=Development and life-history.=--The young insect when just hatched from the egg either resembles, except for the absence of wings, its parent in general appearance as in the case of the locust, or it may, as in the b.u.t.terfly, emerge in a form very unlike the parent. In the first case the young has simply to grow, that is, to increase in size, to develop wings, and to make some other not very obvious developmental changes in order to become fully grown. But in the case of the b.u.t.terfly, and similarly in the case of all other insects as the flies, beetles, bees _et al._, whose young hatch in a larval condition differing markedly from the adult, some radical and striking developmental changes occur before maturity is reached. Such insects are said to undergo complete metamorphosis in their development, while those insects like the locusts, the sucking-bugs, white ants, and others, the just hatched young of which resemble their parents, are said to have an incomplete metamorphosis (fig. 52).

[Ill.u.s.tration: FIG. 53.--The larva of the violet tip b.u.t.terfly, _Polygonia interragationis_, making its last molt, i.e. pupating.

(Photograph from life.)]

In the case of insects with complete metamorphosis, the young hatches as an active grub or worm-like feeding larva which increases in size, casting its skin or molting several times in its growth. Finally after the last larval molt (fig. 53) called pupation the insect appears in a quiescent non-feeding stage called the pupa (fig. 54), and encased in an extra thick and firm chitinous exoskeleton. The immovable pupa is sometimes concealed underground, sometimes enclosed in a silken coc.o.o.n spun by the larva just before pupation, or is in some other way specially protected. It is in this pupal condition that the great changes from wingless, often legless, worm-like larva to winged, six-legged, graceful imago of adult stage are completed, and with the molting of the chitinous pupal cuticle the metamorphosis or development of the insect is completed. As a matter of fact many of the special organs of the adult, the legs and wings, for example, begin to develop as little buds or groups of cells in the body of the larva, and when the larva is ready to pupate these imaginal wings and legs are drawn out to the external surface of the body, and may be readily recognized as they lie on the ventral surface of the pupa folded and closely pressed to the body surface. In recent years the study of the post-embryonic development of insects with complete metamorphosis has revealed some remarkable changes of the internal organs which result in a nearly complete disintegration or breaking down of most of the internal organs of the larva (fig. 55) and a rebuilding of the organs of the adult from primitive beginnings.

[Ill.u.s.tration: FIG. 54.--Chrysalid (pupa) of the violet tip b.u.t.terfly, _Polygonia interragationis_. From this chrysalid issues the full fledged b.u.t.terfly. (Photograph from life.)]

[Ill.u.s.tration: FIG. 55.--A cross-section of the body of the pupa of a honey-bee, showing the body cavity filled with disintegrated tissues, and (at the bottom) a budding pair of legs of the adult, the larva being wholly legless. (Photo-micrograph by Geo. O. Mitch.e.l.l.)]

The habits of the larvae of insects with complete metamorphosis and of the young of some insects with incomplete metamorphosis often differ markedly from the habits of the adults, and as the habits and instincts of insects are remarkably specialized, the study of their behavior and of the structural and physiological modification which their varied habits of life have brought about is of much interest and significance. In later paragraphs this phase of insect study will be again referred to.

=Cla.s.sification.=--Much attention has been paid to the cla.s.sification of insects and the 300,000 (approximately) known species have been variously grouped together into orders by different entomologists. A subdivision of the cla.s.s Insecta into five orders was proposed by Linnaeus about 1750 and was used until comparatively recently. Since then, however, numerous other arrangements have been proposed, all of them agreeing in increasing the number of orders by breaking up some of the old ones into two or more new ones. The cla.s.sification adopted in the text-book[11] of zoology which we have made our reference in cla.s.sification is an 8-order system. The latest English[12] text-book in entomology adopts a 9-order system, while the princ.i.p.al American[13]

text-book on this subject divides the insects into nineteen orders.

The cla.s.sification depends chiefly on the character of the post-embryonic development, that is, on whether the metamorphosis is complete or incomplete, and on the structural character of the mouth-parts and wings. In the following paragraphs a few of the larger insect orders, with some special representatives of each, will be briefly considered.

The best American text-book of the cla.s.sification and habits of insects is Comstocks' "Manual of Insects." For an account of the structure of the wings and mouth-parts of various insects see Comstock and Kellogg's "Elements of Insect Anatomy."

