Spider morphology

The class Arachnida contains predatory spiders and scorpions, as well as the ticks and mites, some of which are blood-sucking parasites that feed on vertebrates. All species have a body consisting of two tagmata: a fused head and thorax called a cephalothorax, and an abdomen.

In spiders (order Araneae), the two body parts are joined by a pedicel, a narrow, waistlike point of attachment. Spiders have six pairs of appendages: the chelicerae, or fangs; a pair of pedipalps, which have various sensory, predatory, or reproductive functions; and four pairs of walking legs. The fangs are supplied with venom from poison glands. Most spider bites are harmless to humans, although they are very effective in immobilizing or killing their insect prey. Venom from some species, including the black widow (Latrodectus mactans) and the brown recluse (Loxosceles reclusa), are potentially, although rarely, fatal to humans. The toxin of the black widow is a neurotoxin, which interferes with the functioning of the nervous system, while that of the brown recluse is hemolytic, meaning it destroys tissue around the bite. After the spider has subdued its prey, it pumps digestive fluid into the tissues via the fangs and sucks out the partially digested meal.