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.
[Illustration: 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.)]
[Illustration: 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.
[Illustration: 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.)]
[Illustration: 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.)]
[Illustration: 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.