  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 InsectsOnly 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 |
| Body stout, wings broad at base, the front and hind wings different in shape. Strong fliers. Dragonflies | Suborder Anisoptera |
| Body slender, wings narrowed at base, the front and hind wings similar in shape. Weaker fliers than dragonflies but nonetheless elusive. Damselflies | Suborder Zygoptera |
Fig. 28.—Odonata. Nymph of Anax junius, a dragonfly widely distributed in Illinois. Actual length of full-grown nymph about 2.3 inches. (Drawing courtesy of C. O. Mohr.)
Fig. 29.—Odonata. Tramea lacerata, a dragonfly commonly found near ponds and drainage ditches in Illinois. Wingspread about 3.2 inches. (Drawing courtesy of C. O. Mohr.)
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
Fig. 30.—Cursoria. Supella supellectilium, the brown-banded cockroach. Actual length about 0.6 inch. (Drawing courtesy of C. O. Mohr.)
Fig. 31.—Cursoria. Stagmomantis carolina, a praying mantis. Common in southern and central Illinois. Actual length of adult about 1.5 to 2.0 inches.
Fig. 32.—Cursoria. Diapheromera femorata, a walkingstick insect. This insect lacks wings. Actual length about 3.0 inches. (Drawing courtesy of C. O. Mohr.)
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.
Fig. 33.—Isoptera. Reticulitermes flavipes, the commonest kind of termite found in Illinois: A, first form queen with wings spread, many times natural size (this is the form that lays eggs); B, worker nymph, natural size; C, first form queen, approximately natural size, with wings placed in their natural resting position. (Drawing courtesy of C. O. Mohr.)
Fig. 34.—Orthoptera. Melanoplus bilituratus, the migratory locust, a common Illinois grasshopper. Actual length about 1.0 inch.
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.
Fig. 35.—Orthoptera. Ceuthophilus maculatus, a wingless cave cricket. Crickets of this kind are found in caves, under rocks, and in basements. Actual length about 1.0 inch.
Fig. 36.—Dermaptera. Labia minor, an earwig frequently abundant in Illinois. Actual length of adult about 0.2 inch.
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,
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.
Fig. 37.—Plecoptera. Isoperla confusa, one of the typical stoneflies found in Illinois; adult form. Actual length about 0.8 inch. Illinois stoneflies range in length from 0.25 inch to 1.5 inches.
Fig. 38.—Plecoptera. Isoperla confusa; the nymph of the species shown in fig. 37. The nymph lives in streams. Actual length about 0.6 inch.
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:
Fig. 39.—Zoraptera. Zorotypus hubbardi, the only zorapteron recorded from Illinois. The specimen on the left is a winged adult female; the specimen on the right is a wingless adult female. Actual length of adults about 0.08 inch.
Fig. 40.—Corrodentia. Psocus striatus, a common bark louse found on many trees. Actual length 0.2 inch.
Fig. 41.—Phthiraptera. Suborder Mallophaga. Cuclotogaster heterographus, a chewing louse found on the heads of poultry. Actual length about 0.1 inch.
Fig. 42.—Phthiraptera. Suborder Anoplura. Haematopinus asini, the blood-sucking horse louse. Actual length 0.1 inch.
| Legs fitted for running, as in fig. 41, without large pincers at their ends; mouthparts situated near the middle of under side of the head and fitted for chewing. Occurring on birds and mammals. The chewing lice | Suborder Mallophaga |
| Legs fitted for clinging to hairs, each leg ending in a large pincer, as in fig. 42; mouthparts retracted within head, consisting of a set of thin, needle-like parts fitted for sucking blood. Normally occurring only on mammals. The sucking lice | Suborder Anoplura |
Suborder Mallophaga.
Individuals of many species of chewing lice move about with considerable rapidity. Many of them are very prettily banded and colored, as is the chicken head louse, Cuclotogaster heterographus (Nitzsch), fig. 41. Anyone who has worked with domestic fowls or animals has seen members of this order scurrying among the feathers or hair. These insects feed on what they can chew from the surface of the skin and in some cases are known to injure their hosts.