=Orthoptera: the locusts, c.o.c.kroaches, crickets, katydids, etc.=--TECHNICAL NOTE.--Obtain specimens of crickets or katydids, and c.o.c.kroaches, and compare the external body structure with that of the gra.s.shopper; examine especially the wings, mouth-parts, legs, and egg-laying organs. Note that the hindmost legs of the c.o.c.kroach are not fitted for leaping but for running. Note the sound-making (stridulating) organs on the bases of the fore wings of the male katydids and crickets. Note the auditory organs (tympana) in the fore tibiae of the katydids and crickets. Crickets can be easily kept alive in breeding-cages in the laboratory and their feeding habits and much of their life-history observed. The growth of the young and the development of the wings can be noted, and will be found to be essentially similar to the conditions already found in the case of the locust.

[Ill.u.s.tration: FIG. 56.--The house cricket, male (_a_) and female (_b_). (From Marlatt.)]

The locust studied as one of the examples of the cla.s.s Insecta belongs to the order Orthoptera, which also includes the c.o.c.kroaches, crickets (fig. 56), katydids and green gra.s.shoppers, the walking-stick or twig insects, the praying mantis and others. The members of this order all have an incomplete metamorphosis, and in all the mouth-parts are fitted for biting and the fore wings are more or less thickened and modified to serve as covers or protecting organs for the broad, plaited, membranous hind wings, which are the true flight organs. The hind legs of locusts, gra.s.shoppers, crickets, and katydids are very large, and enable the insects to leap; the legs of the c.o.c.kroaches are fitted for swift running; the fore legs of the praying mantis are fitted for grasping other insects which serve as their food, and the legs of the walking-stick (fig. 162) are long and slender and fitted for slow walking. The shrill singing of the crickets and katydids and the loud "clacking" of the locusts are all made by stridulation, that is, by rubbing two roughened parts of the body together. The sounds of insects are not made by vocal cords in the throat. The male crickets and katydids (for only the males sing) have the veins of the fore wings modified so that when the bases of the wings are rubbed together (and when the cricket or katydid is at rest the base of one fore wing overlaps the base of the other) a part of one wing called the "sc.r.a.per"

rubs against a part of the other called the "file" and the shrilling is produced. The sounds of locusts are produced by the rubbing of the inside of the hind leg against the outside of the fore wing when the insect is at rest, or by striking the front margin of each hind wing against the hind margin of each fore wing when the locust is flying. For hearing the Orthoptera are provided with auditory organs having the character of tympana or vibrating membranes. In the locusts these ears (fig. 51) are situated on the dorsal surface of the first abdominal segment; in the katydids and crickets they are in the tibiae of the fore legs. The food of locusts, crickets, and katydids is vegetable, being usually green leaves; the c.o.c.kroaches eat either plant or animal substances fresh or dry, while the praying mantis is predaceous, feeding on other insects which it catches in its strong grasping fore legs. The walking-stick or twig insect is an excellent example of what is called "protective resemblance" among animals. Indeed most of the Orthoptera are so colored and patterned as to be almost indistinguishable when on their usual resting- or feeding-grounds. Some of the tropical Orthoptera carry to a marvelous degree this modification for the sake of protection. (In this connection read Chapter x.x.xI referring to "Protective Resemblances".)

[Ill.u.s.tration: FIG. 57.--A bird louse, _Nirmus praestans_, from a tern, _Sterna maxima_. Most birds are infested with small, wingless, biting insects, called bird-lice, which are external parasites feeding on the feathers of the bird host. The bird louse figured is about 1/12 in.

long. (Photo-micrograph by Geo. O. Mitch.e.l.l.)]

=Odonata and Ephemerida: the dragon-flies and May-flies.=--TECHNICAL NOTE.--Obtain specimens of adult and immature dragon-flies. The young dragon-flies (fig. 59) may be got by raking out some of the slime and aquatic vegetation from the bottom of a small pond. Compare the external structure of the adult dragonflies with that of the gra.s.shopper; note the large eyes, the narrow nerve-veined wings, the biting mouth-parts, and the short antennae. Compare the young dragon-flies with the adults; note the developing wings and the peculiar modification of the lower lip into a protrusible, grasping organ which when at rest is folded like a mask over the face. Examine the interior of the posterior part of the alimentary ca.n.a.l to find the rectal gills. Obtain specimens of adult and young May-flies. The young may be found on the under side of stones in a "riffle" in almost any stream. They live also in ponds. They may be recognized by reference to fig. 61. Compare adult May-flies with the dragon-flies; note the weakly chitinized, delicate body-wall, and the difference in size between fore and hind wings; note the biting mouth-parts of the young and their absence or presence in vestigial condition only in the adults.

