphology

and/or molecular evidence cladistically (or using other

numerical analytical methods). Parsimony analysis of morphological

data from extant groups by different researchers

generally agrees with the topology presented in figure 18.1.

However, most of the morphological phylogenetic analyses of

Arachnida published so far are based on groundplan codings

for each order instead of using multiple representatives of each

order showing the particular combinations of character states

in those terminals. This alternative way of coding terminals

has been recently discussed by Prendini (2001a), and it is

Table 18.1

Arachnid Diversity at the Family, Genus, and Species (Described and

Estimated) Levels.

Species

Families Genera Described Estimated

Arachnida 650 9500 100,000 ~1 million

Acari ~430 ~3300–4000 ~50,000 0.5–1 million

Araneae 109 3471 37,596 76,000–170,000

Opiliones 43 1500 5000 7500–10,000

Pseudoscorpiones 24 425 3261 3500–5000

Scorpiones 17 163 1340 4,000

Solifugae 12 141 1084 1,115

Amblypygi 5 17 142 ?

Schizomida 2 39 237 ?

Palpigradi 2 6 78 100

Uropygi 1 16 101 ?

Ricinulei 1 3 55 85

From Adis and Harvey (2000), Harvey (2003), Platnick (2002), Fet et al. (2000).

298 The Relationships of Animals: Ecdysozoans

clearly superior at least in the sense that it allows testing for

monophyly of the arachnid orders. Such an exemplar coding

has been recently attempted (although with some groundplan

codings remaining) in the context of arachnid phylogeny by

Giribet et al. (2002).

Recent analyses based on molecular data neither confirm

much of the tree based on morphology nor agree on an alternative.

Two nuclear loci, 18S and 28S ribosomal RNA are usually

employed at the interordinal level (Wheeler and Hayashi

1998, Giribet and Ribera 2000, Giribet et al. 2001, 2002), on

the grounds that rates of change in these loci seem appropriate

for reconstructing divergences this old. Elongation factor-

1a (EF-1a), EF-2, and RNA polymerase II have also been

studied at the level of arthropod relationships (Regier and

Shultz 1997, 1998, 2001, Shultz and Regier 2000), but few

data are available for the interordinal chelicerate relationships.

The Uropygi–Schizomida doublet is always corroborated, but

the molecular data either deny Acari–Ricinulei (Wheeler and

Hayashi 1998, Giribet et al. 2002) or include them in a trichotomy

with sea spiders (Wheeler 1998). The monophyly

of Tetrapulmonata is strongly supported by morphology,

contradicted by some molecular-only analyses (Wheeler and

Hayashi 1998, Giribet et al. 2001) and confirmed by others

(Giribet et al. 2002). But even the latter found a novel internal

topology for Tetrapulmonata (Amblypygi (Araneae

(Uropygi, Schizomida))). If viewed as an unrooted network,

its spider subclade was correct, but morphology clearly roots

the subclade differently (see below). Wheeler and Hayashi

(1998) did recover Opiliones–Acari (but excluding Ricinulei).

However, this clade was sister to horseshoe crabs, requiring

another hypothesis of secondary marine invasion.

In general, the molecular results to date tend to agree with

morphology on fairly low-level relationships (monophyly of

harvestmen, haplocnemates, camarostomes, scorpions, spiders,

etc.) but to disagree with some morphologically based

deeper nodes. Besides nesting exclusively marine groups

inside terrestrial arachnids, examples include scorpions as

sister to Camarostomata, Acari falling outside a group including

mollusks, myriapods, and chelicerates (Wheeler

and Hayashi 1998), scorpions as sister to spiders (Giribet

et al. 2002), a diphyletic Acari (Giribet et al. 2002; although

monophyly of Acari is, of course, not universally agreed upon

even among acarologists), amblypygids and pseudoscorpions

as sister to the remaining chelicerates, palpigrades nested

within spiders (Wheeler 1998), scorpions as sister to ricinuleids,

or spiders as sister to uropygids exclusive of schizomids

(Giribet and Ribera 2000). The lack of consistency in molecular

results at the ordinal level from one study to the next

casts doubt on the robustness and accuracy of the molecular

data gathered to date. On the other hand, molecular data

have tested the monophyly of arachnid orders more strictly

than has morphology by including multiple exemplars within

each order. Furthermore, very few molecular analyses specifically

address arachnid interrelationships, and the same loci

(18S and 28S rRNA) have been used consistently. Studies of

metazoan or arthropod phylogeny tend to include only a few

chelicerates, and the topological incongruities seen are probably

due at least in part to sparse taxon sampling.

When the currently available molecular data are combined

with morphology (Wheeler 1998, Wheeler and Hayashi 1998,

Giribet et al. 2001, 2002), the latter tend to dominate at the

deepest nodes. The ordinal topology of the combined analysis

by Wheeler and Hayashi (1998: fig. 7) agrees almost perfectly

with the morphology based analysis of Shultz (1990) and

differs strongly from the molecules-only tree. This is not as

true of the largest analysis to date by Giribet et al. (2002).

Figure 18.1. Phylogeny of

arachnid orders based on the

morphological analysis of Shultz

(1990).

remaining Arthropoda

XIPHOSURA ACARI

RICINULEI: Ricinoididae

PALPIGRADI

ARANEAE

UROPYGI: Thelyphonidae

Hubbardiidae

Protoschizomidae

Charinidae

Charontidae

Phrynidae

OPILIONES Phrynichidae

SCORPIONES

PSEUDOSCORPIONES

SCHIZOMIDA

AMBLYPYGI

ARACHNIDA

Acaromorpha

Tetrapulmonata

Pedipalpi

Dromopoda

Novogenuata

Micrura

Camarostomata

Haplocnemata

SOLIFUGAE

PYCNOGONIDA

OPILIOACARIFORMES

ACARIFORMES

PARASITIFORMES

Prokoeneniidae

Eukoeneniidae

Paleoamblypygi: Paracharontidae

Euamblypygi

Neoamblypygi

Phrynoidea

Megoperculata

Arachnida 299

However, given the conflict in molecules alone, it seems

wiser to recommend the morphological cladogram of Shultz

(fig. 18.1) as a working hypothesis for arachnid phylogeny.

Although this review focuses more on the controversies

than the consensus, some nodes in figure 18.1 are well supported.

The tetrapulmonates share the subchelate condition of

the mouthparts, the unique 9 + 3 axoneme sperm morphology,

the narrow or petiolar connection between cephalothorax

and abdomen, the reduction to four prosomal endosternal

components, and the complex coxo-trochanteral joint. According

to a recent anatomical study of the musculoskeletal system,

Pedipalpi share 31 morphological synapomorphies (Shultz

1999), although many of these characters are not independent,

and the extent of homoplasy in other arachnids is unclear.

Camarostomata is also strongly supported by at least

six synapomorphies. Haplocnemata (= Pseudoscorpiones-

Solifugae) also has substantial morphological support. Dromopoda

(= Scorpiones-Pseudoscorpiones-Solifugae-Opiliones)

and Micrura (= Tetrapulmonata-Ricinulei-Acari) have been considered

the weakest nodes morphologically (Weygoldt 1998).

Mites and Ticks (Acari or Acarina)

Mites are the “go anywhere, do anything” arachnids (Walter

and Proctor 1999). They occur on every continent, including

Antarctica, where they dominate the endemic terrestrial

fauna (Pugh 1993). On land, they form a minute, scurrying

plankton that coats the vegetation, from the canopies of the

tallest rainforests down into the soil, at least as deep as roots

can penetrate (Walter 1996, Walter and Behan-Pelletier

1999). Every bird, mammal, reptile, and social insect species

plays host to symbiotic mites, as do many amphibians,

slugs, spiders, scorpions, opilionids, myriapods, and nonsocial

insects. Animal- and plant-associated mites are commonly

commensals that scavenge a living on their hosts’

surfaces, and sometimes provide beneficial services, but all

too often are parasites capable of damaging or killing their

hosts. Although originating on land, mites have reinvaded

and radiated into both freshwater (around six invasions,

>5000 described species) and marine systems (around three

invasions, hundreds of known species) from the intertidal

to the deepest marine trenches (Walter and Proctor 1999).

More than 50,000 species of the “subclass” Acari have

been described and distributed across three superorders,

six orders, more than two dozen suborders and “cohorts”

(~infrasuborders), >400 families, and 3000–4000 genera (see

Table 18.2). Roughly 90 fossil species have been described

(Selden 1993a). Like the artificial assemblage that we call

reptiles, mites are easily recognized as such, but the monophyly

of Acari is open to question. Mites have long been studied

in isolation from other arachnids, and characters that once

appeared to unite the Acari are now known to be more general.

For example, the hexapod larva and the headlike capitulum

(gnathosoma) were once thought unique to mites, but

both are also found in ricinuleids (Lindquist 1984). Other

supposedly unique characters, such as the ventral fusion of

the palpal coxae, occur in many arachnids (e.g., ricinuleids,

schizomids, pseudoscorpions) and may even have evolved

twice within mites (Walter and Proctor 1999). Modern phylogenetic

methods, especially using molecules, have only

recently been applied to Acari, but most of these studies have

been restricted to economically important parasites (Navajas

and Fenton 2000).

Although Acari are not clearly monophyletic (van der

Hammen 1989), each of the three acarine superorders probably

is (Grandjean 1936). Opilioacarans are fairly large

(2–3 mm) tracheate mites, superficially resembling small

opilionids, which retain a number of plesiomorphic characters.

Like early derivative acariform mites and most

opilionids, opilioacarans ingest solid food, using large, threesegmented

chelicerae to grasp small arthropods or fungi, and

Table 18.2

Systematic Synopsis and Distribution of Major Mite Lineages.

