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SUBPHYLUM CHELICERIFORMES

INTRODUCTION TO THE CHELICERIFORMES

Cheliceriformes (ke-LIS-er-i FORM-es) is derived from three roots than mean "clawed horn forms" [clawed -cheli Gr. (χηλή); horn -ceros (κέρας); formes (Latin for forms)]. This is a reference to the clawed feeding structures (chelicerae) that are common to this subphylum.

The chelicerates are part of a large group of animals that includes horseshoe crabs, eurypterids, spiders, ticks, scorpions, whip scorpions, and ambylopygids (Figures A-I), all of which share the derived trait of chelicerae as feeding structures. Almost all are terrestrial with a few notable exceptions (e.g. Limulus; Figure A), and likewise almost all are carnivores. The relationships of the two classes has been problematic since the discovery of the sea spiders (pycnogonids; Figure J) at the beginning of the 20th Century. Brusca and Brusca (2003) indicate that the sea spiders have been allied with almost every member of the panarthropods. Although modern cladistic methods that employ anatomical, developmental, and molecular evidence associate the pycnogonids and the chelicerates, they may be sisters rather than members of the same clade (Tudge 2000). More problematic is the likely association of the cheliceriformes with the trilobites.

A. Limulus, the horseshoe crab, a crustacean-like animal with chelicerae and book gills.

B. Fossils of Eurypterus and a reconstruction of the animal.

C. The trap door spider, Bothriocyntum.

D. The Eastern Daddy Long-legs, Leiobunum.

E. Jumping Spider (Salticid).

F. A common tick, Dermacenter.

G. A scorpion, Opistothalmus.

H. A whip scorpion, Uropygio.

I. An ambylopygid.

J. The skeleton-like sea spider.

Images taken from:
A,C-D,G-I: http://www.amnh.org/exhibitions/hall_tour/spectrum/a2395h.html
B: http://www.fettes.com/orkney/Geology/Devonian%20Life/devonian%20euripterids.htm
E: http://www.palaeos.com/Invertebrates/Arthropods/Chelicerata.htm
F. The Systematics Biodiversity collection.
J: http://sprott.physics.wisc.edu/pickover/pycno.htm

SYNOPTIC DESCRIPTION OF THE SUBPHYLUM CHELICERIFORMES

The following information came from Margulis and Schwartz (1998), Buchsbaum (1938), Barnes (1980), Barnes (1984), Brusca and Brusca (2003), Hickman (1973), Storer and Usinger (1965), and Tudge (2000).

I. SYNONYMS: arachnids, chelicerates, arthropods.

II. NUMBER: >63,000 species known.

III. SUBPHYLUM CHARACTERISTICS:

A. Structure

Symmetry: Bilateral; segmented body regions organized into cephalothorax and abdomen.

Body Cavity: True coelom reduced and absent in adults. Haemocoel the only body cavity.

Body Covering: Covered by chitinous exoskeleton.

Support: Hardened exoskeleton.

Digestive System: Food tube simple. Mouth at anterioventral end. Flanked by modified legs that serve as mandibles in some and a terminal anus in front of the telson (last body segment).

Circulatory System: Open. Haemocoel of blood sinuses with a dorsal heart. Respiratory pigments (haemocyanin sometimes) in the plasma.

Locomotion: Six cephalothoracic pairs of appendages: chelae, pedipalps and 4 pairs of walking legs).

Excretory System: Malpighian tubules or tubules that discharge at the bases of appendages.

Nervous System: Usually the circumesophagial ring and ventral cords united into brain from which nerves issue directly. Eyes usually simple (compound in some), sensory hairs.

Endocrine System:

Respiratory System Modifications of appendages that function as gills book gills or book lungs from which tracheae arise.

B. Reproduction:

Reproductive System: Dioecious. Gonads paired. Fertilization external or internal (by spermatophores). Oviparous.

Development: Eggs are centrolecithal. Development does not include larval form that is markedly different from adult.

C. Ecology: Mostly free-living predators, terrestrial or marine.

HIERARCHICAL CLASSIFICATION OF THE CHELICERIFORMES

This taxonomic system comes from Brusca and Brusca (2003), but it is fairly typical of modern taxonomic treatments. The question of the relationship of the Pycnogonids with the Chelicerates has yet to be resolved, and they may be separated into different subphyla. Also, the chelicerata may be a sister group to the trilobites.

