Part 4

Five existing orders of insects, all found in Illinois, are representative of the primeval wingless insects. In all five, the individual molts at intervals, even after becoming adult and sexually mature, and the old and young are extremely similar. Three of the primitive orders, the campodeids, proturans, and springtails, belong to a specialized early branch of the insect family tree, fig. 18, in which the cheeks have grown forward to form a pouch surrounding the mouthparts. In the other two primitive orders, the bristletails and silverfish, the mouthparts are in a normally exposed condition, but the legs are larger and the insects are rapid runners.

From one of these ancestral, silverfish-like insects arose a form in which wings evolved. In birds and bats, the wings are converted front legs with membranes or feathers attached to form planing or flying surfaces. In insects, on the other hand, the wings are outgrowths of the edge of the body where the sides and top come together, fig. 19. The wings probably began as side flanges of the thorax and permitted a certain amount of planing. Whatever their origin, two pairs of flying wings did evolve, one pair on the second and another on the third segment of the thorax.

In the early winged forms, the wings could not be folded back in repose over the body, but were held out from the sides like airplane wings or together above the body like sails. Two living orders of insects represent this type, the mayflies and the dragonflies. In both of these, as in other winged insects, the wings form as small pads during the early stages of the individual; then at a final molt they are unfolded as functional units. In the mayflies, one more molt occurs after the wings are formed; in this molt the old outside covering of the wings is shed along with that of the rest of the insect. In all other winged insects no molt occurs after the wings are formed and the individual becomes sexually mature.

Insects having erect, nonfolding wings were abundant some 300 million years ago. Fossil remains of many of these early forms have been found in the Mazon Creek area in Illinois, fig. 20.

From one of these early winged types a form evolved in which the wings could be folded compactly over the body; this form gave rise to a great many of our present-day insects. In the first insects that evolved from this form the wing pads of the immature stages, called nymphs, grew as external and often inconspicuous flaps held close to the body. These primitive insects comprise three distinctive groups, characterized mainly by differences in leg and body structure. One group contains the cockroaches, grasshoppers, and their allies; the second includes only the stoneflies; and the third group includes the barklice, true lice, bugs, and their allies.

One of the primitive lines of wing-folding insects, possibly an offshoot from the base of the barklouse-bug line, evolved into a distinctly different type in which the wing pads of the immature stages developed internally and appeared as external pads only in the stage before the adult form. The type was characterized by a marked difference of appearance between the various stages of the life history; these stages have been given distinctive names. The first immature stage, which is without external wing pads, is called a _larva_; the single stage with the external wing pads is called the _pupa_; and the final winged, sexually mature stage is called the _adult_. The larva is essentially a growing stage, the pupa is a quiescent stage of internal reorganization, and the adult is the _egg_-producing stage. This type of insect gradually gave rise to the orders which now contain the largest number of species, including the beetles, moths, and flies. In many lines of this neuropteroid branch, as it is called, the larva has become adapted to a mode of life quite different from that of the adult. Many fly larvae, for example, live in rotting organic material or live as parasites within the tissues of other kinds of insects, whereas the adult flies often feed at flowers on pollen and nectar, visiting the site of the larval habitat only to lay eggs. As a result of this type of evolution, members of the neuropteroid orders exhibit many bizarre and complicated life histories.

Occasionally certain groups of winged insects evolved new types which had small wings or were wingless. The new types resulted because the groups were subjected to environmental situations where wings were of little survival value. Within the groups, individuals with smaller wings were favored; they survived and reproduced in greater numbers than other individuals. Over many generations then, the wings in some groups became very small or were entirely lost. Sometimes this loss of wings occurred in all the adult forms, as in the fleas. Individuals of only one sex may be wingless, as are the females of two Illinois moths, one of which produces bagworms and the other cankerworms. In two groups, the termites and the ants, a wingless worker or soldier caste is produced; in these groups the normal adults which swarm and reproduce are fully winged. These winged forms establish new colonies.

Wings were lost in the evolutionary development of two insect groups that became parasites of warm-blooded animals, both birds and mammals. Each of these insect groups developed into a large, distinctive order. One order, the true lice, evolved from the barklice, and the other, the fleas, evolved possibly from a primeval fly group. Although these two orders, the lice and the fleas, are without wings, the structure of their bodies and their life histories provide adequate testimony of their evolutionary affinities.