Class Arachnida, Acari (Acarina): mites and ticks

Superorder Opilioacariformes: Order Opilioacarida—1 family, 9 genera, ~20 species

Superorder Acariformes: mitelike mites

Order Sarcoptiformes: Endeostigmata, “Oribatida,” Astigmata—~230 families, >15,000 described species, including the paraphyletic

oribatid mites (~1100 genera in >150 families); stored product mites; house dust, feather, and fur mites; and scabies and

their relatives

Order Trombidiformes: Sphaerolichida, Prostigmata—~125 families, >22,000 described species, including spider mites and their

relatives (Tetranychoidea); earth mites and their relatives (Eupodoidea); gall and rust mites (Eriophyoidea); soil predators and

fungivores; hair, skin, and follicle mites (Cheyletoidea); straw itch mites (Pyemotidae); chiggers, velvet mites, water mites, and

their relatives (Parasitengona)

Superorder Parasitiformes: ticks and ticklike mites

Order Ixodida (Metastigmata)—ticks—3 families, <900 described species

Order Holothyrida: holothyrans—3 families, <35 described species

Order Mesostigmata (Gamasida): Monogynaspida + Trigynaspida sensu lato (often treated as 3–4 separate suborders)—~70

families, <12,000 described species, including poultry mites, nasal mites, bird mites, and rat mites (Dermanyssoidea); major soil

predators; biocontrol agents (Phytoseiidae); tortoise mites (Uropodoidea)

300 The Relationships of Animals: Ecdysozoans

serrated hypertrophied palpal coxal setae (rutella) on either

side of the buccal opening to saw the food into bite-sized

chunks that can be swallowed. Fossil opilioacarans are unknown,

although Dunlop (1995) speculates that they may

be related to the curious Carboniferous Phalangiotarbida.

Opilioacariformes may be a sister group to Parasitiformes,

but convincing synapomorphies have yet to be demonstrated.

A sister-group relationship of Opilioacariformes and Parasitiformes

has been recently proposed based on molecular

data (Giribet et al. 2002).

Acariformes are supported by several synapomorphies

unique within Arachnida, including prodorsal trichobothria,

the loss of all primary respiratory structures or remnants (e.g.,

the ventral sacs in Palpigradi), the fusion of the tritosternum

to the palpal coxal endites to form a subcapitulum, and genital

papillae (osmoregulatory structures). Acariformes share

the nonfeeding, hexapod prelarval stage, the rutella, and

particulate feeding with Opilioacariformes. Particulate feeding

also occurs in Opiliones (see above) and in horseshoe

crabs.

Acariformes consist of two orders, Sarcoptiformes and

Trombidiformes, both corroborated by a morphological cladistic

analysis (OConnor 1984) that established the relationship

between the suborders Sphaerolichida (two families

previously attributed to the basal suborder Endeostigmata)

and Prostigmata. Although no comprehensive analysis of

Prostigmata has been published, five cohorts (fig. 18.2) are

well supported by morphological characters. Of these, only

Heterostigmata have received a thorough morphological cladistic

analysis (Lindquist 1986), but parts of Parasitengona

are currently under molecular and morphological review

(e.g., Soeller et al. 2001).

Prostigmatans display anterior dorsal stigmatal openings

and feed only on fluids. Almost half of all known mite

species belong to Prostigmata, including major radiations

of mites parasitic on vertebrates, invertebrates, and plants

(Table 18.2). All of the major acarine plant parasites belong

here, including the smallest known terrestrial animals, gall

mites (Eupodina: Eriophyoidea) as small as 0.07 mm in

length as adults (Walter and Proctor 1999). In contrast,

Parasitengona contains more than 7000 described species

of terrestrial and aquatic mites, including some of the largest

known (16 mm long).

Some traditional subdivisions of the Sarcoptiformes are

obviously paraphyletic, but few cladistic analyses, even of a

preliminary nature, have been published. Astigmata, often

given subordinal rank, is monophyletic (Norton 1998) but

derived from within the traditional suborder Oribatida (also

Oribatei, Cryptostigmata), thereby rendering Oribatida paraphyletic.

Oribatida consist of the beetle mites that form a

dominant part of the soil fauna. Sarcoptiformans were among

the earliest terrestrial animals and probably invaded land

directly from the ocean by way of interstices in moist beach

sand as minute animals that exchanged gases across their

cuticles (Walter and Proctor 1999). By the Early Devonian

(380–400 million years ago), sarcoptiformans were diverse

members of the soil fauna, and 11 species are known from

the Gilboa shales and Rhynie Chert (Norton et al. 1988,

Kethley et al. 1989). Based on extensive fecal remains, it

appears that sarcoptiform mites were major components of

Figure 18.2. Phylogeny of Acari.

Endeostigmata (pars)

Palaeosomata

Enarthronota

Parhyposomata

Mixonomata

Desmononomata

Astigmata

Euoribatida

Sphaerolichida

Eupodina

Parasitengona

Anystae

Raphignathina

Heterostigmata

Ixodida

Holothyrida

Antennophorina

Cercomegistina

Uropodina

Sejina

Microgyniina

Zerconina

Epicriina

Arctacarina

Parasitina

Demanyssina

Prostigmata

Acariformes

Trombidiformes

Sarcoptiformes

Opilioacariformes

Mesostigmata

Parasitiformes

Anystina

Arachnida 301

the detritivore system in Palaeozoic coal swamps (Labandeira

et al. 1997). A later radiation in association with animals

(Astigmata: Psoroptida) has produced a dazzling diversity of

nest, feather, fur and skin inhabitants and a source of some

interesting host–symbiont analyses (e.g., Klompen 1992,

Dabert et al. 2001).

Parasitiformes are supported by a number of unique character

states, including a plate above or behind leg IV bearing

a stigmatal opening and peritreme, a biflagellate tritosternum,

a sclerotized ring formed by fusion of the palps around the

chelicerae (possibly representing a fusion of a ricinuleid-like

cucullus to the palpal coxae), horn-shaped corniculi (possibly

homologous with the rutella) that support the salivary

stylets, a recessed sensory array on leg I (called Haller’s organ

in ticks), and by the use of the chelicerae to transfer

sperm. Additional characters supporting Parasitiformes include

suppression of the prelarval stage and widespread fluid

feeding (the general condition among arachnids).

The internal relationships of Parasitiformes are the best

studied of any of the three acarine superorders, but this is

faint praise indeed. Relationships among ticks (Ixodida) and

between ticks and other suborders are the best resolved (e.g.,

Klompen et al. 1996). However, some exemplary morphological

and molecular analyses of parts of Mesostigmata are

starting to appear (e.g., Naskrecki and Colwell 1998, Cruickshank

and Thomas 1999). The monophyly of ticks, perhaps

the most familiar of all mites because of their large size and

bloodthirsty habits, is supported by several modifications of

the chelicerae and hypostome for blood-feeding. Molecular

evidence suggests that holothyrans, large (2–7 mm long),

reddish to purplish armored mites, are close relatives of ticks.

Holothyrans are rare, known only from Gondwanan continents

and Indo-Pacific Islands, where they scavenge on

fluids from dead arthropods (Walter and Proctor 1998). A

uniquely formed all-encompassing dorsal shield and lateral

peritrematal plate support the monophyly of Holothyrida.

The group consisting of (Holothyrida + Ixodida) is the

sister to Mesostigmata. Characters supporting the monophyly

of the latter are mostly developmental, for example, suppression

of the tritonymphal stage and of the genital opening until

the adult, and the appearance of sclerotized plates on the

opisthosoma in nymphs. Mesostigmata can be split into two

suborders, each with five cohorts based on variation in the

female genital shield. In Monogynaspida (Gamasina), the

plesiomorphic condition of four genital shields (found in

Holothyrida and Cercomegistina) is reduced to a single genital

shield by fusion of the laterals (latigynials) to the median

genital shield and the loss of the anterior genital shield.

Trigynaspida sensu lato shows a general trend toward fusion

of the latigynials with other shields, and is only weakly

supported. Trigynaspines often have restricted distributions

but are prominent members of tropical forest faunas, as are

members of Uropodina. A group comprising Uropodina,

Sejina, and Microgyniina is supported by the development

of a heteromorphic deutonymph (i.e., a differently formed

phoretic stage) that disperses on insects via an anal attachment

organ.

Within Monogynaspida, the cohort Dermanyssina is

clearly separated by the presence of a secondary insemination

and sperm-storage system in the female and an inseminatory

sperm finger on the male chelicera. Dermanyssines occur

on all continents, including Antarctica. About half of the

described species are free-living predators in soil litter, rotting

wood, compost, herbivore dung, carrion, nests, house

dust, or similar detritus-based systems. These predators are

usually abundant and voracious enough to regulate the populations

of other small invertebrates and are often used in

biocontrol. A few mesostigmatans have switched from external

digestion of prey to ingesting fungal spores and hyphae.

Others feed on pollen, nectar, and other plant fluids. Pollen

feeding is common in the Phytoseiidae, a family that has

successfully colonized the leaf-surface habitat and accounts

for about 15% of described species of Mesostigmata. Many

Ascidae (Naskrecki and Colwell 1998) and Ameroseiidae

have become venereal diseases of plants, that is, pollen- and

nectar-feeding flower mites vectored by insect or bird pollinators.

The Dermanyssoidea contain several massive radiations

of vertebrate and invertebrate parasites, including such

well-known pests as the bird and rat mites and the varroa

mite of bees.