Suborder Anoplura.
The sucking lice are sluggish insects that usually cling to hairs. Human lice are often found clinging to clothing. Various species occur on native and domestic species of mammals in Illinois. Fig. 42 shows the horse louse, Haematopinus asini (Linnaeus).
Thysanoptera
Thrips
Fig. 43.—Thysanoptera. Thrips tabaci, onion thrips. Actual length less than 0.1 inch.
Small, active insects, usually about 0.1 inch long, rarely a quarter-inch long, very slender, usually each with two pairs of narrow wings and with the under side of the head forming a sharp, conelike sucking structure. Each wing has a long fringe on the hind margin; each front wing may have one or two veins running the length of the wing. The young of these insects are somewhat similar to the adults but are softer
Hemiptera
True Bugs and Their Allies
Insects usually with two pairs of wings and with the mouthparts formed for sucking. The order contains two distinct suborders, the Heteroptera and the Homoptera. All species of Hemiptera in North America fall readily into one suborder or the other, but certain species in other parts of the world are intermediate between the two suborders.
Fig. 44.—Hemiptera. A typical stink bug of the family Pentatomidae, showing attachment of beak and arrangement of wings. Actual length about 0.4 inch.
In the suborder Heteroptera, containing the stink bugs, the chinch bugs, and their allies, the beak is attached to the under side of the front part of the head; the base of the front pair of wings is hardened, and only the apical portion is membranous or delicate; the entire hind pair is delicate. In repose, the wings are folded over and flat against the body, the hind pair underneath. These characters are shown in fig. 44, which pictures a stink bug belonging to the family Pentatomidae. The young have the same general appearance and habits as the adults, but they lack wings. This suborder includes many common kinds, such as the water bugs, the water striders (these seldom develop wings even in the adult stage), the ambush bugs, and the lace bugs, as well as the stink bugs and the chinch bugs. The chief
Fig. 45.—Hemiptera. Blissus leucopterus, the chinch bug. Actual length about 0.1 inch.
Fig. 46.—Hemiptera. Lygus lineolaris, the tarnished plant bug. Actual length about 0.2 inch.
Fig. 47.—Hemiptera. Anuraphis maidiradicis, the corn root aphid. The form at the left is the winged form; that at the right is the wingless form. All of the plant lice have these two forms. This species, as well as other kinds of plant lice, is frequently attended by ants, which feed on the honeydew produced by the aphids. Actual length less than 0.1 inch.
Members of one family, the Reduviidae or assassin bugs, prey on other insects. A few species called kissing bugs, some of them an inch long, occasionally attack people, inflicting an extremely painful bite and causing considerable bleeding.
Fig. 48.—Hemiptera. Aspidiotus perniciosus, the destructive San Jose scale. The scale is cut away on upper specimen to show insect proper beneath. Diameter less than 0.1 inch.
Fig. 49.—Hemiptera. Empoasca fabae, the potato leafhopper. This species is pale green. Some species are distinguished by bright red or yellow markings. Actual length about 0.1 inch.
Fig. 50.—Megaloptera. The larva of a species of Sialis, an alderfly. This form is aquatic. Actual length 0.7 inch.
Fig. 51.—Megaloptera. The adult of Sialis mohri, an alderfly. Other members of this order reach a length of 1 or 2 inches. They are mostly black, black and white, or mottled gray in color. Actual length 0.5 inch.
The suborder Homoptera contains the cicadas, aphids, and their allies. All these insects have sucking mouthparts, but in each the beak is attached at the back of the head instead of the front of the head as in the suborder Heteroptera. In many species of the Homoptera, each individual has two pairs of wings, both of which are membranous. Probably as many species are without wings, however, as with them. The nymphs are in most respects similar to the adults. Sexual characters, and in some forms wings, gradually develop as the insects approach the adult stage, when development is complete.
Fig. 52.—Neuroptera. Chrysopa nigricornis, a green lacewing. When handled, members of this genus give out a very penetrating and disagreeable odor. Actual length 0.6 inch.
Fig. 53.—Neuroptera. A larva of the genus Chrysopa. This form uses the long jaws to impale aphids and suck their body juices. Actual length 0.5 inch.