CLASS CHELICERATA

The chelicerate body is made of two major body regions: the prosoma (cephalothorax) and opisthosoma (abdomen). The prosoma is usually covered by a carapace-like shield with simple median eyes and compound lateral eyes. Opisthosoma up to 12 segments with a terminal telson. The prosoma bears 4 pairs of uniramous, multiarticulate walking legs. Modified appendages include chelicerae and pedipalps.

SUBCLASS MEROSTOMATA (2 ORDERS; 1 EXTANT ORDER)

Marine; heavy exoskeleton; prosoma with large horseshoe-shaped carapace, separated from opisthosoma by hinge; telson forms long tail spine; 2 lateral compound eyes and 2 median ocelli; chelicerae small; pedipalps leg-like, chelate; walking legs chelate, except last pair; last pair with leaf-like processes used for burrowing; spiny gnathobases of limbs macerate food; 1^st pair of opisthosomal appendages form flap over reproductive openings; 2^nd-6^th pairs of opisthosomal appendages form swimming and gaseous exchange organs; opisthosoma unsegmented, with lateral spines; excretory organs coxal; fertilization external; with a "trilobite" larva, benthic.

Limulus, Carcinoscorpius, Tachypleus.

Eurypterus, Pterygotus.

SUBCLASS ARACHNIDA (11 ORDERS)

Essentially terrestrial; exoskeleton light to heavy; prosoma wholely or partly covered by a carapace; opisthosoma basically of 13 segments plus telson, often reduced; respiratory organs book lungs or tracheae; without compound eyes; abdominal appendages highly modified or absent; consume liquid (usually pre-liquified animal) food using a pumping pharynx; young stage sometimes with 3 pairs of legs.

Aponomma, Argas, Boophilus, Dermacentor, Ixodes, Ornithodorus, Zeroseius, Demodex, Halotydeus, Penthaleus, Scirus, Tydeus, Acanthophrynus, Damon, Heterophrynus, Stegophrynus, Tarantula, Hepathela, Liphistius, Cyclocosmia, Ummidia, Atypus, Acanthoscurria, Aphonopelma, Diplura, Loxosceles, Argyrodes, Episinus, Latrodectus, Ulesanus, Hyptiotes, Nephila, Uloborus, Araneus, Argiope, Cyrtophora, Mastophora, Pasilobus, Zygiella, Dolichognatha, Eucta, Leucauge, Meta, Pachygnatha, Clubiona, Erigone, Dicymbium, Linyphia, Agelena, Coelotes, Argyroneta, Lycosa, Pardosa, Pirata, Dolomedes, Pisaura, Oxyopes, Thomisus, Xysticus, Heteropoda, Portia, Salticus, Dinopis, Scytodes, Caddo, Leiobunum, Trogulus, Allokoenenia, Eukoeninia, Koeninia, Leptokoeninia, Prokoeninia, Chelifer, Chitrella, Chthonius, Dinocheirus, Garypus, Menthus, Pseudogarypus, Cryptocellus, Pseudocellus, Ricinoides, Agastoschizomus, Megaschizomus, Nyctalops, Protoschizomus, Schizomus, Androctonus, Bothriurus, Buthus, Centuroides, Chaerilus, Diplocentrus, Hadrurus, Hemiscorpion, Nebo, Parabuthus, Paruroctonus, Tityus, Vaejovis, Biton, Branchia, Dinorhax, Galeodes, Solpuga, Abaliella, Chajnus, Mastigoproctus, Charinus, Thelyphonus, Miranda, Latrodectus, Eremobates, Galeodes, Phalangium, Leioburnum, Trombicula.

CLASS PYCNOGONIDA (1 ORDER)

Marine; opisthosoma very small, unsegmented; prosoma divided into a head with cylindrical proboscis and 3 pairs of appendages (chelicerae, pedipalps, ovigerous legs - non-ambulatory legs used for carrying eggs), and a segmented trunk of 4-6 segments each with one pair of legs borne on the end of large lateral trunk processes; legs often very long with a span of 75 cm; 2 pairs of eyes on rounded tubercle on posterior head region; lateral gut caeca and parts of gonads extend into legs; without excretory or respiratory organs; haemocoel divided into upper and lower sections by horizontal membrane; nervous system not concentrated; eggs brooded by male and hatch as protonymphal larval stage with 3 pairs of appendages.

Acheilia, Ascorhynchus, Austrodecus, Colossendeis, Delcopoda, Nymphon, Nymphopsis, Pycnogonum, Tanystylum.

This page is maintained by Jack R. Holt. Last modified: 03/10/08