Ricinuleids (Ricinulei)

Ricinulei are an enigmatic group of curious, slow-moving

arachnids that possess a series of unique modifications, including

a hinged plate, the cucullus, at the front of the

prosoma, which acts as a hood covering the mouthparts; a

locking mechanism between the prosoma and the opisthosoma

(shared with the fossil trigonotarbids) that can be uncoupled

during mating and egg-laying; and a highly modified

male third leg that is used for sperm transfer during mating.

This leg structure is analogous to the modified pedipalp of

male spiders, and provides a series of species-specific character

states helpful in delimiting taxa.

Ricinulei are probably the sister group of mites (Lindquist

1984, Weygoldt and Paulus 1979, Shultz 1990, Wheeler and

Hayashi 1998, Giribet et al. 2002). Savory (1977) proposed

a relationship to Opiliones, even suggesting paraphyly of

Opiliones by including Ricinulei, but that hypothesis remains

quite dubious. More recently, addition of the extinct order

Trigonotarbida as well as molecular data suggested a possible

relationship to tetrapulmonates (Dunlop 1996a, Giribet et al.

2002). Internal relationships of extant Ricinulei have been

explored by Platnick (1980).

Hansen and Sшrensen (1904) provided the first comprehensive

taxonomic account of this order, in which they

recognized a single family, Cyptostemmatoidae, with eight

species grouped in the genera Cryptostemma and Cryptocellus.

The order, as currently defined, contains just a single recent

302 The Relationships of Animals: Ecdysozoans

family, Ricinoididae, with three genera (Harvey 2003). Ricinoides

(10 species) occurs in the rainforests of western and

central Africa. Cryptocellus (27 species) and Pseudocellus (18

species) occur in forest and cave ecosystems of Central

America as far north as Texas and as far south as Peru.

Selden (1992) proposed a classification for the order that

divided it into two suborders, Palaeoricinulei for the two

families of Carboniferous ricinuleids (15 species total) and

the Neoricinulei for Ricinoididae.

Palpigrades (Palpigradi)

Palpigrades or micro-whip scorpions are one of the most

enigmatic arachnid orders, with just 78 species in six genera

and two families (Harvey 2003), and an unresolved phylogenetic

position because of doubts regarding the many reductional

apomorphies these small animals possess. Only one

fossil species is known (Selden and Dunlop 1998). Their

phylogenetic placement based on molecular data is similarly

equivocal (Giribet et al. 2002). Palpigrades bear a long, multisegmented

flagellum, three-segmented chelicerae, subsegented

pedipalpal and pedal tarsi, and a host of other

modifications, including lack of slit sensillae, a dorsal hinged

joint between the trochanter and femur on the walking legs

(Shultz 1989), and a pair of anteromedial sensory organs

(Shultz 1990). Palpigrades occur primarily in endogean habitats—

soil, litter, under rocks, in caves and other subterranean

voids—but the remarkable genus Leptokoenenia occurs

in littoral deposits of Saudi Arabia and Congo.

Until recently only a single family, Eukoeneniidae, was

recognized, but Condй (1996) transferred Prokoenenia and

Triadokoenenia to a separate family, Prokoeneniidae. These two

families can be distinguished by the presence (Prokoeneniidae)

or absence (Eukoeneniidae) of abdominal ventral sacs on sternites

IV–VI. This arrangement has not been tested cladistically,

nor has the monophyly of each of the six genera. The genera

are disproportionately sized: Eukoenenia consists of 60 named

species, and the remaining five genera possess a total of just

18 species. Although the differences between families and

genera are well understood (Condй 1996), their interrelationships

have never been examined cladistically.

Spiders (Araneae)

Spiders currently consist of 110 families, about 3500 genera,

and more than 38,000 species (Platnick 2002). Roughly

600 fossil species have been described (Selden 1996, Selden

and Dunlop 1998). Strong synapomorphies support the

clade: cheliceral venom glands, male pedipalpi modified for

sperm transfer, abdominal spinnerets and silk glands, and

lack of the trochanter-femur depressor muscle (Coddington

and Levi 1991). The advent of the scanning electron microscope

in the 1970s rejuvenated spider systematics: microstructures

on the cuticle (sensory tarsal organs, the kinds and

distributions of silk spigots on spinnerets) are now fundamental

to phylogenetic research. Roughly 67 quantitative

cladistic analyses of spiders have been published to date,

covering about 905 genera (about 25% of the known total),

on the basis of approximately 3200 morphological characters.

Nine of these studies focus on interfamilial relationships

(Coddington 1990a, 1990b, Platnick et al. 1991, Goloboff

1993, Griswold 1993, Griswold et al. 1998, 1999, Bosselaers

and Jocquй 2002, Silva Davila 2003). Many of the others that

focus on single families, however, include multiple outgroups

that overlap from one study to another (Coddington 1986a,

1986b, Jocquй 1991, Rodrigo and Jackson 1992, Hormiga

1994, 2000, Davies 1995, 1998, 1999, Harvey 1995,

Hormiga et al. 1995, Gray 1995, Ramнrez 1995a, 1995b,

1997, Pйrez-Miles et al. 1996, Ramнrez and Grismado 1997,

Scharff and Coddington 1997, Sierwald 1998, Huber 2000,

2001, Platnick 1990, 2000, Davies and Lambkin 2000, 2001,

Griswold 2001, Griswold and Ledford 2001, Wang 2002,

Schьtt 2003). The trend has been to address unknown parts

of the spider tree, thus yielding a first-draft, higher level

phylogeny for the order, rather than repeating or intensifying

lower level analyses. On the one hand, overlap and congruence

have been fortuitously sufficient to permit “adding”

results together manually; on the other, they are so sparse

that many details in figure 18.3 are certain to change with

more data and more detailed taxon sampling. Molecular

work, at least above the species level, is still almost nonexistent

(but see Huber et al. 1993, Hausdorf 1999, Piel and

Nutt 1997, Hedin and Maddison 2001). Some molecular

results are strongly contradicted by morphology, such as

rooting the spider clade among arachnids on an araneomorph

rather than a mesothele (Wheeler and Hayashi 1998).

The comparative data for the most inclusive groupings

of spiders have been known for more than a century, but the

data were not rigorously analyzed from a phylogenetic point

of view until the mid-1970s (Platnick and Gertsch 1976).

This analysis clearly showed a fundamental division between

two suborders: the plesiomorphic mesotheles (one family,

Liphistiidae; two genera; about 85 species) and the derived

opisthotheles. Although mesotheles show substantial traces

of segmentation, for example, in the abdomen and nervous

system, the opisthothele abdomen is usually smooth and

the ventral ganglia fused. Opisthotheles is composed of two

major lineages: the baboon spiders (or tarantulas) and their

allies (Mygalomorphae, 15 families, about 300 genera, 2500

species) and the so-called “true” spiders (Araneomorphae,

94 families, 3200 genera, 36,000 species) (Platnick 2003).

Mygalomorphs resemble mesotheles. They tend to be

fairly large, often hirsute animals with large, powerful chelicerae

that live in burrows and, apparently, rely little on silk

for prey capture, at least compared with many araneomorph

spiders. Within mygalomorphs, the atypoid tarantulas are

probably sister to the remaining lineages (Raven 1985,

Goloboff 1993), although some evidence supports the monoArachnida

303

Figure 18.3. Phylogeny of Araneae.

Liphistiidae

Atypidae

Antrodiaetidae

Mecicobothriidae

Idiopidae

Ctenizidae

Actinopodidae

Migidae

Barychelidae

Theraphosidae (Ischnocolinae)

Paratropididae

Theraphosidae (Theraphosinae)

Hypochilidae

Austrochilidae

Gradungulidae

Filistatidae

Caponiidae

Tetrablemmidae

Dysderidae

Segestriidae

Orsolobidae

Oonopidae

Pholcidae

Diguetidae

Plectreuridae

Ochyroceratidae

Leptonetidae

Telemidae

Drymusidae

Scytodidae

Periegopidae

Sicariidae

Eresidae

Oecobiidae

Hersiliidae

Mimetidae + Malkaridae?

Huttoniidae

Palpimanidae

Stenochilidae

Micropholcommatidae

Holarchaeidae

Pararchaeidae

Archaeidae

Mecysmaucheniidae

Deinopidae

Uloboridae

Araneidae

Tetragnathidae Theridiosomatidae

Mysmenidae

Anapidae

Symphytognathidae

Pimoidae

Linyphiidae

Nesticidae

Theridiidae

Cyatholipidae

Synotaxidae

Nicodamidae

Titanoecidae

Phyxelididae

Dictynidae

Zodariidae

Cryptothelidae

Heteropodidae

Anyphaenidae

Clubionidae

Corinnidae

Zoridae

Liocranidae

Philodromidae

Salticidae

Selenopidae

Thomisidae

Neolanidae

Stiphidiidae

Agelenidae

Amphinectidae

Metaltellinae

Desidae

Amaurobiidae

Tengellidae

Zorocratidae

Ctenidae (Acanthoctenus)

Zoropsidae

Miturgidae

Ctenidae

Psechridae

Senoculidae

Oxyopidae

Miturgidae (Uliodon)

Trechaleidae

Lycosidae

Pisauridae

"Dipluridoids" + Hexathelidae

"Nemesiioids" + Microstigmatidae

Gnaphosidae

Prodidomidae

Ammoxenidae

Lamponidae

Cithaeronidae

Gallieniellidae

Trochanteriidae

Araneomorphae

Neocribellatae

Araneoclada

Entelegynae

Canoe Tapetum Clade

Divided Cribellum Clade

RTA Clade Dionycha

Gnaphosoidea

Zodarioids

Amaurobioids

Lycosoidea

Higher Lycosoids

Ctenoid complex

Fused Paracribellar Clade

Agelenoids

Stiphidioids

Titanoecoids

Orbiculariae

Araneoidea

Derived Araneoids

Reduced Piriform Clade

Araneoid Sheet Web Weavers

Spineless Femur Clade

Cyatholipoids

Theridioids

Linyphioids

Symphytognathoids

Deinopoidea

Palpimanoidea

Eresoidea

Haplogynae

Austrochiloidea

Paleocribellatae

Mygalomorphae

Avicularoidea

Crassitarsae

Theraphosodina

Theraphosoidea

Rastelloidina

Domiothelina

Migoidea

Atypoidea

Mesothelae

Opisthothelae

"Cyrtaucheniioids"

304 The Relationships of Animals: Ecdysozoans

phyly of Mecicobothriidae and the atypoids. The atypoid

sister group is Avicularioidea, of which the basal taxon,

Dipluridae, seems to be a paraphyletic assemblage. One of

the larger problems in mygalomorph taxonomy concerns

Nemesiidae, currently 38 genera and 325 species (Goloboff

1993, 1995). The group is conspicuously paraphyletic. The

remaining mygalomorph families are relatively derived and

more closely related to each other than to the preceding. Two

seemingly distinct groups are the theraphosodines [baboon

spiders or true “tarantulas” and their allies (Pйrez-Miles et al.

