DESCRIPTION OF THE SUBPHYLUM CHELICERIFORMES (SCHRAM AND HEDGEPETH 1978)

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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 name was coined by Schram and Hedgepeth (1978) to recognize a higher taxon with Chelicerata and Pycnogonida as sister groups.

INTRODUCTION TO THE CHELICERIFORMES The chelicerates are part of a large group of animals that includes horseshoe crabs, eurypterids, spiders, ticks, scorpions, whip scorpions, and ambylopygids (Figure 1), all of which share the derived trait of chelicerae as feeding structures (Figure 2). Almost all are terrestrial with a few notable exceptions (e.g. Limulus), and likewise almost all are carnivores. The relationships of the two classes has been problematic since the discovery of the sea spiders (pycnogonids; Figure 3) 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.

	MAJOR CLADES OF THE CHELICERIFORMES Am = Arachnomorpha Ch = Cheliceriformes Eu = Euchelicerata
FIGURE 1. MAJOR CLADES OF THE CHELICERIFORMES. This is a portion of Figure 1 on the Arthropoda page, and the clade designations refer to that cladogram.

FIGURE 2. An illustration of Phidippus audax, a jumping spider. Note the green chelicerae under the head. Image by Patrick Edwin Moran, Wikimedia Commons	FIGURE 3. The skeleton-like pycnogonid or Sea Spider. The determination of the phylogenetic position of this group may change much of our thinking about the chelicerates and the arthropods in general. Image from http://sprott.physics.wisc.edu/pickover/pycno.htm

The Pycnogonids The Sea Spiders (Figure 3) are entirely marine. The opisthosoma (abdomen) is very small and unsegmented. The prosoma (cephalothorax), however, is divided into a head with a cylindrical proboscis and three pairs of appendages (chelicerae, pedipalps, ovigerous legs – non-ambulatory legs used for carrying eggs). The trunk is has four to six segments, each with one pair of legs borne on the end of large lateral trunk processes. The legs often are very long and can have a span of 75 cm. They have two pairs of eyes on a rounded tubercle that projects from the posterior head region. The gut has lateral caecae that extend into parts of the legs, as do parts of the gonads. Despite the size of the Sea Spiders, they have no excretory or respiratory organs. The haemocoel divided into upper and lower sections by a horizontal membrane, and the nervous system is diffuse, not concentrated. The male broods the eggs, which hatch as a protonymphal larval stage with three pairs of appendages. Note that the Sea Spiders were given as sisters to the other living arthropods in the analysis of Waggoner (1996).

The Euchelicerate Clade (Eu) 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. The opisthosoma has up to twelve segments with a terminal telson. The prosoma bears 4 pairs of uniramous, multiarticulate walking legs. Modified appendages include chelicerae and pedipalps.

The Merostomates The Horshoe Crabs (Figure 4) are marine, crab-like animals with a heavy exoskeleton. The prosoma is a large horseshoe-shaped carapace, which is separated from the opisthosoma by a hinge. The telson, which forms the tail spine, is long and substantial. Like most chelicerates, the prosoma has two lateral compound eyes and 2 median ocelli. The chelicerae are small, and the pedipalps are leg-like and end in claws. Likewise, the walking legs have claws, except the last pair, which have leaf-like processes used for burrowing. The spiny gnathobases of the limbs macerate food. The first pair of opisthosomal appendages form a covering over the reproductive openings. The second through the sixth pairs of opisthosomal appendages form swimming and gas exchange organs (called book gills). The opisthosoma is relatively small, unsegmented, and has lateral spines. Excretory organs are associated with the coxae, the most proximal segments of the legs. Fertilization is external, producing a benthic “trilobite” larva. Maturation takes up to ten years. Eurypterids (Figure 5) were among the top predators of the middle and late Paleozoic Seas. Sometimes called Sea Scorpions, eurypterids grew to be more than 2 meters long, though most were smaller. They likely walked over the bottom in search of prey. The placement of legs in some taxa even suggest that they could walk out onto land.

FIGURE 4. Limulus, the horseshoe crab, a crustacean-like animal with chelicerae and book gills. Image from http://www.amnh.org/exhibitions/hall_tour/spectrum/a2395h.html	FIGURE 5. Eurypterus, a Paleozoic Sea Scorpion: fossils and a reconstruction of the animal. Image from http://www.fettes.com/orkney/Geology/Devonian%20Life/devonian%20euripterids.htm

The Arachnids These are the spiders (Figures 6-8), ticks (Figure 9), mites, scorpions (Figure 10), whip scorpions (Figure 11), and ambylopygids (Figure 12), which are all essentially terrestrial animals. The exoskeleton is variable, but generally light. The prosoma is entirely (or partly) covered by a carapace, and they usually have four pairs of walking legs. The opisthosoma has thirteen segments plus a telson, which is often reduced. Furthermore, the abdominal appendages are highly modified or, in many cases, absent. The respiratory organs are book lungs or tracheae. The animals have many eyes, but they usually are simple and without compound eyes. The arachnids consume liquid (usually pre-liquified animal) food using a pumping pharynx. They inject prey with venom through the chelicerae, which begins the process of digestion. Tarantulas can eat prey as large as small rodents and birds, which they reduce to a mass of bones and fur/feathers in less than two days. The toxins can be quite potent and deadly to humans1. Young arachnids look like the adults. That is, they do not go through a larval stage. However, some of the young can have three pairs of walking legs and add the fourth pair during successive molts.

FIGURE 6. The trap door spider, Bothriocyntum. Image from http://www.amnh.org/exhibitions/hall_tour/spectrum/a2395h.html	FIGURE 7. The Eastern Daddy Long-legs, Leiobunum. Image from http://www.amnh.org/exhibitions/hall_tour/spectrum/a2395h.html	FIGURE 8. Jumping Spider (Salticid). Image from http://www.palaeos.com/Invertebrates/Arthropods/Chelicerata.htm

FIGURE 9. A common tick, Dermacenter. Image from The Systematics Biodiversity Image Archive	FIGURE 10. A scorpion, Opistothalmus. Image from http://www.amnh.org/exhibitions/hall_tour/spectrum/a2395h.html	FIGURE 11. A whip scorpion, Uropygio. Image from http://www.amnh.org/exhibitions/hall_tour/spectrum/a2395h.html	FIGURE 12. An ambylopygid. Image from http://www.amnh.org/exhibitions/hall_tour/spectrum/a2395h.html

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1. I received a bite on my right leg from a Brown Recluse Spider when I was a boy. I remember the leg swelling so much that my parents had to cut my jeans off of me. The bite area became necrotic and took quite some time to heal. -J. Holt

By Jack R. Holt and Carlos A. Iudica. Last revised: 02/12/2016