Fig. 66.—Siphonaptera. Pulex irritans, human flea (left), and Nosopsyllus fasciatus, one of the rat fleas. Actual length of body about 0.1 inch.
Fig. 67.—Siphonaptera. Larva of flea found in mouse nest. Length about 0.12 inch.
RELATIVES OF INSECTS
There are many small animals that belong to the same general group as insects and that are frequently collected with them. Spiders, centipedes, and amphipods are a few of many examples of such animals. Together with insects, they form the animal phylum called Arthropoda, characterized by having segmented bodies and jointed legs. A brief description is included here of the common groups of these insect relatives found in Illinois.
Isopoda
Sowbugs, Pillbugs
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.
Fig. 68.—Isopoda. Armadillidium vulgare, a common pillbug or sowbug (two views). Actual length 0.3 inch. (Drawings from U.S.D.A.)