1996), typically vagabond)] and the rastelloidines (typically

trap door spiders). Because of the evident paraphyly of several

large mygalomorph “families” (Dipluridae, Nemesiidae,

Cyrtaucheniidae), the number of mygalomorph family-level

lineages will probably increase dramatically with additional

research.

Araneomorphs include more than 90% of known spider

species; they are derived in numerous ways and are quite

different from mesotheles or mygalomorphs. Although repeatedly

lost, a strong synapomorphy of this clade is the

fusion and specialization of the anterior median spinnerets

into a flat spinning plate (cribellum) with hundreds to thousands

of spigots that produce a dry yet extremely adhesive

silk (cribellate silk). Many araneomorph lineages independently

abandoned the sedentary web-spinning lifestyle to

become vagabond hunters, but the plesiomorphic foraging

mode seems to be a web equipped with dry adhesive silk

(austrochiloids, Filistatidae among the haplogynes, oecobiids

and eresids among eresoids, many entelegyne groups).

Within Araneomorphae, the relictually distributed Hypochilidae

(two genera, 11 species) are sister to the remaining

families (Platnick et al. 1991). Some austrochiloid

genera have lost webs, and most haplogynes are also vagabonds.

These haplogyne taxa tend to live in leaf litter or

other soil habitats (Caponiidae, Tetrablemmidae, Orsolobidae,

Oonopidae, Telemidae, Leptonetidae, Ochyroceratidae,

etc.; Platnick et al. 1991). The haplogyne cellar spiders

(Pholcidae) are exceptional for their relatively elaborate,

large webs. Some of the most common and ubiquitous commensal

spider species are pholcids.

The entelegyne “node” in spiders is supported by several

synapomorphies (Griswold et al. 1999). Among other things,

the copulatory apparatus fundamentally changed in both

males and females. One theory is that the change was driven

by cryptic female choice: the tendency of females to choose

males on the basis of their effectiveness in genitalic stimulation

during copulation (Eberhard 1985). Females evolved

a complex antechamber to their gonopore and acquired a

second opening of the reproductive system to the exterior

coupled with an unusual “flow-through” sperm management

system in which deposited sperm are stored in separate chambers

for later use in fertilizing eggs. Females also evolved a

special sort of silk used only in egg sacs, which is almost

universally present among entelegynes although its function

is unknown. Male genitalia became hydraulically rather than

muscularly activated and more elaborate; the interaction with

the equally complicated female genitalia became more complex.

This “hydraulic bulb” of the male genitalia is so flexible

during its operation that males have evolved various

levers and hooks that seem to serve mainly to stabilize and

orient their own genitalia during copulation. One of these,

the “retrolateral tibial apophysis” has given its name to a fairly

large clade of entelegyne families (the “RTA clade”; Coddington

and Levi 1991, Griswold 1993, Sierwald 1998).

Non-entelegynes, in contrast, have relatively simple male and

female genitalia in which the female anatomy is one or two

pairs or an array of blind receptacula, and the male intromittent

organ is a smooth and simple hypodermiclike structure

operated by tarsal muscles.

Among entelegynes the “eresoid” families seem basal. No

clear synapomorphies define this group; in various analyses,

eresoids may be paraphyletic (Coddington 1990a, Griswold

et al. 1999). Perhaps the hottest current controversy in entelegyne

systematics concerns the Palpimanoidea (10 families,

54 genera). Before their relimitation as a monophyletic group

(Forster and Platnick 1984), palpimanoid families were dispersed

throughout entelegyne classification: mimetids,

archaeids, and micropholcommatids in particular were considered

to be araneoids. The two classic features defining

Palpimanoidea are setae shortened and thickened to function

as cheliceral teeth (very rare in spiders) and the concentration

of cheliceral glands on a raised mound. However, these two

features are homoplasious within palpimanoids, and evidence

is building that some palpimanoid taxa are araneoids after all

(Schьtt 2000).

One of the larger entelegyne lineages is the Orbiculariae.

It unites two robustly monophyletic superfamilies (Araneoidea,

12 families, 980 genera; and Deinopoidea, 2 families,

23 genera) mainly but not entirely on the basis of web

architecture and morphology associated with web spinning

(Coddington 1986b and references therein). Both groups

spin orb webs. Ethological research on orb weavers shows

that orbs are constructed in fundamentally similar ways, although

the deinopoid orb uses the plesiomorphic cribellate

silk, whereas araneoids use the derived viscid silk (Griswold

et al. 1998). Araneoidea are by far the larger taxon and includes

many ecologically dominant web-weaving species.

Interestingly, derived araneoids (the “araneoid sheet web

weavers,” six families, 685 genera) no longer spin orbs (some

may not even spin webs) but rather sheets, tangles, and cobwebs

(Griswold et al. 1998). There is a strong trend among

araneoids to reduce and stylize the spinning apparatus

(Hormiga 1994, 2000).

The sister taxon of Orbiculariae remains a mystery, although

the most recent research suggests that most other

entelegyne lineages are more closely related to each other than

any is to the orb weavers (Griswold et al. 1999). Thus, the

orbicularian sister group at present seems likely to be a very

Arachnida 305

large, hitherto unrecognized lineage consisting of amaurobioids

(Davies 1995, 1998, 1999, Davies and Lambkin 2000,

2001), “wolf” spiders [Lycosoidea (Griswold 1993)], twoclawed

hunters (Dionycha; Platnick 1990, 2000), and other,

smaller groups (Jocquй 1991). Many of these lineages are relictual

austral groups whose diversity is very poorly understood.

The phylogenetic structure among non-orbicularian entelegynes,

therefore, is highly provisional at this point. Because

of a long-standing emphasis on symplesiomorphy,

many of the classical entelegyne families (most seriously

Agelenidae, Amaurobiidae, Clubionidae, Ctenidae, and

Pisauridae) were paraphyletic. Dismembering these assemblages

into monophyletic units has been difficult because the

monophyly of related families is also often doubtful (e.g.,

Amphinectidae, Corinnidae, Desidae, Liocranidae, Miturgidae,

Tengellidae, Stiphidiidae, Titanoecidae). Therefore

neither the RTA clade, nor the two-clawed hunting spider

families (Dionycha) may be strictly monophyletic, although

in each is certainly a large cluster of closely related lineages.

Dionychan relationships are quite unknown, although some

headway has been made in the vicinity of Gnaphosidae

(Platnick 2000). In contrast, Lycosoidea was supposedly

based on a clear apomorphy in eye structure, but recent

results suggest that this feature evolved more than once

or, less likely, has been repeatedly lost (Griswold et al. 1998).

The nominal families Liocranidae and Corinnidae are

massively polyphyletic (Bosselaers and Jocquй 2002). The

nodes surrounding Entelegynae will certainly change in the

future.

In sum, phylogenetic understanding of spiders has advanced

remarkably since the early 1980s. We are on the cusp

of having at least a provisional, quantitatively derived hypothesis

at the level of families, but on the other hand, the density

and consistency of the data for subsidiary taxa will remain

soft for some years to come.

Whip Spiders (Amblypygi)

Whip spiders, also known as tailless whip scorpions, are a

conspicuous group of mostly medium to large, dorsoventrally

flattened arachnids distributed throughout the humid tropics

and subtropics with a few species occurring in the arid

regions of southern Africa. Although most species are epigean,

several troglobite species are known.

Monophyly of Amblypygi is supported by several features,

including the morphology and orientation of the pedipalps,

the enormously elongated antenna-like first legs that

act as tactile organs, and the presence of a cleaning organ on

the palpal tarsus. The order belongs to Pedipalpi as the sister

to Camarostomata (Uropygi + Schizomida) (Shultz 1990,

1999, Giribet et al. 2002), although some treatments place

them as the sister to Araneae (e.g., Platnick and Gertsch 1976,

Weygoldt and Paulus 1979, Wheeler and Hayashi 1998).

Current understanding of the internal phylogeny and

classification of Amblypygi is almost entirely the work of

Weygoldt (1996, 2000), who recognized five families, placed

in two suborders, Paleoamblypygi and Euamblypygi. Paleoamblypygi

contain a single West African species, Paracharon

caecus (Paracharontidae), as well as five Carboniferous species

that remain unplaced in a family. Paleoamblypygi differ

in various features, including an anteriorly produced carapace

and reduced pedipalpal spination. The Euamblypygi

consist of the remaining whip spiders, including the circumtropical

Charinidae, which contains three genera and

43 species. Charinidae may not be monophyletic (Weygoldt

2000). The remaining three families comprise Neoamblypygi,

which is in turn divided into the Charontidae and Phrynoidea;

the latter includes the Phrynidae and Phrynichidae. The

Charontidae consist of two genera and 11 species from Southeast

Asia and Australasia. The Phrynidae contain four genera

and 55 species from the Americas, with a single outlying

species from Indonesia (Harvey 2002a). The Phrynichidae

contain 31 species in seven genera from Africa, Asia, and

South America.

