Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Multicellular Animals
Arthropods
Subphylum Chelicerata - Class Arachnida - Order Araneae

There are approximately 21,000 known species of spiders. Their body is divided into a cephalothorax and an unsegmented abdomen joined by a narrow pedicel. The chelicerae bear a curved claw each, at the base of which lie venom glands. On the abdomen of spiders are spinnerets, which open the ducts of the silk glands.

The most distinctive feature of spider biology is the production and spinning of silk. Spider silk serves A wide variety of purposes. Many spiders use it to build trapping webs. Females wrap their eggs in silk threads and construct cocoons around them. Young spiderlings disperse by flying significant distances on silk threads. This typically happens in early autumn, during the "Indian summer." A small spider climbs onto tree branches, secretes a thread of silk, and lowers itself down on it, then climbs back up to the attachment point and cuts the thread, which is caught by the wind and carries the animal away.

House spiders have adapted to living in human dwellings. They hunt flies, cockroaches, and other insects. European garden spiders (cross spiders) are distinguished by a cross-shaped pattern on the upper side of their abdomen. They live in trees, weaving trapping webs between the branches. The threads of these webs are coated with a sticky substance to which flying insects adhere upon hitting the web. The diving bell spider lives underwater, constructing a bell-shaped silken shelter among submerged plants. The spider fills the shelter with air, carrying it down trapped between the hairs covering its body. The diving bell spider hunts small aquatic animals, breathes, and hides inside its shelter.

All spiders are venomous. They destroy numerous pest insects, thereby providing immense ecological benefits. The fear and aversion inspired by spiders are largely unfounded, although the bites of Central Asian karakurt spiders and tarantulas are dangerous to humans. The karakurt is the most dangerous.