The young of both dragon-flies and May-flies may easily be kept alive in the laboratory aquarium (fruit-jars or battery-jars with pond water in), and their feeding habits, their swimming, their respiration, and much of their development observed. The young May-flies should be got from ponds, not running streams. Put one of these semi-transparent May-fly nymphs into a watch-gla.s.s of water, and examine under the microscope. The movements of the gills, heart, and alimentary ca.n.a.l, and much of the anatomy can be readily made out. The emergence of the adult from the nymphal skin can be seen if close watch is kept. The young dragon-flies may be seen to capture and devour their prey. They may also transform into adults, but for this it will be necessary to obtain nymphs nearly ready for transformation.

Among the most familiar and interesting insects are the dragon-flies (fig. 58), sometimes called "devil's darning-needles." They are commonly seen flying swiftly about over ponds or streams catching other flying insects. The dragon-flies are the insect-hawks; they are predaceous and very voracious, and are probably the most expert flyers of all insects. There are many species, and their bright iridescent colors and striking wing-patterns make them very beautiful. The young dragon-flies (fig. 59) are aquatic, living in streams and ponds, where they feed on the other aquatic insects in their neighborhood. They catch their prey by lying in wait until an insect comes close enough to be reached by the extraordinarily developed protrusible grasping lower lip (fig. 60). When at rest this lower lip lies folded on the face so as to conceal the great jaws. The young dragon-flies breathe by means of gills which do not project from the outside of the body, as do the gills of other aquatic insects, but line the inner wall of the posterior or rectal part of the alimentary ca.n.a.l. Water enters the ca.n.a.l through the a.n.a.l opening and bathes these gills, bringing oxygen to them and taking away carbonic acid gas. The aquatic immature life of the dragon-flies lasts from a few months to two years. When ready to change to adult, the young crawls out of the water and clinging to a rock or plant makes its last molt.

[Ill.u.s.tration: FIG. 58.--A dragon-fly, _Sympetrum illotum_, common in California. (From life.)]

[Ill.u.s.tration: FIG. 59.--The young (nymph) of the dragon-fly, _Sympetrum illotum_. (From Jenkins and Kellogg.)]

[Ill.u.s.tration: FIG. 60.--Young (nymph) dragon-fly, showing lower lip folded and extended. (From Jenkins and Kellogg.)]

[Ill.u.s.tration: FIG. 61.--Young (nymph) of May-fly, showing (_g_) tracheal gills. (From Jenkins and Kellogg.)]

Other abundant and interesting pond and brook insects are the May-flies.

The young May-flies (fig. 61) are aquatic, living in streams and ponds and feeding on minute organisms such as diatoms and other algae. The immature life lasts a year, or even two or three in some species, and then the May-fly crawls out of the water upon a plant-stem or projecting rock and, molting, appears as the winged adult. The adult May-fly, having its mouth-parts atrophied (a few May-flies have functional mouth-parts), takes no food, and lives only a few hours or at most perhaps a few days. It has the shortest life (in adult stage) of all insects. The female drops her eggs into the water.

=Hemiptera: the sucking-bugs.=--TECHNICAL NOTE.--Obtain specimens of water-striders (narrow elongate-bodied insects with long spider-like legs which run quickly about on the surface of ponds or quiet pools in streams), water-boatmen (mottled grayish insects about half an inch long which swim and dive about in ponds and stream-pools), back-swimmers (which are usually in company with the water-boatmen, but which swim with back downwards and are marked with purplish-black and creamy white patches), cicadas (the dog-day locusts), and plant-lice (the "green fly" of rose-bushes and other cultivated plants). Compare the external structure of some of these Hemiptera with the other insects already examined; note especially the sucking beak, composed of the elongate tube-like labium in which lie the greatly modified flexible needle-like maxillae and mandibles, the whole forming an equipment for piercing and sucking. Obtain immature specimens of some of these insects (distinguished by their smaller size and the wing-pads); note that the metamorphosis is incomplete, the young resembling the parents in general appearance.

Both immature and adult specimens of water-boatmen (_Corisa_), back-swimmers (_Notonecta_), and water-striders (_Hygrotrechus_) can be easily kept in the laboratory aquaria- and their swimming, breathing, and feeding habits observed. Note especially the carrying of air down beneath the water.