Whip Scorpions (Uropygi)

Whip scorpions are large, heavily sclerotized arachnids that

have changed little since the Carboniferous. They primarily

inhabit tropical rainforests but some, such as the well-known

North American Mastigoproctus giganteus, occupy arid environments.

Like other members of the Pedipalpi, tarsus I is

subsegmented and is used as a tactile organ. They possess a

number of distinctive features, including palpal chelae with

the movable finger supplied with internal musculature (Barrows

1925), a long, multisegmented flagellum, raptorial pedipalps,

and a long rectangular carapace. The abdomen bears

a pair of glands that discharge at the base of the flagellum

and are used to direct a spray of acetic acid (vinegar) at potential

predators (Eisner et al. 1961; Haupt et al. 1988). On

account of this unusual ability, whip scorpions are known

as vinegaroons (or vinegarones) in the southern United

States.

Uropygi are consistently placed as sister to Schizomida,

and the gross morphology of its members suggests monophyly.

Dunlop and Horrocks (1996) suggested that the Carboniferous

uropygid Proschizomus may represent the sister

to Schizomida, rendering Uropygi paraphyletic. The sole

family Thelyphonidae is divided into four subfamilies:

Hypoctoninae (4 genera, 25 species: Southeast Asia, South

America, west Africa), Mastigoproctinae (4 genera, 18 species:

Americas, Southeast Asia), Typopeltinae (1 genus, 10

species), and Thelyphoninae (7 genera, 48 species: Southeast

Asia and Pacific) (Rowland and Cooke 1973, Harvey

2003). Eight fossil species have been described (Selden

and Dunlop 1998, Harvey 2003). Only Typopeltinae and

306 The Relationships of Animals: Ecdysozoans

Thelyphoninae are well supported by apomorphic character

states; Hypoctoninae and Mastigoproctinae appear to be

solely defined by plesiomorphies (M. Harvey, unpubl. obs.).

Schizomids (Schizomida)

Schizomids are small (<1 cm), weakly sclerotized arachnids

that can be recognized by the presence of a short abdominal

flagellum that generally in females consists of three or four

segments and in males is single segmented. The shape and

setation of the male flagellum are species specific (e.g., Rowland

and Reddell 1979, Harvey 1992b, Reddell and Cokendolpher

1995), probably reflecting its use during courtship and mating,

in which it is gripped in the mouthparts of the female

(Sturm 1958).

The order contains two families, the Central American

Protoschizomidae and the widely distributed Hubbardiidae.

Three fossil species have been described (Selden and Dunlop

1998). Protoschizomidae are represented by two genera and

11 species from Mexico or Texas, many from caves (Rowland

and Reddell 1979, Reddell and Cokendolpher 1995). The

Hubbardiidae consist of two subfamilies. Megaschizominae are

represented by two species of Megaschizomus from Mozambique

and South Africa. The widespread Hubbardiinae consists

of 205 species in 35 genera (Harvey 2003), the vast

proportion of which have been named in the last 40 years

because of an increased awareness of previously overlooked

character systems such as female genitalia. Cokendolpher and

Reddell (1992) presented a cladistic analysis of the basal clades

of Schizomida but refrained from including individual hubbardiine

genera, whose systematics are still in a state of flux.

Harvestmen (Opiliones)

Commonly known as “daddy longlegs,” harvestmen, shepherd

spiders, or harvest spiders (among other names), the Opiliones

were well known to North Temperate farmers and shepherds

because of their abundance at harvest time. These are the only

nonacarine arachnids known to ingest vegetable matter, but

generally they prey on insects, other arachnids, snails, and

worms. They can ingest particulate food, unlike most arachnids,

which are liquid, external digesters. The order is reasonably

well studied, although many of the Southern Hemisphere

families are still poorly understood taxonomically.

Opiliones contain 43 families, about 1500 genera, and

about 5000 species, but many more species await discovery

and description. Most members of Opiliones are small to

medium in size (<1 mm to almost 2.5 cm in the European

species Trogulus torosus) and inhabit moist to wet habitats

on all continents except Antarctica. Laniatores include large

(>2 cm), colorful, well-armored Opiliones, most diverse in

tropical regions of the Southern Hemisphere, but many

laniatorids are also very small. Eupnoi and Dyspnoi are more

widely distributed and are especially abundant in the Northern

Hemisphere. Members of Cyphophthalmi are distributed

worldwide but are among the smallest (down to 1 mm) and

most obscure members of the Opiliones.

Opilionids are typical arachnids with two basic body

regions, and their junction is not constricted, giving them

the appearance of “waistless” spiders. The cephalothorax

generally has a pair of median simple eyes surmounting the

ocular tubercle. Cyphophthalmi either lack eyes entirely or

have a pair of eyes (some stylocellids), possibly lateral eyes.

The anterior rim of the cephalothorax bears the large openings

of a pair of secretory organs, known as repugnatorial

glands. These differ in position and type among different

groups within Opiliones, being most obvious in the suborder

Cyphophthalmi, whose members take the shape of cones,

named ozophores. The cephalothorax bears one pair of chelate

three-segmented chelicerae for manipulating the food

particles, one pair of pedipalps of either tactile or prehensile

function, and four pairs of walking legs. The legs can be

enormously long (>15 cm) in some Eupnoi and Laniatores

species. Laniatorid palps are usually large and equipped with

parallel rows of ventral spines that act as a grasping organ.

The second pair of walking legs is sometimes modified for a

tactile or sensory function.

The abdomen is clearly segmented in most species, although

some segments may appear fused to different degrees.

One pair of trachea for respiration opens ventrally on the

sternite of the first abdominal segment. The genital aperture

and its associated structures (operculum) open on the same

segment. The anal region is very often modified; certain

Cyphophthalmi males have anal glands, secondary sexual

characters that are probably secretory. Females may have a

long ovipositor with sensory organs on the tip that is used

to check the soil quality for egg deposition. Males have a

muscular or hydraulically operated penis, or copulatory

organ. Some mites have vaguely similar structures, but otherwise

ovipositors and penises are unique to Opiliones. Fertilization

is thus internal and direct.

The monophyly of Opiliones is strongly supported by the

presence of five unambiguous synapomorphies: (1) the presence

of repugnatorial glands, (2) the special vertical bicondylar

joint between the trochanter and femur of the

walking legs, (3) the paired tracheal stigmata on the genital

segment, (4) the male penis, and (5) the female ovipositor

(Shultz 1990, Giribet et al. 2002). Opilionid taxonomy supposes

a basic division between Cyphophthalmi (no common

name, six families) and the remaining harvestmen

(“Phalangida”), consisting of Eupnoi (six families), Dyspnoi

(seven families), and Laniatores (24 families). Eupnoi and

Dyspnoi have been traditionally grouped in Palpatores

(fig. 18.4).

Cyphophthalmids are small (1–6 mm), hard-bodied, soil

animals that superficially resemble mites. Six families are recognized

(Shear 1980, 1993, Giribet 2000), although some

Arachnida 307

do not withstand cladistic tests (Giribet and Boyer 2002).

“Palpatores” are diverse and heterogeneous; their monophyly

is disputed. The component Eupnoi and Dyspnoi, however,

are well-supported monophyletic clades, each with two superfamilies.

Eupnoi includes Caddoidea (one family) and

Phalangioidea (five families), and Dyspnoi includes Ischyropsalidoidea

(three families) and Troguloidea (four families).

The caddoids and especially the phalangioids include the

typical “daddy long legs” of the Holarctic region, although

Gondwanan families of both groups also exist. Ischyropsalidoids

and troguloids are diverse but more poorly known.

Laniatores, in contrast, are heavily sclerotized, usually shortlegged,

often fantastically armored animals with diversity

concentrated in the Southern Hemisphere.

Only recently have workers focused on the internal phylogenetic

structure of Opiliones. Five modern quantitative

cladistic studies have been published to date, covering about

50 genera and directed mainly at interfamilial relationships

(Shultz 1998, Giribet et al. 1999, 2002, Giribet and Wheeler

1999, Shultz and Regier 2001). In contrast to the situation

in spiders, molecular data are strongly represented and largely

agree with morphology. Despite the relatively small size of

the group, no phylogeny to date has included all families.

Martens and coworkers (Martens 1976, 1980, 1986, Martens

et al. 1981) and Shear (1986) provided an early overview

of aspects of opilionid phylogeny and emphasized the

phylogenetic value of the male genital organs. Martens rejected

the division between Cyphophthalmi and Phalangida,

instead suggesting the taxon “Cyphopalpatores,” consisting

of Cyphophthalmi nested within a paraphyletic Palpatores.

The idea depended largely on penis morphology, but

because a penis among arachnids is unique to Opiliones

(convergent in some mites), the character transformation

was polarized and ordered by evolutionary speculations

rather than outgroups. If the features are left unordered,

Cyphopalpatores disappear under parsimony (Shultz 1998,

Giribet et al. 2002). All later work has decisively rejected

the Cyphopalpatores hypothesis and agrees that Phalangida

are monophyletic.

