Chelicerata

Lore & Background

Chelicerates have segmented bodies with jointed limbs covered in a cuticle of chitin and proteins. Their body plan consists of two tagmata: the prosoma (cephalothorax) and opisthosoma (abdomen), except in mites where the division is lost. The chelicerae, which give the group its name, are the only appendages before the mouth; in most subgroups they are pincers used for feeding, but in spiders they form fangs for venom injection. Marine chelicerates have gills, while air-breathing forms have book lungs and tracheae. Most rely on modified bristles for touch and sensing vibrations, air currents, and chemical changes; active hunting spiders have acute eyesight.

Reader's Guide

Chelicerates are significant as one of the two major living subdivisions of arthropods, with a fossil record extending to the mid-Cambrian. They include medically and ecologically important groups: spiders and scorpions produce venoms that are dangerous to humans but are studied for potential treatments for cancer and erectile dysfunction; horseshoe crab blood is used to test for contaminant bacteria; mites cause allergies, transmit diseases, and are agricultural pests. The group originally were predators but now use all major feeding strategies: predation, parasitism, herbivory, scavenging, and eating decaying matter. Most modern chelicerates liquidize food using chelicerae and pedipalps and digestive enzymes. Air-breathing chelicerates conserve water by excreting solid waste via Malpighian tubules. Reproduction varies: marine horseshoe crabs use external fertilization, while air-breathing chelicerates use internal but usually indirect fertilization, often with elaborate courtship. Most lay eggs that hatch as miniature adults, but scorpions and some mites retain eggs internally until young emerge. The inclusion of Pycnogonida (sea spiders) in Chelicerata has been questioned, and the position of scorpions remains controversial.

Did You Know?

• Chelicerates split from Mandibulata by the mid-Cambrian, as evidenced by stem-group chelicerates like Habeliida, Mollisonia, and Megachelicerax.
• Venom has evolved three times in chelicerates: in spiders, scorpions, and pseudoscorpions; possibly four times if tick secretions are included.
• The blood of horseshoe crabs is used in the medical industry to test for contaminant bacteria.
• Mites can cause allergies in humans, transmit several diseases to humans and livestock, and are serious agricultural pests.