[Ill.u.s.tration: FIG. 62.--The female red orange scale insect, _Aspidiotus aurantii_, very injurious to orange-trees. It has no wings, legs, nor eyes, but remains motionless on a leaf, stem, or fruit, holding fast by its long slender beak, through which it sucks up the plant-sap. The male is winged, and has no mouth-parts, taking no food. (Photo-micrograph by Geo. O. Mitch.e.l.l.)]

[Ill.u.s.tration: FIG. 63.--The female rose-scale, _Diaspis rosae_, a pest of rose-bushes, without eyes, wings, or legs, but with slender sucking proboscis. The male is winged and without mouth-parts. (Photo-micrograph by Geo. O. Mitch.e.l.l.)]

The Hemiptera are characterized particularly by their highly specialized sucking mouth-parts, no other of the sucking insects having the proboscis composed in the same manner. The palpi of both maxillae and labium are wholly wanting in Hemiptera and the flexible needle-like maxillae and mandibles are enclosed in the tubular labium.

This order is a large one and includes many well-known injurious species, as the chinch-bug (_Blissus leucopterus_), which occurs in immense numbers in the grain-fields of the Mississippi valley, sucking the juices from the leaves of corn and wheat, the grape Phylloxera (_Phylloxera vastatrix_), so destructive to the vines of Europe and California, the scale insects (_Coccidae_) (figs. 62 and 63), the worst insect pests of oranges, the squash-bugs and cabbage-bug and a host of others. Some of the Hemiptera, for example, the lice and bed-bugs, are predaceous, sucking the blood of other animals.

[Ill.u.s.tration: FIG. 64.--A water-strider, _Hygrotrechus_ sp. (From Jenkins and Kellogg.)]

The water-striders (fig. 64) catch other insects, both those that live in the water and those which fall on to its surface, and holding the prey with their seizing fore legs they pierce its body with their sharp beak and suck its blood. They lay their eggs in the spring glued fast to water-plants. The young water-striders are shorter and stouter in shape than the adults.

[Ill.u.s.tration: FIG. 65.--A water-boatman, _Corisa_ sp. (From Jenkins and Kellogg.)]

The water-boatmen (fig. 65) and back-swimmers swim and dive about in the water, coming more or less frequently to the surface to get a supply of air. This air they hold under the wings, or on the sides and under part of the body entangled in the fine hairs on the surface. The insects appear to have silvery spots on the body, due to the presence of this air. The "rowing" legs of the water-boatmen (_Corisa_) are the hindmost pair; in the back-swimmers (_Notonecta_) they are the middle legs.

The cicadas (fig. 66) are the familiar insects of summer which sing so shrilly from the trees, the seventeen-year cicada (_Cicada septendecim_) (oftentimes called locust) being the best known of this family. Its eggs are laid in slits cut by the female in live twigs.

The young, which hatch in about six weeks, do not feed on the green foliage, but fall to the ground, burrow down to the roots of the tree and there live, sucking the juices from the roots, for sixteen years and ten or eleven months. When about to become adult, the young cicada crawls up out of the ground and clinging to the tree-trunk molts for the last time, and flies to the tree-tops.

[Ill.u.s.tration: FIG. 66.--The seventeen-year cicada, _Cicada septendecim_; the specimen at left showing sound-making organ, _v.

p._, ventral plate; _t_, tympanum. (From specimen.)]

The plant-lice (_Aphididae_) are small soft-bodied Hemiptera which have both winged and wingless individuals. In the early spring a wingless female hatches from an egg which, laid in the preceding fall, has pa.s.sed the winter in slow development. This wingless female, called the stem-mother, lays unfertilized eggs or more often perhaps gives birth to live young, all of which are similarly wingless females which reproduce parthenogenetically. This reproduction goes on so rapidly that the plant-lice become overcrowded on the food-plant and then a generation of winged[14] individuals is produced from time to time.

These winged plant-lice fly away to new plants. In the autumn a generation of males and females is produced; these individuals mate and each female lays a single large egg which goes over the winter, and produces in the spring the wingless agamic stem-mother. Plant-lice produce honey-dew, a sweetish substance much liked by ants, and the lice are often visited, and sometimes specially cared for, by the ants for the sake of this honey-dew. Small as they are, plant-lice occur in such numbers as to do great damage to the plants on which they feed.

The apple-aphis, cherry-aphis, pear-aphis, cabbage-aphis and others are well-known pests. The most notoriously destructive plant-louse is the grape _Phylloxera_, which lives on the roots and leaves of the grape-vine. Immense losses have been caused by this pest, especially in the wine-producing countries of southern Europe.

=Diptera: the flies.=--TECHNICAL NOTE.--Obtain specimens of the adult and young stages of the blowfly and the mosquito. All the young stages of the blowfly may be obtained, and its life-history studied, by exposing a piece of meat to decay in an open gla.s.s jar. The larvae of the mosquito are the familiar wrigglers of puddles and ponds, and by collecting some of them and keeping them in a gla.s.s jar of water covered with a bit of mosquito-netting, the life-history of the mosquito is easily studied. If the eggs can be obtained from the pond so much the better; they are in little black ma.s.ses floating on the surface of the water, and resemble at first glance nothing so much as a floating bit of soot. The external structure of the adult flies should be compared with that of the other insects studied, noting especially the condition of mouth-parts and wings, and the subst.i.tution of balancers for the hind wings. The mouth-parts of the mosquito are in the form of a long proboscis composed of six slender needle-like stylets lying in a tube narrowly open along its dorsal surface. The tube is the labium, and the stylets are the two maxillae, two mandibles, and two other parts known as the epipharynx and the hypopharynx. Two additional thicker elongate segmented processes lying outside of and parallel with the tube are the maxillary palpi. The male mosquito (distinguished from the female by the more hairy or bushier antennae) lacks the pair of needle-like mandibles. The mouth-parts of the blowfly are composed almost exclusively of the thick fleshy proboscis-like labium, which is expanded at the tip to form a rasping organ.

The Diptera or true flies are readily distinguishable from other insects by their having a single pair of wings instead of two pairs, the hind wings being transformed into small k.n.o.b-headed pedicels called balancers or halteres. The flies undergo complete metamorphosis, and their mouth-parts are fitted for piercing and sucking (as in the mosquito) or for rasping and lapping (as in the blowfly). Nearly 50,000 species of flies are known, more than 4,000 being known in North America alone.

The blowfly (_Calliphora vomitoria_) is common in houses, but can be distinguished from the house-fly by its larger size and its steel-blue abdomen. It lays its eggs on decaying meat (or other organic matter) and the white footless larvae (maggots) hatch in about twenty-four hours. They feed voraciously and become full grown in a few days. They then change into pupae which are brown and seed-like, being completely enclosed in a uniform chitinized case which wholly conceals the form of the developing fly. The house-fly has a life-history and immature stages like the blowfly, but its eggs are deposited on manure.

[Ill.u.s.tration: FIG. 67.--The mosquito, _Culex_ sp.; showing eggs (on surface of water), larvae (long and slender, in water), pupa (large headed, at surface), and adult (in air). (From living specimens.)]

The mosquito (_Culex_ sp.) (fig. 67) lays its eggs in a sooty-black little boat-shaped ma.s.s which floats lightly on the surface of the water. In a few days the larvae, or "wrigglers," issue and swim about vigorously by bending the body. The head end of the body is much broader than the other, the thoracic segments being markedly larger than the abdominal ones. The head bears a pair of vibrating tufts of hairs, which set up currents of air that bring microscopic organic particles in the water into the wriggler's mouth. At the posterior tip of the body are two projections, one the breathing-tube (the wriggler coming often to the surface to breathe), and the other the real tip of the abdomen. The wriggler, although heavier than water, can hang suspended from the surface film by the tip of its breathing-tube. It changes in a few days into the pupa, which, instead of being quiescent as with most flies, can swim about. It has a large bulbous head end and the posterior end of the body bears a pair of swimming-flaps. It takes no food. When ready to change to the adult mosquito the pupa (which, unlike the wriggler, is lighter than water) floats at the surface of the water, back uppermost. The chitinous cuticle splits along the back and the delicate mosquito comes out, rests on the floating pupal skin until its wings are dry, and then flies away. Only the female mosquitoes suck blood. If they cannot find animals, mosquitoes live on the juices of plants. They are world-wide in their distribution, being serious pests even in Arctic regions, where they are often intolerably numerous and greedy. Recent investigations have shown that the germs which cause malaria in man live also in the bodies of mosquitoes, and are introduced into the blood of human beings by the biting (piercing) of the mosquitoes. It is probable also that the germs of yellow fever are distributed by mosquitoes in the same way. By pouring a little kerosene on the surface of a puddle no mosquitoes will be able to escape from the water.

