Spider behavior Spider behavior Spider behavior refers to the range of behaviors and activities performed by spiders. Spiders are air-breathing arthropods that have eight legs and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms which is reflected in their large diversity of behavior. Almost all known spider species are predators, mostly preying on insects and on other spiders, although a few species also take vertebrates such as frogs, lizards, fish, and even birds and bats. Spiders' guts are too narrow to take solids, and
Arthropod micrograms (millionths of a gram), while the heaviest weigh over . Some living crustaceans are much larger; for example, the legs of the Japanese spider crab may span up to , with the heaviest of all living arthropods being the American lobster, topping out at over 20 kg (44 lbs). The embryos of all arthropods are segmented, built from a series of repeated modules. The last common ancestor of living arthropods probably consisted of a series of undifferentiated segments, each with a pair of appendages that functioned as limbs. However, all known living and fossil arthropods have grouped segments into