This suborder contains a large number of economic pests, including scale insects and leafhoppers, as well as many aphids. In many species of aphids each insect has a pair of tubular structures near the end of the body; these are called cornicles and can be seen in fig. 47,
Megaloptera
Alderflies, Dobsonflies
In this and in the following orders of insects, the life history includes four distinct stages, the egg, the larva, the pupa, and the adult. In the Megaloptera, which include alderflies and dobsonflies, the larvae are caterpillar-like or grublike, and the pupae represent a transformation stage in which the tissues of the larvae are converted to those of the adults. The larvae never have external wing pads; in winged species, these pads first appear externally in the pupae. The adults have long antennae, two similar pairs of net-veined wings, and chewing mouthparts. They are moderately strong fliers. The larvae, fig. 50, live in streams and lakes; when fully grown they migrate to dry land and pupate in the ground or under the bark of rotten logs.
Typical of the appearance of Illinois alderflies is the adult of Sialis mohri Ross, shown in fig. 51. Well known to the fisherman is the hellgrammite, the tough, ferocious, leathery larva found under rocks in streams and prized for bait. This larva matures into the large dobsonfly, Corydalis cornuta (Linnaeus), which often attains a wingspread of 4 inches.
Neuroptera
Lacewings and Their Allies
Insects with two pairs of wings, both pairs about the same size and shape and intricately netted with veins; antennae long and slender, mouthparts fitted for chewing, posterior end of body without tails. The green lacewings, including Chrysopa nigricornis Burmeister, fig. 52, are our commonest members of this order. The young or larvae of this order are entirely unlike the adults and are somewhat grublike in form. The aphid lion, the interesting larva of Chrysopa, fig. 53, is frequently collected by the sweeping method. Another interesting larva of this order is the doodlebug or ant lion, of Huckleberry Finn fame. The adult insects that mature from these ant lion larvae are very similar in appearance to the chrysopids or lacewings. The larva of each of these insects sinks its long, sharp, curved mandibles into the body of its prey and sucks out the body juices. The female Chrysopa has the curious habit of forming a long, slender
When the larva is mature, it spins a globular, silken cocoon or cell around itself and in this changes into the pupal, or quiescent, stage. While the pupa itself does not appear active, within it the larval tissues are reorganized into the structures of the adult, and the final growth of the wings and reproductive organs occurs. When this change is completed, the adult insect emerges from the cocoon.
Fig. 54.—Coleoptera. Copris minutus, one of the scarab beetles. The drawing shows one of the elytra upraised and illustrates the method of folding the hind pair of wings under the elytra. Actual length 0.4 inch.
Fig. 55.—Coleoptera. A weevil belonging to the genus Curculio, which feeds on nuts and acorns. In this genus the beak is exceptionally long. In most of the Illinois weevils the beak is shorter and stouter. Actual length 0.4 inch.
Coleoptera
Beetles, Weevils
Insects with two pairs of wings, the second pair delicate and folded under the first pair, which are hard and thickened and folded back against the body, touching each other along the midline to form a hard shell, as shown in Copris minutus (Drury), fig. 54. The upper wings are not used for locomotion, but form part of the body armor and are called elytra. In most beetles they cover the entire posterior part of the body; in many others they are abbreviated and cover only part of the abdomen. The immature stages of the beetles
A great many of the serious insect pests, including kinds that attack field crops, stored products, and household goods, are beetles. Beetles of one group having the front of the head produced into a snoutlike structure, as in the genus Curculio, fig. 55, are called weevils or snout beetles. This group has maggot-like larvae and contains many of our worst pests, such as the plum curculio, cotton boll weevil, alfalfa weevil, and clover weevil. Bizarre and striking forms occur in many beetle groups, notably among the scarab and long-horn beetles. The largest in Illinois is the rhinoceros beetle, Dynastes tityus (Linnaeus); the males (one shown on the cover of this circular) have long projections on both head and thorax; the larvae live in rotten wood.
Tree-boring beetle larvae are destructive to many orchard, ornamental, and native trees. These include chiefly the round-headed borers, adults of which are long-horn beetles; flat-headed borers, adults of which are metallic wood borers; and engraver or shot-hole types, adults of which are small and bullet shaped and are called bark beetles.
In a few families of beetles, both the adults and larvae are fitted for aquatic life. Well known among these are the shining whirligig beetles.
Hymenoptera
Bees, Wasps, Ants, Sawflies
Insects typically with two pairs of wings; antennae of various lengths; chewing mouthparts; without tails. A typical member of this group is the wasp Vespula maculata (Linnaeus), fig. 56. Many adult members of the group are atypical in that they lack wings; these include all the true ants, fig. 57, which are without wings except for the sexual forms produced at the time of the nuptial flights. Forms of one species, Lasius interjectus Mayr, are shown in fig. 58. The wings, when developed, are without scales; the venation is much less extensive than in the Neuroptera; and the hind wings differ in shape and size from the front wings. The young stages of the Hymenoptera are caterpillar-like or grublike, entirely different from the adults.
This very large order includes such well-known forms as the bees, as well as the wasps and the ants, mentioned above. In addition, it includes the sawflies, whose caterpillar-like larvae are extensive defoliators of a large number of native and cultivated plants and shrubs; the large and varied groups of parasitic wasps that exert great influence in the natural control of a tremendous number of other insects; and a large number of gall-making wasps, whose galls are
Fig. 56.—Hymenoptera. Vespula maculata, the common bald-faced hornet. Actual length 0.8 inch.
Fig. 57.—Hymenoptera. A worker ant belonging to the genus Formica. This form lacks wings. Actual length 0.3 inch.
Mecoptera
Scorpionflies
Insects typically with two similar pairs of delicate wings, each wing with a network of veins. In repose the wings are laid either tentlike over the back or almost flat. The mouthparts are fitted for chewing and usually are lengthened into a beaklike structure, as in Panorpa chelata Carpenter, fig. 60. The larvae, seldom found, live in damp woods. The adults of most winged species occurring in Illinois are about 0.5 inch long. They are active in early summer in shady woods, flying through the undergrowth. Adults of the genus Boreus are smaller and they have very short, veinless wings. They emerge in the winter and early spring; these little metallic black insects often hop around on late winter snow. In certain genera, the adult male genitalia form a bulb-like structure at the end of the body, as in fig. 60. This structure is harmless but, because it resembles a scorpion’s sting, insects of these genera are given the name scorpionflies.
Fig. 58.—Hymenoptera. Lasius interjectus, a harmless winged ant, the yellow ant, with which the winged termite is often confused: A, queen with wings spread, many times natural size; B, worker ant, natural size; C, queen, approximately natural size, with wings partially closed and as usually seen. Ant has a narrower waist and shorter wings than termite. Actual length of queen about 0.3 inch.
Fig. 59.—Hymenoptera. A parasitic wasp, Opius, sp. Actual length 0.1 inch. (Drawing from U. S. D. A.)
Fig. 60.—Mecoptera. Panorpa chelata, one of about 15 Illinois species of scorpionflies. Only the male has the “scorpion” tail. Actual length of insect about 0.5 inch.
Trichoptera
Caddisflies
Insects with two pairs of wings, poorly developed mouthparts of the chewing type, and long antennae; without tails on the posterior end of the body. In repose, the wings are held rooflike over the body and have only a moderate number of longitudinal veins, which are not connected by crossveins into any resemblance of a network. Neither body nor wings are covered with scales. The larvae are wormlike and they live in streams, ponds, and lakes. Many of them build cases of sticks, stones, or sand and move about with only the front end of the body protruding from the case. When disturbed, the larvae withdraw completely into the cases and are then very difficult to see. The adult fly and larva of Rhyacophila fenestra Ross illustrate this order, figs. 61 and 62. In
Fig. 61.—Trichoptera. Rhyacophila fenestra; the adult form of this caddisfly is shown here. Actual length about 0.4 inch.
Fig. 62.—Trichoptera. Rhyacophila fenestra; the larva, shown here, is aquatic and builds no case. The larvae of some other kinds of caddisflies live in cases made of sticks and stones.
Lepidoptera
Butterflies, Moths
Insects typically with two pairs of wings, with long antennae, and with mouthparts forming a long sucking tube. The body and wings are covered with a dense mass of scales, which are characteristic of this order, fig. 63. The young are known as caterpillars or grubs. The larval stage in this order is well exemplified by the fairly smooth, cylindrical caterpillar of the fall armyworm, Laphygma frugiperda (Smith), fig. 64. Some other larvae are hairy; still others are sluglike.
Fig. 63.—Lepidoptera. A typical moth, showing scales on wings and body, and sucking tube, which is coiled up under the head when not in use. Species of this order occurring in Illinois include specimens that vary in size from 0.1 inch to several inches. The largest of these insects have a wingspread of over 5 inches.
To this order belong not only a very large number of species, but also a very large number that are especially injurious to agriculture. These include such species as the codling moth, cabbage moth, butterflies, the entire cutworm group, and a host of others. In addition, the various clothes moths, which are a constant source of loss to householders, and various species of meal moths, which cause tremendous damage to stored grain every year, are members of this order.
Fig. 64.—Lepidoptera. Laphygma frugiperda, the fall armyworm: A, egg mass on leaf; B, top and side view of individual eggs; C, adult; D, larva; E, pupae. Wing span of adult about 1.5 inches. (Drawing from U. S. D. A.)
In one group of moths, there are clear “windows” on the wings, but these are always surrounded by areas or lines of scales. A few species of the Lepidoptera are very odd in having wingless females. Examples are the bagworms and some of the cankerworms. In these species, however, the body of the female is densely clothed with scales, which will serve to identify her as one of the Lepidoptera.
The habits of Lepidoptera larvae are very diverse. Most of these larvae are leaf eaters, but some bore into trunks of trees and stems of herbaceous plants. Some of the small ones mine within leaf tissue, others live in the ground, where they eat roots, and a few are aquatic, living in clear, rapidly flowing streams.
Diptera
Flies, Mosquitoes, and Their Allies
Insects with only one pair of wings, each wing with a limited number of veins. Other characters of the order, including antennae and mouthparts, are extremely varied. Most immature stages are wormlike or maggot-like. They live in protected situations, such as within the tissues of a plant, in water, in leaf mold, or in the tissues of animals. A typical life cycle is that shown for the house fly, Musca domestica Linnaeus, fig. 65. The ubiquitous house fly is undoubtedly the best known representative of this order. It is also one of the most persistent and dangerous insect pests, being a possible carrier of many diseases.
Fig. 65.—Diptera. Musca domestica, the house fly. The fly has only a single pair of wings; the pale, maggot-like larva is without legs; the darker, egg-shaped puparium contains the pupal or quiescent stage. Length of adult 0.2 inch. (Drawing by Alice Ann Prickett.)
Mosquitoes, punkies, black flies, and horse flies are likewise well known members of this order. In addition to economic forms, the order Diptera includes midges, crane flies, bee flies, robber flies, bluebottle flies, and a great assortment of other kinds of insects. Interesting are the bee flies, which mimic other insects such as honey bees, bumble
Siphonaptera
Fleas
Wingless insects that evolved from folding-wing insects; conspicuously flattened from side to side; with stout spiny legs, and with numerous spines over the body; without conspicuous antennae or tails or a forked posterior appendage like that of the springtails; usually hard; ranging in color from yellowish brown to almost black.
The human flea, Pulex irritants Linnaeus, and a widespread Illinois rat flea, Nosopsyllus fasciatus (Bosc), are shown in fig. 66.
All the fleas, which feed on the blood of birds and other animals, have sucking mouthparts. They are powerful jumpers. The young stages are slender, white larvae, fig. 67, which live in the nests of various animals; these larvae are seldom collected. The fleas are found on the animals themselves or around their nests. Several species of fleas, including the cat and dog flea, the human flea, and the rat fleas, attack man. One of the rat fleas, Xenopsylla cheopis (Rothschild) is the common transmitter of the organism causing bubonic plague.
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.