[Ill.u.s.tration: FIG. 68.--The house-flea, _Pulex irritans_; _a_, larva; _b_, pupa; _c_, adult. (The fleas are probably more nearly related to the Diptera than to any other order of insects.) (After Beneden.)]

=Lepidoptera: the moths and b.u.t.terflies.=--TECHNICAL NOTE.--Obtain specimens of a few moths, and compare with the b.u.t.terfly already studied; note especially the character of antennae. Obtain miscellaneous specimens of larvae, pupae, and coc.o.o.ns of any moths or b.u.t.terflies. Note the variety in colors, markings, and skin coverings of the larvae; note the shape and markings of the pupae.

Rear from eggs, larvae, or pupae in breeding-cages any moths and b.u.t.terflies obtainable (for directions for rearing moths and b.u.t.terflies see Chapter x.x.xIV), keeping note of the times of molting and of the duration of the various immature stages. If the eggs of silkworms can be obtained the whole life cycle of the silkworm moth can be observed in the schoolroom. The larvae (worms) feed on mulberry or osage orange leaves, feeding voraciously, growing rapidly and making no attempts to escape. The molting of the larvae can be observed, the spinning of the silken coc.o.o.n, and the final emergence of the moth. The moths after emergence will not fly away, but if put on a bit of cloth will mate, and lay their eggs on it.

From these eggs, which should be kept well aired and dry, larvae will hatch in nine or ten months (if the race is an "annual").

The Lepidoptera (figs. 69-74) include all those insects familiarly known to us as moths and b.u.t.terflies; they are characterized by their scale-covered wings (fig. 69) and long nectar-sucking proboscis composed of the two interlocking maxillae. They undergo a complete metamorphosis (fig. 70) and their larvae are the familiar caterpillars of garden and field. These larvae have biting mouth-parts and feed on vegetation, some of them being very injurious, for example the army-worms, cut-worms, codlin moth worms, etc. The adult moths and b.u.t.terflies take only liquid food, or no food at all, and are wholly harmless to vegetation. The structure and life-history of a b.u.t.terfly has already been studied, and in the more general conditions of structure and life-history there is much similarity in the many insects of this order. The eggs are usually laid on the food-plant of the larva; the larva feeds on the leaves of this plant, grows, molts several times, and pupates either in the ground or in a silken coc.o.o.n or simply attached to a branch or leaf. There are about six thousand species of moths and b.u.t.terflies known in North America, and they are our most beautiful insects.

[Ill.u.s.tration: FIG. 69.--A small, partly denuded part, much magnified, of a wing of a "blue" b.u.t.terfly, _Lycaena_ sp., showing the wing, scales and the pits in the wing-membrane, in which the tiny stems of the scales are inserted. (Photo-micrograph by Geo. O. Mitch.e.l.l.)]

=Coleoptera: the beetles.=--TECHNICAL NOTE.--Obtain specimens of various beetles, among them some water-beetles and June-beetles with their young stages, if possible; if not, then the young stages and adults of any beetle common in the neighborhood of the school. Of the swimming and diving water-beetles there are three families, viz., the Gyrinidae or whirligig beetles, with four eyes (each compound eye divided in two), the Hydrophilidae, or water-scavengers with two eyes and antennae with the terminal segments thicker than the others, and the Dytiscidae or predaceous water-beetles with two eyes and slender thread-like antennae. Try to find Dytiscidae, large, oval, shining black beetles; the larvae are called water-tigers and are long, slim, active creatures with six legs and slender curving jaws (see fig. 76). The June-beetles are the heavy brown buzzing "June-bugs" and their larvae are the common "white grubs" found underground in lawns and pastures. Have live water-tigers and predaceous water-beetles in the aquarium.

Note their feeding and breathing. Compare the external structure of the beetles with that of the other insects, noting especially the biting mouth-parts, and their thickened h.o.r.n.y fore wings serving as covers for the folded membranous hind wings.

[Ill.u.s.tration: FIG. 70.--The forest tent-caterpillar moth, _Clisiocampa disstria_, in its various stages; _m_, male moth; _f_, female moth; _p_, pupa; _e_, eggs (in a ring) recently laid; _g_, eggs hatched; _c_, larva or caterpillar. Moths and caterpillar are natural size, eggs and pupa slightly enlarged. (Photograph by M. V. Slingerland.)]