Convex, many-legged animals having conspicuous antennae; several of the posterior segments short and joined rather closely to form an abdomen. Of the Illinois forms, about one-half are aquatic, living in streams and ponds. The others live in terrestrial situations that are humid and dark. They are frequently found under boards and in soil in greenhouses. One species of this group is _Armadillidium vulgare_ (Latreille), fig. 68, which possesses the ability to curl up in a hard shell-like ball when disturbed. The isopods, relatives of crabs, shrimps, and crayfish, belong to the general group known as crustaceans. The crustacean groups are abundant in the ocean. In past geologic ages, the early ancestors of such predominantly terrestrial groups as insects and spiders resembled ancestors of the present crustacean marine forms.
[Illustration: Fig. 68.--Isopoda. _Armadillidium vulgare_, a common pillbug or sowbug (two views). Actual length 0.3 inch. (Drawings from U.S.D.A.)]
Humpbacked, many-legged crustaceans, fig. 69, that are, unlike the isopods, flattened from side to side like the fleas. They are all aquatic but are seldom found swimming in open water; they prefer to live in tangled masses of vegetation, under stones or logs, and among debris in the very shallow water where it touches the bank. These little shrimps are never more than about one-half inch long and are frequently collected in large numbers along with aquatic beetles. As is the case with the aquatic sowbugs, certain species of amphipod shrimps occur in subterranean water sources and frequently are found in wells. Most of these species are blind.
[Illustration: Fig. 69.--Amphipoda. _Gammarus_ sp., a common small water shrimp. Actual length 0.4 inch.]
[Illustration: Fig. 70.--Scorpionida. _Centruroides vittatus_, the only scorpion known to occur in Illinois. Actual length 2.0 inches. (Drawing by Alice Ann Prickett.)]
Animals belonging to the spider group; characterized by a pair of stout pincers at the end of each front leg and a long tail-like extension of the abdomen ending in a sharp sting, as in _Centruroides vittatus_ (Say), fig. 70. They have fairly long bodies and five pairs of legs each, including the chelate pair. During the day they live under stones and bark, moving about at night in search of insects and other small organisms on which they prey. Many species occur in the Southwest; the range of one of these extends as far northeastward as southwestern Illinois.