Primitive Wingless Insects
Only five orders of primitive wingless insects are known; species of each order occur in Illinois. These orders represent the stages in insect evolution before wings had appeared.
Fig. 21.—Diplura. A campodeid belonging to the genus Campodea, found under stones in moist places. Actual length of adult about 0.1 inch. (Drawing from E. O. Essig.)
Fig. 22.—Protura. Acerentulus barberi, a proturan found on sticks and leaves in the leaf mold of forests. Actual length of adult about 0.02 inch. (Drawing from H. E. Ewing.)
Diplura
Campodeids, Japygids
Small, wingless, fragile, blind, whitish insects that run fairly rapidly. They have long antennae and either two fairly long tails or a pair of forceps-like structures at the end of the abdomen. They are terrestrial and are found chiefly under stones in humid and shady situations. [Fig. 21] shows a common campodeid, a species of Campodea, occurring commonly in Illinois.
The Diplura feed on fungi and other soil microorganisms. Although they are found most frequently under stones, they live also in the soil and in matted leaves or duff on the floor of woods. About a dozen species of Diplura occur in Illinois.
Protura
Proturans
Minute, wingless, blind insects that never grow to more than 0.05 inch long. They have no antennae and use the front legs to some extent for feeling. They are terrestrial and are found inhabiting dead twigs and leaves on the forest floor. [Fig. 22] shows Acerentulus barberi Ewing, a member of a genus which occurs in many localities in Illinois. Only a few species of proturans have been taken in the state.
Collembola
Springtails
Small, wingless insects that jump and crawl when disturbed. They have short antennae and usually a springing structure on the under side near the posterior end of the body. They live in moist places and are abundant under leaf mold and similar material. Illustrated in [fig. 23] is Achorutes armatus Nicolet, which often becomes a major pest in mushroom cellars and greenhouses.
About a hundred different species of Collembola occur in Illinois; they include some of our smallest insects. A few never grow longer than 0.007 inch; the largest approach half an inch in length. These hardy animals are active all year and are surprisingly resistant to cold. Certain species occur on snow in winter. In Illinois a small, bluish gray species, Podura aquatica Linnaeus, is found on the surface of still water at the margins of ponds and small streams.
Microcoryphia
Bristletails
Wingless, somewhat cylindrical insects that run and jump with extreme rapidity. They have long antennae and three long tails. The under side of the abdomen bears several pairs of short projections called styli, which are vestiges of abdominal legs. Bristletails live in rocky places or in ground cover. A type occurring in some parts of Illinois is shown in [fig. 24]. It often occurs on rocky exposures, where it resembles the lichens and is difficult to detect.
Fig. 23.—Collembola. A, Isotoma andrei; B, Achorutes armatus; C, Neosminthurus clavatus. A and C are found in woodland leaf molds; B is frequently abundant in commercial mushroom cellars. Actual length of adults ranges from 0.03 to 0.05 inch. (Drawings A and C after Harlow B. Mills.)
Fig. 24.—Microcoryphia. A bristletail belonging to the genus Machilis, often found on lichen-covered, shaded rocks. Actual length including tail is sometimes 0.5 inch. (Drawing after R. E. Snodgrass.)
Fig. 25.—Thysanura. Thermobia domestica, a common Silverfish. Actual length about 0.3 inch.
Thysanura
Silverfish
Wingless, flat insects that run rapidly. They have long antennae and three long tails. They are terrestrial and are commonly found in dwellings. [Fig. 25] shows one of the common Silverfish, Thermobia domestica (Packard); it frequently eats book bindings and other starchy materials. Some out-of-door rare forms live in the soil and are seldom collected.