The following synopsis of Illinois insects treats the various kinds in the sequence in which we believe they evolved, from the extremely primitive bristletails to the highly specialized flies.

HOW TO IDENTIFY SPECIMENS

As an aid to the beginner in making preliminary identification of his specimens and also as an aid in arranging his collection, a short descriptive synopsis of the orders of living insects is given below. In this description are noted the most distinctive features of the common insects occurring in Illinois. There are rare and obscure forms, seldom met by the collector, that require a more technical key for their identification; for these the collector will need to consult some of the more nearly complete books listed on page 70. The collector will find, however, that this synopsis will afford a beginning for his classification of the common forms.

Various characters are used to identify an insect to family, genus, and species. Among these characters are the antennae, wings (if present), legs, and mouthparts. Frequently important for identification are such minute details as hair or scales covering the body or wings and the texture of these parts. In most cases good microscopic equipment is necessary to see clearly the characters used in the diagnosis of insects.

SYNOPSIS OF ILLINOIS INSECT ORDERS

Of the 28 orders of insects recognized in North America, 26 have been collected in Illinois. The two orders not found here are the Embioptera or webspinners, a tropical and subtropical order, and the Raphidiodea or snakeflies, which occur in the western mountainous region of the continent.

The Illinois fauna thus contains a remarkable variety of insects, including forms such as the bristletails, mayflies, and cockroaches, which are practically "living fossils" of insects that lived hundreds of millions of years ago.

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.

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.

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.

Primitive Winged Insects

The primitive winged insects cannot fold their wings, which in repose are held erect over the body or straight out from the sides, as illustrated in figs. 26 and 29. Although many types of these insects lived in bygone ages, only two orders have survived to the present. Both occur abundantly in Illinois.

Ephemeroptera Mayflies

A group of insects in which the nymphs or young live in streams and lakes; the adults are found along the edges of the streams or lakes from which they have emerged. Mayflies are unique in that the full-grown nymphs molt into winged insects that are not quite mature and that molt again, usually the next day, when they emerge as fully mature adults. The nymphs are varied in shape and have short antennae, long legs, which are often flattened, and three tails at the end of the body. The adult flies have very long front legs, short antennae, practically no mouthparts, usually two pairs of wings, and two or three long tails. When a mayfly is at rest, the wings are held together above the body. _Hexagenia limbata_ (Serville), figs. 26 and 27, is one of the very common Illinois mayflies and is an important factor in the food economy of many fish.

Mayflies, formerly called Plectoptera, together with stoneflies, caddisflies, and midges, constitute a very large portion of the life of our lakes and streams; all four groups are important as fish food.

Odonata Dragonflies, Damselflies

Another order in which the nymphs develop in streams, lakes, or ponds, and in which the adults are aerial. The nymphs have short antennae, long legs, and either a stout body with no tail, as in _Anax junius_ (Drury), fig. 28 (dragonfly nymph), or a slender body with three large leaflike gills projecting from the end of the body (damselfly nymph). A most distinctive feature of this order is an extensile, highly modified lower lip that fits like a mask over the face of a nymph. The lower lip is hinged to extend forward and seize the small animals upon which the nymph lives. The adults are large, often beautifully colored, as is the _Tramea lacerata_ Hagen, fig. 29. They have chewing mouthparts and two pairs of large wings, very finely and intricately netted with veins.

The order is divided into two types; the adult flies are told apart as follows:

Body stout, wings broad at base, the front and Suborder hind wings different in shape. Strong fliers. Anisoptera Dragonflies Body slender, wings narrowed at base, the front Suborder Zygoptera and hind wings similar in shape. Weaker fliers than dragonflies but nonetheless elusive. Damselflies

Folding-Wing Insects

Almost all insects in this category can fold their wings in repose back over their bodies, as illustrated in fig. 31. A few kinds, notably some of the moths and butterflies, have lost this wing action and in repose hold their wings erect. In some of these kinds, the male is winged and the female is wingless; in others, certain generations may be wingless and others winged; and, in still others, the species may be wingless in all stages. No members of the orders of folding-wing insects molt after becoming winged or sexually mature.