Opinions diverge on groups within Phalangida. Three

monophyletic groups clearly exist: Eupnoi, Dyspnoi, and

Laniatores, as recognized by Hansen and Sшrensen (1904),

but the monophyly of Palpatores is still disputed. Molecular

data (18S rRNA and 28S rRNA) separately and combined

with morphology suggest Dyspnoi as sister to Laniatores, thus

rendering Palpatores paraphyletic (Giribet 1997, Giribet et al.

1999). The morphological codings employed in these studies

were later criticized by Shultz and Regier (2001), who

presented new molecular data to support Palpatores monophyly

but dismissed the morphological evidence. A more

inclusive analysis of morphology and molecular data including

35 genera of Opiliones recently reaffirmed Palpatores

paraphyly, a result stable under a wide variety of analytical

parameters (Giribet et al. 2002). This result also accords with

a study of internal Cyphophthalmi relationships (Giribet and

Boyer 2002). The studies of Shultz and Regier (2001) and of

Figure 18.4. Phylogeny of Opiliones.

Sironidae

Stylocellidae

"Metopilio" group

Phalangiidae

Sclerosomatidae

Caddidae

Ischyropsalidae

Ceratolasmatidae

Sabaconidae

Dicranolasmatidae

Nemastomatidae

Nipponopsalidae

Trogulidae

Triaenonychidae

Oncopodidae

Phalangodidae

Gonyleptidae

Stygnopsidae

Cosmetidae

Phalangioidea

Oncopodoidea

Travunioidea

Ischyropsalidoidea

Laniatores

Cyphophthalmi

Eupnoi

Caddoidea

Gonyleptoidea

Troguloidea

Sironoidea

Dyspnoi

Phalangida

Palpatores

308 The Relationships of Animals: Ecdysozoans

Giribet et al. (2002) disagree on the internal resolution of

Troguloidea and Ischyopsalidoidea, possibly because of the

sparser taxon sampling in Shultz and Regier’s analysis or

differences in information content between the genes used.

Phylogeny of Laniatores is still in its infancy. No analysis

has yet included a large sample with the exception of a study

on Gonyleptoidea (Kury 1993) and the more recent molecular

(Shultz and Regier 2001) and total evidence (Giribet et al.

1999, 2002) analyses considering Opiliones as a whole. The

Laniatores are a well-supported monophyletic group originally

divided into two groups, Oncopodomorphi and

Gonyleptomorphi, by Šilhavy (1961). Martens (1976) later

divided Laniatores into the three superfamilies Travunioidea,

Oncopodoidea, and Gonyleptoidea, although it has been

suggested (A. B. Kury, unpubl. obs.) suggests that Gonyleptoidea

could be paraphyletic with respect to Oncopodoidea,

constituting a clade informally named “Grassatores.” The

tripartite relationship proposed by Martens (1976) for

Laniatores was also corroborated by total evidence analyses

(Giribet et al. 1999, 2002), but many laniatorean families

remain untested and their phylogenetic affinities unexplored.

Fossil members of Opiliones are rare, and their fossil

record is currently restricted to a few Paleozoic and Mesozoic

examples plus a more diverse Tertiary record based

principally on the Florissant Formation and on Baltic and

Dominican ambers (for reviews, see Cokendolpher and

Cokendolpher 1982, Selden 1993b). The majority of known

fossil harvestmen strongly resemble members of Eupnoi and

Dyspnoi. Laniatores is currently only known from Tertiary

ambers, and all the Dominican amber harvestmen described

so far are Laniatores (Cokendolpher and Poinar 1998). A

single fossil of the suborder Cyphophthalmi is known from

Bitterfeld amber, Sachsen-Anhalt, Germany (Dunlop and

Giribet in press).

Scorpions (Scorpiones)

Although their placement in Arachnida remains controversial

(Weygoldt and Paulus 1979, Shultz 1990, 2000, Sissom

1990, Starobogatov 1990, Wheeler et al. 1993, Dunlop 1998,

Dunlop and Selden 1998, Jeram 1998, Weygoldt 1998,

Wheeler and Hayashi 1998, Dunlop and Webster 1999,

Dunlop and Braddy 2001, Giribet et al. 2002), scorpions

are unquestionably monophyletic. The clade is supported by

11 synapomorphies, including pectines (ventral abdominal

sensory appendages), chelate pedipalps, and a five-segmented

postabdomen (metasoma) terminating with a modified telson,

including a pair of venom glands internally and a sharp

aculeus distally, which functions as a stinging apparatus for

offense and defense (Shultz 1990, Wheeler et al. 1993,

Wheeler and Hayashi 1998, Giribet et al. 2002).

The approximately 1340 extant (Recent) scorpion species

in 163 genera and 17 families (Fet et al. 2000, Lourenзo

2000, Prendini 2000, Fet and Selden 2001, Soleglad and

Sissom 2001, Kovarнk 2001, 2002) constitute a monophyletic

crown group with a post-Carboniferous common ancestor

(Jeram 1994a, 1998). Fossil representatives comprise 92

species assigned to 71 genera and 42 families (Fet et al. 2000),

of which only six species can be placed in two extant families.

All Paleozoic scorpions form the stem group of this clade,

with Palaeopisthacanthus the most crownward stem taxon

(Jeram 1994b, 1998), sister to recent scorpions (Soleglad and

Fet 2001). Paleozoic scorpions were far more diverse than

present forms and are pivotal to resolving the phylogenetic

placement of the order (Jeram 1998, Dunlop and Braddy

2001), but their phylogeny and classification are controversial

and largely decoupled from that of Recent scorpions.

Some classifications (Kjellesvig-Waering 1986, Starobogatov

1990) were typological and overly detailed (Sissom 1990, Fet

et al. 2000). Kjellesvig-Waering (1986) placed Paleozoic

scorpions into two suborders, five infraorders, 21 superfamilies,

and 48 families; only Palaeopisthacanthidae was

placed with the suborder containing Recent period scorpions.

Starobogatov (1990) treated scorpions and eurypterids

as two superorders and recognized two orders and seven suborders

of scorpions. Other classifications, although based on

phylogenetic analysis (Stockwell 1989, Selden 1993a, Jeram

1994a, 1994b, 1998), were hampered by the limited quantity

and quality of data obtainable from fragmentary fossils.

These treat scorpions as a class Scorpionida, with two extinct

and one Recent order, the latter containing several suborders

and infraorders, of which, again, only one contains all living

representatives. In the latest classification of Paleozoic scorpions

(Jeram 1998), hierarchical ranks are not established

because the rank of the crown group is uncertain and there

is no point of reference for the stem group clades.

Stockwell (1989) conducted the first quantitative phylogenetic

analysis of Recent scorpions, excluding Buthidae,

and proposed a new higher classification. Stockwell retrieved

four major clades of Recent scorpions, ranked as superfamilies:

Buthoidea (Buthidae and Chaerilidae), Chactoidea

(Chactidae, Euscorpiidae, and Scorpiopidae), Scorpionoidea

(Bothriuridae, Diplocentridae, Ischnuridae, Scorpionidae,

and Urodacidae), and Vaejovoidea (Iuridae, Superstitioniidae,

and Vaejovidae). However, Stockwell used groundplans

derived from often paraphyletic genera as terminals (Prendini

2001b), casting doubt on his cladistic findings and resulting

classification. Further, only his proposed revisions to the

suprageneric classification of North American Chactoidea

and Vaejovoidea were actually published (Stockwell 1992),

although others, notably Lourenзo (1998a, 1998b, 2000),

have since implemented some of his other unpublished

revisions.

Only two significant family-level morphological analyses

appeared since Stockwell (1989). One treats Scorpionoidea

using exemplar species (Prendini 2000). The other treats

the chactoid family Euscorpiidae using genera as terminals

(Soleglad and Sissom 2001). Soleglad and Fet (2001) recently

attempted to illuminate basal relationships among extant

Arachnida 309

scorpions (placement of the enigmatic Chaerilidae and monotypic

Pseudochactidae), in an analysis based solely on

trichobothrial characters, and Fet et al. (2003) presented an

analysis of 17 buthid exemplar species based on 400–450

bp of 165 rDNA. A molecular analysis of the entire order,

based on nuclear and mitochondrial DNA loci, to be combined

with available morphological data, is underway (L.

Prendini and W. Wheeler, unpubl. obs.).

Stockwell’s (1989) unpublished cladogram remains the

only comprehensive hypothesis for nonbuthid families and

genera. Addressing the internal relationships of Buthidae

(~50% and 43% of all generic and species diversity, respectively)

is a major goal of future research. Although it will

certainly change, the most reasonable working hypothesis

of scorpion phylogeny is basically Stockwell’s (1989) cladogram

for nonbuthids as emended by Prendini (2000),

Soleglad and Sissom (2001), and Soleglad and Fet (2001) and

including the little that is known about buthid phylogeny

(fig. 18.5). Most of Lourenзo’s (1996, 1998b, 1998c, 1999,

2000) proposed familial and superfamilial emendations

cannot be justified phylogenetically (Prendini 2001b, 2003a,

2003b, Soleglad and Sissom 2001, Volschenk 2002) but are

included here because they represent the most recent published

opinion.

Most authorities agree that the basal dichotomy among

Recent scorpions separates buthids (Buthoidea) from nonbuthids,

a hypothesis supported by morphological, embryological,

toxicological, and DNA sequence data (Lamoral 1980,

Stockwell 1989, Sissom 1990, Fet and Lowe 2000, Soleglad

and Fet 2001, Fet et al. 2003, L. Prendini and W. Wheeler,

unpubl. obs.). The divergence predates the breakup of Pangaea.

Similarly, it is clear that the buthoid clade is monophyletic,

although the monogeneric Microcharmidae (Lourenзo 1996,

1998c, 2000) renders Buthidae paraphyletic (Volschenk

2002). Within Buthidae sensu lato, a basal dichotomy between

New and Old World genera has also been retrieved with toxicological

and DNA sequence data (Froy et al. 1999, Tytgat

et al. 2000, L. Prendini and W. Wheeler, unpubl. obs.).

The Buthidae are the largest and most widely distributed

scorpion family (81 genera, 570 species). Buthids are characterized

by eight chelal carinae, the type A trichobothrial

pattern, and flagelliform hemispermatophore, whereas most

also display a triangular sternum (Vachon 1973, Stockwell

1989, Sissom 1990, Prendini 2000). Buthidae include the

majority of species known to be highly venomous to humans.

Buthid scorpion toxins block sodium and potassium channels,

preventing transmission of action potentials across synapses

(Tytgat et al. 2000). At the clinical level, this results in

severe systemic symptoms and signs of neurotoxicosis (extreme

pain extending beyond the site of envenomation, disorientation,

salivation, convulsions, paralysis, asphyxia, and

often death). Toxins affecting sodium channels are better

known and divided into two major classes, alpha and beta,

according to physiological effects and binding properties

(Froy et al. 1999). Alpha toxins occur among Old and New

World buthids, whereas beta toxins occur only among New

World buthids.

Examining the phylogenetic placements of the enigmatic

Chaerilidae (one genus and 19 species, Khatoon 1999,

Kovarнk 2000) from tropical South and Southeast Asia, and

recently described monotypic Pseudochactidae (Gromov

1998), known only from Central Asia, is critical for resolving

basal relationships of scorpions. Both display autapomorphic

trichobothrial patterns, dubbed type B (Vachon

1973) and type D (Soleglad and Fet 2001), respectively, along

with a peculiar mix of buthid and nonbuthid character states.

Chaerilidae additionally exhibit an autapomorphic, fusiform

hemispermatophore (Stockwell 1989, Prendini 2000). Although

Stockwell (1989) placed Chaerilidae as sister taxon

of Buthidae, mounting evidence confirms earlier opinions

Pseudochactidae Figure 18.5. Phylogeny of Scorpiones.

New World Buthidae

Old World Buthidae

Microcharmidae

Belisariinae (Troglotayosicidae)

Chactidae

Euscorpiinae

Megacorminae

Scorpiopinae

Troglotayosicinae (Troglotayosicidae)

Superstitioniinae

Typhlochactinae

Vaejovidae

Iurinae

Caraboctoninae

Hadrurinae

Bothriuridae

Heteroscorpionidae

Urodacidae

Hemiscorpiidae

Ischnuridae

Scorpionidae

Diplocentrinae

Nebinae

Buthoidea

Scorpionoidea

Vaejovoidea

Chactoidea

Diplocentridae

Euscorpiidae

Superstitioniidae

Iuridae

Chaeriloidea: Chaerilidae

310 The Relationships of Animals: Ecdysozoans

that they are sister group of nonbuthids (Lamoral 1980,

Lourenзo 1985, Prendini 2000, Soleglad and Fet 2001),

whereas Pseudochactidae may, instead, be sister group of

buthids (Fet 2000, Soleglad and Fet 2001, Fet et al. 2003).

Neither hypothesis based on evidence supports Lourenзo’s

(2000) proposal to place Pseudochactidae with Chaerilidae

in a unique subfamily, Chaeriloidea.

All remaining scorpions are characterized by the type C

trichobothrial pattern and the lamelliform hemispermatophore,

whereas most display 10 chelal carinae and a pentagonal sternum

(Vachon 1973, Stockwell 1989, Sissom 1990, Prendini

2000). According to morphological and molecular evidence

(Stockwell 1989, L. Prendini and W. Wheeler, unpubl. obs.),

the type C scorpions comprise two distinct clades, corresponding

to Stockwell’s (1989) superfamilies Scorpionoidea and

(Chactoidea + Vaejovoidea).

Relationships in the scorpionoid clade (37 genera and

380 species, or 23% and 29% of generic and species diversity)

are better understood. All scorpionoid families are

monophyletic according to morphological and molecular

evidence (Stockwell 1989, Prendini 2000, L. Prendini and

W. Wheeler, unpubl. obs.). Placement of Bothriuridae, a

Gondwanan group with species in South America, Africa, India,

and Australia, remains contentious. Bothriuridae was placed

as sister to the chactoid-vaejovoid clade in some reconstructions

(Lamoral 1980, Lourenзo 1985) and, more recently

(Lourenзo 2000), in a unique superfamily Bothriuroidea. However,

quantitative analyses (Stockwell 1989, Prendini 2000)

place it as sister to the remaining scorpionoid families, monophyly

of which is, in turn, well supported by embryological and

reproductive characters, the most important being katoikogenic

development. Embryos develop in ovariuterine diverticula and

obtain nutrition through specialized connections with digestive

caeca, rather than developing in the lumen of the

ovariuterus (apoikogenic development) as in other scorpions.

Katoikogenic scorpions occur mostly in the Old World and

include some of the largest and most impressive scorpions.

Relationships among the katoikogenic scorpionoid families,

portrayed in figure 18.5, are well supported, except for the

sister group relationship of Malagasy Heteroscorpionidae and

Australian Urodacidae, which warrants additional testing

(Prendini 2000).

Monophyly of the chactoid-vaejovoid clade (42 genera

and 360 species, or 26% and 27% of generic and species

diversity) appears well supported by morphological and

molecular data (Stockwell 1989, L. Prendini and W. Wheeler,

unpubl. obs.), but relationships among its component families,

and monophyly thereof, are uncertain. Chactoidea and

Vaejovoidea, as conceptualized by Stockwell (1989), may not

withstand further analysis.

The chactoid-vaejovoid lineage includes the traditional

and severely paraphyletic families Chactidae, Iuridae, and

Vaejovidae. In an attempt to achieve monophyly, Stockwell

(1989, 1992) removed Scorpiopidae from Vaejovidae, and

Superstitioniidae and Euscorpiidae (to which he transferred

the chactid subfamily Megacorminae) from Chactidae.

Soleglad and Sissom (2001) further altered these families by

placing the scorpiopid genera into Euscorpiidae and transferring

Chactopsis from Chactidae to Euscorpiidae. Chactid

monophyly, particularly inclusion of the North American

Nullibrotheas in an otherwise exclusively neotropical group,

is untested. Euscorpiidae, comprising species from Europe,

Asia, and the Americas, and Vaejovidae, including most

North American species, now appears to be monophyletic.

This cannot be said for Superstitioniidae, a family consisting

almost entirely of eyeless, depigmented troglobites from

Mexico that, in Stockwell’s (1989, 1992) view, included two

additional troglobites: Troglotayosicus from Ecuador and Belisarius

from the Pyrenees (France and Spain). Sissom (2000)

questioned their inclusion in Superstitioniidae. Lourenзo

(1998b) placed them in a new family, Troglotayosicidae, because

of their eyeless, troglobite habitus. Notwithstanding

that eyelessness may have evolved convergently in the caves

of Ecuador and the Pyrenees, morphological and molecular

evidence (Soleglad and Sissom 2001, L. Prendini and

W. Wheeler, unpubl. obs.) indicates that Belisarius is more

closely related to Euscorpiidae than to Troglotayosicus, which

probably is a superstitioniid.

Iuridae, also in the chactoid-vaejovoid clade, include six

genera from North America, South America, and southwestern

Eurasia, formerly distributed among two families and four

subfamilies. This heterogeneous group is united by a single

synapomorphy—a large, ventral tooth on the cheliceral movable

finger (Francke and Soleglad 1981, Stockwell 1989).

However, mounting morphological and molecular evidence

(L. Prendini and W. Wheeler, unpubl. obs.) suggests that it

is paraphyletic. Few agree on placement of the monotypic

North American Anuroctonus in the chactoid-vaejovoid clade

at large, although it might be related to Hadrurus, also from

North America (Stockwell 1989, 1992). The South American

Caraboctonus and Hadruroides form a monophyletic

group, as do the Eurasian Calchas and Iurus, but the Eurasian

genera display significant trichobothrial and pedipalp carinal

differences, suggesting that their putative relationship to the

other genera is spurious.

Pseudoscorpions (Pseudoscorpiones)

Pseudoscorpions, false scorpions, or book scorpions are a

cosmopolitan group that consists of 24 families, 425 genera,

and 3261 species (Harvey 1991, 2002b, M. S. Harvey, unpubl.

obs.). They represent a monophyletic clade strongly supported

by several features, but only one, the presence of a silk producing

apparatus discharging through the movable cheliceral

finger, is deemed to be autapomorphic. Other important

features include the presence of chelate pedipalps, loss of the

median eyes, median claw absent from all legs but replaced

by an arolium, and two-segmented chelicerae. They represent

the sister group of Solifugae, together comprising the

Arachnida 311

Haplocnemata (Shultz 1990, Wheeler et al. 1993, Wheeler

and Hayashi 1998, Giribet et al. 2002).

Chamberlin (1931) provided the first modern classification

of the order, recognizing the groups Heterosphyronida

and Homosphyronida. The former consisted solely of the

Chthonioidea, whereas the latter consisted of two suborders,

Diplosphyronida (Neobisioidea and Garypoidea) and Monosphyronida

(Feaelloidea, Cheiridioidea, and Cheliferoidea).

Beier (1932a, 1932b) adopted this classification but changed

the subordinal names to Chthoniinea, Neobisiinea, and

Cheliferiinea. These complementary classifications remained

in place, with various new families being added or synonymized,

until Harvey (1992a) presented a cladistic analysis of

the group based upon 200 morphological and behavioral characters.

Harvey’s analysis (fig. 18.6) hypothesized a different

arrangement, with the suborder Epiocheirata, composed of the

superfamilies Chthonioidea and Feaelloidea, representing the

sister to the remaining Iocheirata. Epiocheiratans lack a venom

apparatus in the chelal fingers, and adults and later nymphal

instars always possess a small unique diploid trichobothrium

on the distal end of the fixed chelal finger. Chthonioidea are

dominated by the cosmopolitan Chthoniidae (30 genera, 612

species). Tridenchthoniidae (15 genera, 70 species) is largely

tropical, whereas Lechytiidae (Lechytia, 22 species) is sporadically

distributed. Whereas the superfamily Chthonioidea and

the families Tridenchthoniidae and Lechytiidae are each clearly

monophyletic, Chthoniidae probably are not. The Pseudotyrannochthoniinae

and some other apparently basal taxa such

as Sathrochthonius may warrant removal from the family. Feaelloidea

are curiously distributed with Pseudogarypidae (seven

species, two genera) in North America and Tasmania, and

Feaellidae (11 species, one genus), on continents bordering

the Indian Ocean. These distributions are undoubtedly

vicariant (Harvey 1996). The group was once more widely

distributed, because three species of Pseudogarypus are known

from Oligocene Baltic amber deposits.

The larger suborder, Iocheirata, is characterized by the

presence of a venom apparatus in the chelal fingers (later lost

in one finger in several lineages) and absence of the diploid

trichobothrium. Iocheirata contains Hemictenata (Neobisioidea)

and Panctenata (Olpioidea, Garypoidea, Sternophoroidea,

and Cheliferoidea).

Neobisioidea are a basal clade containing Bochicidae (10

genera, 38 species) and Ideoroncidae (9 genera, 54 species),

successively followed by the Hyidae (three genera, nine species),

Gymnobisiidae (four genera, 11 species), Neobisiidae

(33 genera, 499 species), Syarinidae (16 genera, 96 species),

and Parahyidae (one genus, one species). Olpioidea contain

two families, Olpiidae (52 genera, 324 species) and

Menthidae (four genera, eight species), but there is little support

for the monophyly of the former. Garypoidea consist

of the basal Geogarypidae (three genera, 59 species), the

Holarctic Larcidae (two genera, 12 species), the Garypidae

(10 genera, 75 species), and two families previously placed

in Cheiridioidea: Cheiridiidae (six genera, 71 species) and

Pseudochiridiidae (two genera, 12 species). Cheiridioidea

were recently reinstated as a separate superfamily by Judson

(2000) but without a full reanalysis of the character set provided

by Harvey (1992a).

The remaining taxa are placed in Elassommatina—consisting

of the monofamilial Sternophoroidea (three genera,

20 species), a group of pallid, flattened, corticolous species

distributed in various disparate regions of the world (Harvey

1991)—and Cheliferoidea. The perceived relationship of

Sternophoridae with Cheliferoidea is only tentatively supported

(Harvey 1992a), and knowledge of the mating be-

Figure 18.6. Phylogeny of

Pseudoscorpiones.

Chthoniidae

Tridenchthoniidae

Lechytiidae

Feaellidae

Pseudogarypidae

Ideoroncidae

Bochicidae

Hyidae

Gymnobisiidae

Neobisiidae

Syarinidae

Parahyidae

Garypidae

Larcidae

Cheiridiidae

Pseudochiridiidae

Geogarypidae

Olpiidae

Menthidae

Sternophoroidea: Sternophoridae

Withiidae

Cheliferidae

Chernetidae

Atemnidae

Iocheirata

Epiocheirata

Feaelloidea

Chthonioidea

Hemictenata

(Neobisioidea)

Panctenata

Mestommatina

Elassommatina

Cheliferoidea

Garypoidea

Olpioidea

312 The Relationships of Animals: Ecdysozoans

havior of sternophorids may assist in determining their

phylogenetic status. Cheliferoidea consist of Withiidae (34

genera, 153 species), Cheliferidae (59 genera, 274 species),

and Chernetidae (111 genera, 646 species). The resolution

of this clade depends on mating behavior and spermatophore

morphology (Proctor 1993). Cheliferoids are the only pseudoscorpions

with sperm storage receptacula (spermathecae)

in females.

The fossil fauna consists of 35 named species, most of

which were found as inclusions in Tertiary ambers. Cretaceous

pseudoscorpions are known (Schawaller 1991), but

the earliest known taxon is Dracochela deprehendor from

Devonian shales in New York (Schawaller et al. 1991).

Harvey (1992a) confirmed the monophyly of most families,

but the original analysis is currently being extended to

include more taxa to test further the monophyly and internal

phylogeny of various clades.

Solifuges, Camel Spiders (Solifugae)

Solifuges or solpugids are a bizarre group of specialized,

mostly nocturnal, errant hunting arachnids notable for their

huge powerful chelicerae and voracious appetite (Punzo

1998). Besides their large powerful chelicerae, solifuges are

unique in having sensory malleoli (or racket organs) on the

fourth coxae and trochanters, and many other peculiar features

(prosomal stigmata, male cheliceral flagellae, palpal

coxal gland orifices, adhesive palpal organs, a monocondylar

walking leg joint between the femur and patella).

The Solifugae contain 1,084 species in 141 genera and

12 families (Harvey 2003): Ammotrechidae (22 genera, 81

species), Ceromidae (three genera, 20 species), Daesiidae (28

genera, 189 species), Eremobatidae (eight genera, 183 species),

Galeodidae (eight genera 199 species), Gylippidae (five

genera, 26 species), Hexisopodidae (two genera, 23 species),

Karschiidae (four genera, 40 species), Melanoblossiidae (six

genera, 16 species), Mummuciidae (10 genera, 18 species),

Rhagodidae (27 genera, 98 species), and Solpugidae (17

genera, 191 species). Only three fossil species are known

(Selden and Dunlop 1998). They primarily occur in Old and

New World semi-arid to hyperarid ecosystems but are absent

from Australia and Madagascar. The Southeast Asian

melanoblossiid Dinorhax rostrumpsittaci is unusual in residing

in rainforest, whereas the peculiar mole solifuges (Hexisopodidae)

from the deserts of southern Africa are highly modified

for burrowing through soil (Lamoral 1972, 1973).

Relationships within the order are very poorly understood,

largely because of the chaotic familial and generic classification

promulgated by Roewer (1932, 1933, 1934) and

continued with many reservations by later workers (e.g.,

Muma 1976, Panouse 1961, Turk 1960). The current classification

is a flat structure devoid of any phylogenetic signal

(Harvey 2002b, 2003). There has been no detailed

phylogenetic work on any solifuge group, let alone a synopsis,

and no monophyly arguments exist for any family, although

some (e.g., Hexisopodidae) seem to be defined by

obvious autapomorphies. The group urgently needs higher

level cladistic analysis.

Conclusions

The last decade has seen substantial progress in research

on major arachnid clades. Considering family rank as indicating

“major” lineages, at least preliminary hypotheses are

available for five of the 13 “orders” (Araneae, Amblypygi,

Opiliones, Scorpiones, and Pseudoscorpiones), but an additional

four (Ricinulei, Palpigradi, Uropygi, and Schizomida)

have only one or two clades ranked as families, so

relationships at that level are trivial. Solifugae (12 families,

141 genera) and Acari (~400 families, ~4000 genera) remain

as substantial lineages without explicit family-level phylogenies.

Although solifuge taxonomy is so completely artificial

that it is difficult to know how to begin, the main reason is

lack of workers: only two or three solifuge specialists exist

worldwide. Mites similarly suffer from a lack of taxonomists,

but the few acarologists must deal with a much greater taxonomic

tangle. There are so many autapomorphic mite lineages

and so much diversity that relationships are obscured,

resulting in an overly split higher classification. The very small

size of mites makes molecular work difficult, although not

impossible (e.g., Dabert et al. 2001), and they are so morphologically

diverse (and often highly simplified) that morphological

work is no easier.

The current conflict between molecules and morphology

at the ordinal level in arachnid phylogeny is intriguing but

probably temporary. Deeper nodes in arachnid phylogeny

are hard to recover consistently with 18S and 28S rRNA sequence

data. Curiously, the same loci do provide robust signal

on still deeper nodes (e.g., arthropods; see Wheeler et al.,

ch. 17 in this vol.), as well as shallower nodes such as

Opiliones (Giribet et al. 2002) and Scorpiones (L. Prendini

and W. Wheeler, unpubl. obs.). The problem, therefore,

seems to be, on the one hand, exploratory—loci robustly

informative for these presumably Lower Palaeozoic divergences

are as yet unknown—and on the other, technical,

because the few loci that seem to have worked in other taxa

at comparable levels have not been studied in arachnids.

Edgecombe et al. (2000) also point out that the “anomalous”

nodes in molecular results are usually weakly supported. The

sheer quantity of molecular data make a single, most parsimonious

tree almost inevitable, but that obscures the often

very tenuous support for some nodes. Because fewer comparisons

are usually possible, morphological data are more

likely to produce multiple most parsimonious trees so that

dubious nodes disappear in the strict consensus tree. No

doubt as more genes are analyzed and taxon sampling improves,

the discrepancies will decrease and the congruence

of the total evidence will improve.

Arachnida 313

Acknowledgments

We thank Heather Proctor, Jeff Shultz, Jeremy Miller, and Greg

Edgecombe for comments on the manuscript, and the National

Science Foundation (EAR-0228699, DEB-9712353, and DEB-

9707744 to J.A.C.) and the Smithsonian Neotropical Lowlands

and Biodiversity Programs for funding.

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