Cursoria Cockroaches, Mantids, Walkingsticks

An order that includes three groups of terrestrial insects, each group markedly different in appearance from the others: (1) rapidly running insects usually having two pairs of wings, each with a dense network of fine veins, the front pair of wings thick and leathery, fig. 30 (cockroaches); (2) winged insects having long, grasping front legs, fig. 31 (praying mantids); and long, wingless insects resembling sticks, fig. 32 (walkingsticks). The mouthparts are fitted for chewing. The young look and act like the adults except that they do not have wings. The cockroaches are omnivorous, feeding chiefly on organic foods rich in carbohydrates, or on fungus growth. Cockroaches are among our most persistent indoor pests, eating a wide variety of domestic foods. The praying mantids feed on other insects, which they capture in their enlarged front legs. The walkingsticks eat leaves. The cockroaches and mantids lay eggs that are glued together and form pods or capsules, each containing 30 or more eggs. The walkingsticks lay their eggs singly.

Isoptera Termites

Fragile or soft insects with chewing mouthparts. The mating forms are dark brown and have two similar pairs of wings; both pairs are delicate and have a fine network of veins. The workers are white and soft bodied. Termites, which live in colonies in wood, are also called "white ants," although they are not true ants. The common native species in Illinois is _Reticulitermes flavipes_ (Kollar), fig. 33, which lives in rotten logs and is destructive to buildings of wooden construction throughout Illinois; it is most destructive in the southern part of the state.

Orthoptera Grasshoppers, Crickets

Terrestrial insects usually with two pairs of wings, each wing with a very fine, dense network of veins, the front pair thick and leathery, the hind pair delicate and fanlike. The mouthparts, fitted for chewing, have stout mandibles. The young look and act like the adults but do not have wings. This order includes all the grasshoppers, crickets, and katydids. Fig. 34 shows the migratory locust or grasshopper, _Melanoplus bilituratus_ (Walker). Adults of several of the groups of Orthoptera never develop wings. These include such odd forms as the cave crickets, exemplified by _Ceuthophilus maculatus_ (Harris), fig. 35.

Dermaptera Earwigs

Insects with two pairs of wings, the front pair forming short, hard covers, the second pair large, membranous, many-veined, and in repose folded intricately beneath the front pair. The abdomen ends in a pair of pincer-like structures. A common Illinois form is _Labia minor_ (Linnaeus), shown in fig. 36. Of the half-dozen species known from Illinois, all but one were originally from Europe or Asia. Earwig females lay eggs in chambers in the ground and guard them.

Plecoptera Stoneflies

Insects that pass the young or nymphal stage in streams. They have slender, soft bodies and long tails; they move about rapidly. The adults are terrestrial in habit and occur along streams. In most species, the adults have two pairs of wings that are folded flat over the back; the number of crossveins varies from many to few. The antennae are long; the mouthparts are of the chewing type but greatly reduced. Of exceptional interest are stonefly adults that emerge in winter and are active from November through March. The winter forms can often be collected on bridges. Figs. 37 and 38 illustrate a spring species, _Isoperla confusa_ Frison.

Zoraptera Zorapterans

Small, whitish insects, about a twelfth of an inch long, that run rapidly. They live in small colonies in rotting sawdust, in rotten logs, and under the bark of stumps. They have fairly long antennae, chewing mouthparts, and a pair of short, inconspicuous tails. Most members of a colony are wingless, but occasionally there occurs a darker specimen having two pairs of wings. These wings have only a few veins. In Illinois only a single rare species has been found; it is called _Zorotypus hubbardi_ Caudell, fig. 39.

Corrodentia Booklice, Barklice

Small, rounded or flattened insects, rarely a quarter-inch long, usually about 0.13 inch. In many species, adults have two pairs of wings, which have only a few zigzagging veins. Winged forms, such as _Psocus striatus_ Walker, fig. 40, are found in crevices of bark and on dead leaves. Common species found in houses and on stored grain are usually wingless and louselike, similar in general appearance to fig. 41. Corrodentia eat fungus growth on bark, dead leaves, moldy grain, damp books, and similar materials. Some of the outdoor species become very abundant on drying corn leaves during autumn and may breed in immense numbers. They do little harm, feeding chiefly on fungus strands.

Phthiraptera Chewing Lice, Sucking Lice

Wingless, blind, flattened insects with short antennae, short legs, inconspicuous mouthparts, and no tails on the posterior end of body. They are found exclusively on the bodies of birds and other warm-blooded animals. The young have the same general shape and habits as the adults and are found with them. Illinois species of lice belong to two distinct suborders, which may be differentiated through use of the following key: