Insects, Their Ways and Means of Living/Chapter VI

CHAPTER VI

PLANT LICE

"Plant lice! Ugh," you say, "who wants to read about those nasty things! All I want to know is how to get rid of them." Yes, but the very fact that those soft green bugs that cover your roses, your nasturtiums, your cabbages, and your fruit trees at certain seasons reappear so persistently, after you think you have exterminated them, shows that they possess some hidden source of power; and the secrets of a resourceful enemy are at least worth knowing—besides, they may be interesting.

Really, however, insects are not our enemies; they are only living their appointed lives, and it just happens that we want to eat some of the same plants that they and their ancestors have always fed on. Our trouble with the insects is just that same old economic conflict that has bred the majority of wars; and, in the case between us and the insects, it is we who are the aggressors and the enemies of the insects. We are the newcomers on the earth, but we fume around because we find it already occupied by a host of other creatures, and we ask what right have they to be here to interfere with us! Insects existed millions of years before we attained the human form and aspirations, and they have a perfectly legitimate right to everything they feed on. Of course, it must be admitted, they do not respect the rights of private property; and therein lies their hard luck, and ours.

The plant lice are well known to anyone who has a garden, a greenhouse, an orchard, or a field of grain. Some call them "green bugs"; entomologists usually call them aphids. A single plant louse is an aphis, or an aphid; more than one are usually called aphides, or aphids.

The distinguishing feature of the plant lice, or aphids, as we shall by preference call them, is their manner of feeding. All the insects described in the preceding chapters eat in the usual fashion of biting off pieces of their food, chewing them, and swallowing the masticated bits. The

Fig. 88. Group of green apple aphids feeding along a rib on under surface of an apple leaf

aphids are sucking insects; they feed on the juices of the plants they inhabit. Instead of jaws, they have a piercing and sucking beak (Fig. 89), consisting of an outer sheath inclosing four slender, sharp-pointed bristles which can be thrust deep into the tissues of a leaf or stem (Fig. 89 B). Between the bristles of the innermost pair (Fig. 90, Mx) are two canals. Through one canal, the lower one (b), a liquid secretion from glands of the head is injected into the plant, perhaps breaking down its tissues; through the other (a) the plant sap and probably some of the protoplasmic contents of the plant cells are drawn up into the mouth. A sucking apparatus like that of the aphids is possessed by all insects related to the aphids, comprising the order Hemiptera, and will be more fully described in the next chapter, which treats of the cicada, a large cousin of the aphids.

When we observe, now, that different insects feed in two quite different ways, some by means of the biting type of mouth parts, and others by means of the sucking type, it becomes evident that we must know which kind of insect we are dealing with in the case of pests we may be trying to control. A biting and chewing insect can be killed by the mere expedient of putting poison on the outside of its food, if it does not become aware of the poison and desist from eating it; but this method would not work with the piercing and sucking insects, which extract their food from beneath the surface of the plants on which they feed. Sucking insects are, therefore, to be destroyed by means of sprays or dusts that will kill them by contact with their bodies. Aphids are usually attacked with irritant sprays, and in general it is not a difficult matter to rid infested plants of them, though in most cases the spraying must be repeated through the season.

Fig. 89. The way an aphis feeds on the juices of a plant

A, an aphis with its beak thrust into a rib of a leaf. B, section through the midrib of a young apple leaf, showing the mouth bristles from the beak of an aphis penetrating between the cells of the leaf tissue to the vascular bundles, while the sheath of the beak is retracted by folding back beneath the head

When any species of aphis becomes well established on a plant, the infested leaves (Fig. 88) may be almost as crowded as an East Side street on a hot summer afternoon. But there is

PLATE 2

The green apple aphis (Aphis pomi)
A, adult sexual female; B, adult male; C, young femme; D, female laving an egg; E, eggs, which turn from green to black after they are laid.
(Enlarged about 20 times)

no bustle, no commotion, for each insect has its sucking bristles buried in the leaf, and its pump is busy keeping the stomach supplied with liquid food. The aphis crowds are mere herds, not communities or social groups as in the case of the termites, ants, or bees.

Wherever there are aphids there are ants, and in contrast to the aphids, the ants are always rushing about all over the place as if they were looking for something and each wanted to be the first to find it. Suddenly one spies a droplet of some clear liquid lying on the leaf and gobbles it up, swallowing it so quickly that the spherule seems to vanish by magic, and then the ant is off again in the same excited manner. The explanation of the presence and the actions of the ants among the aphids is this: the sap of the plants furnishes an unbalanced diet, the sugar content being far too great in proportion to the protein. Consequently the aphids eject from their bodies drops of sweet liquid, and it is this liquid, called "honey dew," that the ants search out so eagerly. Some of the ants induce the aphids to give up the honey dew by stroking the bodies of the latter. The glistening coat often seen on the leaves of city shade trees and the shiny liquid that bespatters the sidewalks beneath is honey dew discharged from innumerable aphids infesting the under surfaces of the leaves.

In studying the termites, we learned that it is possible for a single pair of insects to produce regularly several kinds of offspring differing in other ways than those of sex. In the aphids, a somewhat similar thing occurs in that each species may be represented by a number of forms; but with the aphids these different forms constitute successive generations. If events took place in a human family as they do in an aphid family, children born of normal parents would grow up to be quite different from either their father or their mother; the children of these children would again be different from their parents and also from their grandparents, and when mature they perhaps would migrate to some other part of the country; here they would have children of their own, and the new fourth generation would be unlike any of the three preceding; this generation would then produce another, again different; and the latter would return to the home

Fig. 90. Cross-section through the base of the beak of an aphis.
(From Davidson)
The outer sheath of the beak is the labium (Lb), covered basally by the labrum (Lm). The four in-closed bristles are the mandibles (Md) and the maxillae (Mx), the latter containing between them a food canal (a) and a salivary canal (b). Only the inner walls of the labrum and labium are shown in the section

town of their grandparents and great-grandparents, and here bring forth children that would grow up in the likeness of their great-great-great grandparents! This seems like a fantastic tale of fiction, too preposterous to be taken seriously, but it is a commonplace fact among the aphids, and the actual genealogy may be even more complicated than that above outlined. Moreover, the story is not yet complete, for it must be added that all the generations of the aphids, except one in each series, are composed entirely of females capable in themselves of reproduction. In warm climates, it appears, the female succession may be uninterrupted.

How insects do upset out generalizations and our peace of mind! We have heard of feminist reformers who would abolish men. With patient scorn we have listened to their predictions of a millenium where males will be unknown and unneeded—and here the insects show us not only that the thing is possible but that it is practicable, at least for a certain length of time, and that the time can be indefinitely extended under favorable conditions.

Since special cases are always more convincing than general statements, let us follow the seasonal history of some particular aphids, taking as examples the species that commonly infest the apple.

Let the time be a day in the early part of March. Probably a raw, gusty wind is blowing from the northwest, and only the silver maples with their dark purplish clusters of frowzy flowers already open give any suggestion of the approach of spring. Find an old apple tree somewhere that has not been sprayed, the kind of tree an entomologist always likes to have around, since it is sure to be full of insects. Look closely at the ends

Fig. 91. Aphis eggs on apple twigs in March; an enlarged egg below

of some of the twigs and you will probably find a number of little shiny black things stuck close to the bark, especially about the bases of the buds, or tucked under the projecting edges of scars and tiny crevices (Fig. 91). Each little speck is oval and about one thirty-sixth of an inch in length.

To the touch the objects are firm, but elastic, and if you puncture one a pulpy liquid issues from it; or so it appears, at least, to the naked eye—a microscope would show that in this liquid there is organization. In short, the tiny capsule contains a young aphid, because it is an aphid egg. The egg was deposited on the twig last fall by a female aphis, and its living contents have remained alive since then, though fully exposed to the inclemencies of winter.

Immediately after being laid in the fall, the germ nucleus of the aphis egg begins development, and soon forms a band of tissue lying lengthwise on the under surface of the yolk. Then this scarcely-formed embryo undergoes a curious process of revolution in the egg, turning on a crosswise axis head foremost into the yolk and finally stretching out within the latter with the back down and the head toward the original rear end of the egg. Thus it remains through the winter. In March it again becomes active, reverses itself to its first position, and now completes its development.

The date of hatching of the apple aphis eggs depends much upon the weather and will vary, therefore, according to the season, the elevation, and the latitude; but in latitudes from that of Washington north, it is some time in April, usually from the first to the third week of the month. The eggs of most insects resemble seeds in

Fig. 92. Eggs of the green apple aphis with outer coverings split before hatching; below, an egg removed from its covering

their capacity for lying inert until proper conditions of warmth and moisture bring forth the creature biding its time within. The eggs of one of the apple aphids, however, are killed by premature warm weather, or if artificially warmed too long before the normal time of hatching. In general, the final development of the aphis embryos keeps pace with the development of the apple buds, since both are controlled by the same weather conditions, and this coordination usually insures the young aphids against starvation; but the eggs commonly hatch a little in advance of the opening of the buds, and a subsequent spell of cold weather may give the young lice a long wait for their first meal.

The approaching time of hatching is signaled in most

Fig. 93. Hatching of the green apple aphis, Aphis pomi

A, the egg. B, an egg with the outer coat split. C, the same egg with the inner shell split at one end. D-F, three successive stages in the emergence of the young insect. G-J, shedding the hatching membrane. K, the empty eggshell. L, the young aphid

cases by the splitting of an outer sheath of the egg (Fig. 92), exposing the glistening, black, true shell of the egg within. Then, from one to several days later, the shell itself shows a cleft within the rupture of the outer coat, extending along hall the length of the exposed egg surface and down around the forward end (Fig. 93 C). From this split emerges the sort head of the young aphis (D), bearing a hard, toothed crest, evidently the instrument by which the leathery shell was broken open, and for this reason known as the "egg burster." Once exposed, the head continues to swell out farther and farther as if the creature had been compressed within the egg. Soon the shoulders appear, and now the young aphis begins squirming, bending, inflating its fore parts and contracting its rear parts, until it works its body mostly out of the egg (E, F) and stands finally upright on the tip of its abdomen which is still held in the cleft of the shell (G).

The young aphis at this stage, however, like the young roach, is still inclosed in a thin, tight-fitting, membranous bag having no pouches for the legs or other members, which are all cramped within it. The closely swathed head swells and contracts, especially the facial part, and suddenly the top of the bag splits close to the right side of the egg burster (Fig. 93 H). The cleft pulls down over the head, enlarges to a circle, slides along over the shoulders, and then slips down the body. As the tightly stretched membrane rapidly contracts, the appendages are freed and swing out from the body (I). The shrunken pellicle is reduced at last to a small goblet supporting the aphid upright on its stalk, still held by the tip of the abdomen and the hind feet (I). To liberate itself entirely the insect must make a few more exertions (J), when, finally, it pulls its legs and body from the grip of the drying skin, and is at last a free young aphid (L).

The emergence from the egg and from the hatching membrane is a critical period in the life of an aphid. The

PLATE 3

The rosy apple aphis (Anuraphis roseus)
A, apple leaves and young fruit distorted by the aphids; B, under surface of an infested leaf; C, immature wingless aphid (greatly enlarged); D, immature winged aphis

process may be completed in a few minutes, or it may take as long as hall an hour, but if the feeble creature should be unable to free itself at last from the drying and contracting tissue, it remains a captive struggling in the grip of its embryonic vestment until it expires. The young aphid successfully delivered takes a few uncertain, staggering steps on its weak and colorless legs, and then complacently rests awhile; but after about twenty min-

Fig. 94. Young aphids on apple buds in spring

utes or half an hour it is able to walk in proper insect fashion, and it proceeds upward on its twig, a course sure eventually to lead it to a bud.

While the aphid eggs are hatching, or shortly thereafter, the apple buds are opening and unfolding their delicate, pale-green leaves, and from everywhere now the young aphids come swarming upon them, till the tips are often blackened by their numbers (Fig. 94). The hungry horde plunges into the hearts of the buds, and soon the new leaves are punctured with tiny beaks that rob them of their food; and the young foliage, upon which the tree depends for a stunted and yellowed. Now is the time for the orchardist to spray if he has not already done so.

The entomologist, however, takes note that all the young aphids on the apple trees are not alike; perhaps there are three kinds of them in the orchard (Fig. 95), differing slightly, but enough to show that each belongs to a separate species. When the first buds infested are

Fig. 95. Three species of young aphids round on apples in the spring
A, the apple-grain aphis, Rhopalosiphum prunifoliae. B, the green apple aphis, Aphis pomi. C, the rosy apple aphis, Anuraphis roseus

exhausted, the insects migrate to others, and later they spread to the larger leaves, the blossoms, and the young fruit. The aphids all grow rapidly, and in the course of two or three weeks they reach maturity.

The full-grown insects of this first generation, those produced from the winter eggs, are entirely wingless, and they are all females. But this state of affairs in no wise hinders the multiplication of the species, for these remarkable females are able of themselves to produce offspring (a faculty known as parthenogenesis), and furthermore, they do not lay eggs, but give birth to active young. Since they are destined to give rise to a long line of summer generations, they are known as the stem mothers.

One of the three aphid species of the apple buds is known as the green apple aphis (Fig. 95 B). During the early part of the season the individuals of this species are round particularly on the under surfaces of the apple leaves. They cause the infested leaves to curl and to become distorted in a characteristic manner (Fig. 96). The stem mothers (Fig. 97 A, B) begin giving birth to young (C) about twenty-four hours after reaching maturity, and any one of the mothers, during the course of her life of from ten to thirty days, may produce an average family of fifty or more daughters, for all her offspring are females, too. When these daughters grow up, however, none of them is exactly like their mother. They all have one more segment in each antenna; most of them are wingless (D), but many of them have wings—some, mere padlike stumps, but others well developed organs capable of flight (Fig. 97 E).

Fig. 96. Leaves of apple infested and distorted by the green apple aphis on under surfaces

Both the wingless and the winged individuals of this second generation are also parthenogenetic, and they give birth to a third generation like themselves, including wing- less, half-winged, and fully-winged forms, but with a greater proportion of the last. From now on there follows a large number of such generations continuing through the season. The winged forms fly from one tree to another, or to a distant orchard, and round new colonies. In summer, the green apple aphis is found principally on young shoots of the apple twigs, and on water sprouts growing in the orchard.

During the early part of the summer, the rate of production rapidly increases in the aphid colonies, and individuals of the summer generations sometimes give birth to young a week after they themselves were born. In the fall, however, the period of growth again is lengthened, and the families drop off in size; until the last females of the season produce each a scant half dozen young, though they may live to a much greater age than do the summer individuals.

The young summer aphids born as active insects are inclosed at birth in a tight-fitting, seamless, sleeveless, and legless tunic, as are those hatched from the winter eggs. Thus swathed, each emerges, rear end first, from the body of the mother, but is finally held fast by the face when it is nearly free. In this position, the embryonic bag splits over the head and contracts over the body of the young aphid to the tip of the abdomen, where it remains as a cap of shriveled membrane until it finally drops off or is pushed away by the feet. The infant, now vigorously kicking, is still held in the maternal grasp, and eventually liberates itself only after some rather violent struggling; but soon after if is free it walks away to find a feeding place among its companions on the leaf. The mother is but little concerned with the birth of her child, and she usually continues to feed during its delivery, though she may be somewhat annoyed by its kicking. The average summer female gives birth to two or three young aphids every day.

The succession of forms in the families is one of the most interesting phases of aphid life. Investigations have shown that the winged individuals are produced principally by wingless forms, and experiments have demonstrated that the occurrence of the winged forms is correlated with changes in the temperature, the food supply, and the duration of light. At a temperature around sixty-five degrees few winged individuals ever appear, but they are produced at temperatures either below or above this point. Likewise it has been found that when the food supply gives out through the drying

Fig. 97. The green apple aphis, Aphis pomi. A, B, adult stem mothers. C, a newly-born young of the summer forms. D, a wingless summer form. E, a winged summer form

of the leaves or by the crowding of the aphids on them, winged forms appear, thus making possible a migration to fresh feeding grounds. Then, too, certain chemical substances, particularly salts of magnesium, added to the water or wet sand in which are growing cuttings of plants infested with aphids, will cause an increase of winged forms in the insects subsequently born. This does not happen if the plants are rooted, but it shows that a change in the food can have an effect on wing production.

Finally, it has recently been shown experimentally by Dr. A. Franklin Shull that winged and wingless conditions in the potato aphis may be produced artificially by a variation in the relative amount of alternating light and darkness the aphids receive during each twenty-four hours. Shortening the illumination period to twelve hours or less results in a marked increase in the number of winged forms born of wingless parents. Continuous darkness, however, produces few winged offspring. Maximum results perhaps are obtained with eight hours of light. The effect of decreased light appears from Doctor Shull's experiments to be directly operative on the young from thirty-four to sixteen hours before birth, and it is not to be attributed to any physiological effect on the plant on which the insects are feeding.

It is evident, therefore, that various unfavorable local conditions may give rise to winged individuals in a colony of wingless aphids, thus enabling representatives of the colony to migrate in the chance of finding a more suitable place for the continuance of their line. The regular production of spring and fall migrants is brought about possibly by the shorter periods of daylight in the earlier and later parts of the season.

The final chapter of the aphid story opens in the fall and, like all last chapters done according to the rules, it contains the sequel to the plot and brings everything out right in the end.

All through the spring and summer the aphid colonies have consisted exclusively of virgin females, winged and wingless, that give birth to virgin females in ever-increasing numbers. A prosperous, self-supporting feminist dominion appears to be established. When summer's warmth, however, gives way to the chills of autumn, when the food supply begins to fail, the birth rate slackens and falls off steadily, until extermination seems to threaten. By the end of September conditions have reached a desperate state. October arrives, and the surviving virgins give birth in forlorn hope to a brood that must be destined for the end. But now, it appears, another of those miraculous events that occur so frequently in the lives of insects has happened here, for the members of this new brood are seen at once to be quite different creatures from their parents. When they grow up, it develops that they constitute a sexual generation, composed of females and males! (Plate 2 A, B.)

Feminism is dethroned. The race is saved. The marriage instinct now is dominant, and if marital relations in this new generation are pretty loose, the time is October, and there is much to be accomplished before winter comes.

The sexual females differ from their virgin mothers and grandmothers in being of darker green color and in having a broadly pear-shaped body, widest near the end (Plate 2 A). The males (B) are much smaller than the females, their color is yellowish brown or brownish green, and they have long spiderlike legs on which they actively run about. Neither the males nor the females of the green apple aphis have wings. Soon the females begin to produce, not active young, but eggs (D). The eggs are deposited most anywhere along the apple twigs, in crevices where the bark is rough, and about the bases of the buds. The newly-laid eggs are yellowish or greenish (D), but they soon turn to green, then to dark green, and finally become deep black (E). There are not many of them, for each female lays only from one to a dozen; but it is these eggs that are to remain on the trees through the winter to produce the stem mothers of next spring, who will start another cycle of aphid lire repeating the history of that just closed.

The production of sexual forms in the fall in temperate climates seems to have some immediate connection with the lowered temperature, for in the tropics, it is said, the aphid succession continues indefinitely through parthenogenetic females, and in most tropical species sexual

Fig. 98. The rosy apple aphis, Anuraphis roseus, on apple

A, a cluster of infested and distorted leaves. B, an adult stem mother. C, young apples dwarfed and distorted by the feeding of the aphids

males and females are unknown. In the warmer regions of the West Coast of the United States, species that regularly produce males and females every fall in the East continue without a reversion to the sexual forms.

Of the other two species of apple aphids that infest the buds in the spring, one is known as the rosy apple aphis (Fig. 95 C). The name comes from the fact that the early summer individuals of this species have a waxy pink tint more or less spread over the ground color of green (Plate 3), though many of the adult stem mothers (Fig. 98 B) are of a deep purplish color. The early generations of the rosy aphis infest the leaves (Fig. 98 A, Plate 3 A) and the young fruit (Fig. 98 C, Plate 3 A), causing the former to curl up in tightly rolled spirals, and the latter to become dwarfed and distorted in form.

The stem mothers of the rosy aphis give birth parthenogenetically to a second generation of females which are mostly wingless like their mothers; but in the next generation many individuals have wings. Several more generations now rapidly follow, all females; in fact, as with the green aphis, no males are produced till late in the season. The winged forms, however, appear in increasing numbers, and by the first of July almost all the individuals born have wings. Heretofore, the species has remained on the apple trees, but now the winged ones are possessed with a desire for a change, a complete change both of scenery and of diet. They leave the apples, and when next discovered they are found to have established themselves in summer colonies on those common weeds known as plantains, and mostly on the narrow-leaved variety, the rib-grass, or English plantain (Fig. 99).

Fig. 99. The rosy apple aphis on narrow-leaved plantain in summer; above, a wingless summer form (enlarged)

As soon as the migrants land upon the plantains they give birth to offspring quite unlike themselves or any of the preceding generations. These individuals are of a yellowish-green color and nearly all of them are wingless (Fig. 99). So well do they disguise their species that entomologists were a long time in discovering their identity. Generations of wingless yellow females now follow upon the plantain. But a weed is no fit place for the storage of winter eggs, so, with the advent of fall, winged forms again appear in abundance, and these migrate back to the apples. The fall migrants, however, are of two varieties: one is simply a winged female like the earlier migrants that came to the plantain from the apple, but the other is a winged male (Fig. 100 A). Both forms go back to the apple trees, and there the females give birth to a generation of wingless sexual females (B), which, when mature, mate with the males and produce the winter eggs.

The third of the aphid species that infest the spring buds of the apple is known as the apple-grain aphis, so called because, being a migratory species like the rosy

Fig. 100. The winged male (A) and the wingless sexual female (B) of the rosy apple aphis

aphis, it spends the summer upon the leaves of grains and grasses. The eggs of the apple-grain aphis are usually the first to hatch in the spring, and the young aphids of this species (Fig. 95 A) are distinguished by their very dark green color, which gives them a blackish appearance when massed upon the buds. Later they spread to the older leaves and to the petals of the apple blossoms, but on the whole their damage to the apple trees is less than that of either of the other two species. The summer history of the apple-grain aphids is similar to that of the rosy aphis, excepting that they make their summer home on grains and grasses instead of on plantains. In the fall, the winged female migrants (Plate 4) come back to the apple and there give birth to wingless sexual females, which are later sought out by the winged males.

It would be impossible here even to enumerate the

PLATE 4

The apple-grain aphis (Rhopalosiphum Prunifoliae)
The winged form produced in the fall that migrates from the grain to the apple trees. (Enlarged 20 times)

Fig. 101. Some common aphids of the garden
A, winged form of the potato aphis, Illinoia solanifolii, one of the largest of the garden aphids. B, winged form of the peach aphis, Myzus persicae, which infests peach trees and various garden plants. C, wingless form of the peach aphis. D, wingless form of the melon aphis, Aphis gossypii. E, winged form of the melon aphis

many species of aphids that infest our common field and garden plants (Fig. 101) and cultivated shrubs and trees, to say nothing of those that inhabit the weeds, the wild shrubbery, and the forest trees. Almost every natural group of plants has its particular kind of aphid, and many of them are migratory species like the rosy and grain aphis of the apple. There are root-inhabiting species as well as those that live on the leaves and stems. The Phylloxera, that pest of vineyards in California and France, is a root aphid. Those cottony masses that often appear on the apple twigs in late summer mark the presence of the woolly aphis, the individuals of which exude a fleecy covering of white waxy threads from their backs. The woolly aphis is more common on the roots of apple trees, being especially a pest of nursery stock, but it migrates to both the twigs and the roots of the apple from the elm, which is the home of its winter eggs.

An underground aphid of particular interest is one that lives on the roots of corn. We have seen that all aphids are much sought after by ants because of the honey dew they excrete, a substance greatly relished and prized by the ants. It is said that some ants protect groups of aphids on twigs by building earthen sheds over them; but the corn-root aphis owes its very existence to the ants. A species of ant that makes its nests in cornfields runs tunnels from the underground chambers of the nests to the bases of nearby cornstalks. In the fall the ants gather the winter eggs of the aphids from the corn roots and take them into their nests where they are protected from freezing during the winter. Then in the spring the ants bring the eggs up from the storage cellars and place them on the roots of various early weeds. Here the stem mothers hatch and give rise to several spring generations; but, as the new corn begins to sprout, the ants transfer many of the aphids to the corn roots, where they live and multiply during the summer and, in the fall, give birth to the sexual males and females, which produce the winter eggs. The eggs are again collected by the ants and carried to safety for the winter into the depths of their underground abodes. All this the ants do for the aphids in exchange for the honey dew they receive from them. The ants have so domesticated these corn-root aphids that the aphids would perish without their care. The farmer, therefore, who would rid his cornfield of the aphid pest, proceeds with extermination measures against the ants.

The crowded aphid colonies exposed on stems and leaves naturally form the happy hunting grounds for a

Fig. 102. A common ladybird beetle, Coccinella novemnotata, that feeds on aphids. (Enlarged 5 times)
A, the larva. B, the adult beetle

host of predacious insects. Here are thousands of soft- bodied creatures, all herded together, and each tethered to one spot by the bristles of its beak thrust deep into the tissues of the plant—a pot-hunter's paradise, trulv. Consequently, the placid lives of the aphids have many interruptions, and vast numbers of the succulent creatures serve only as half-way stages in the food cycle of some other insect. The aphids have small powers of active defense. A pair of slender tubes, the cornicles, projecting from the rear end of the body, eject a sticky liquid which the aphids are said to smear on the faces of attacking insects; but the ruse at best probably does not give much protection. Parthenogenesis and large families are the principal policies by which the aphids insure their race against extinction.

The presence of "evil" in the world has always been a thorn for those who would preserve their faith in the idea of beneficence in nature. The irritation, however, is not

Fig. 103. The aphis-lion, feeding on an aphis held in its jaws

in the flesh but in a distorted growth of the mind, and consequently may be alleviated by a change of mental attitude. The thorn itself, however, is real and can not be explained away. Beneficence is not a part of the scheme by which plants and animals have attained through evolution their present conditions and relations. On the other hand, there are not good species and bad species; for every creature, including ourselves, is a thorn to some other, since each attacks a weaker that may contribute to its existence. There are many insects that destroy the aphids, but these are "enemies" of the aphids only in the sense that we are enemies of chickens and of cabbages, or of any other thing we kill for food or other purposes.

Recognizing, then, that evil, like everything else, is a matter of relativity and depends upon whose standpoint it is from which we take our view, it becomes only a pardonable bias in a writer if he views the subject from the standpoint of the heroes of his story. With this understanding we may note a few of the "enemies" of the aphids.

Everybody knows the "ladybirds," those little oval, hard-shelled beetles, usually of a dark red color with black spots on their rounded backs (Fig. 102 B). The female ladybirds, or better, lady-beetles, lay their orange-colored eggs in small groups stuck usually to the under surfaces of leaves (Fig. 132 B) and in the neighborhood of aphids. When the eggs hatch, they give forth, not ornate insects resembling lady-beetles, but blackish little beasts with thick bodies and six short legs. The young creatures at once seek out the aphids, for aphids are their natural food, and begin ruthlessly feeding upon them. As the young lady-beetles mature, they grow even uglier in form, some of them becoming conspicuously spiny, but their bodies are variegated with areas of brilliant color—red, blue, and yellow—the pattern differing according to the species. A common one is shown at A of Figure 102. When one of these miniature monsters becomes full-grown, it ceases its depredations on the aphid flocks, enters a period of quietude, and fixes the rear end of its body to a leaf by exuding a glue from the extremity of its abdomen. Then it sheds its skin, which shrinks down over the body and forms a spiny mat adhering to the leaf and supporting the former occupant by only the tip of the body (Fig. 132 E). With the shedding of the skin, the insect has changed from a larva to a pupa, and after a short time it will transform into a perfect lady-beetle like its father or mother.

Another little villian, a remarkably good imitation of a small dragon (Fig. 103), with long, curved, sicklelike jaws extending forward from the head, and a vicious tem

Fig. 104. The golden-eye, Chrysopa, the parent of the aphis-lion, and its eggs
A, the adult insect. B, a group of eggs supported on long threadlike stalks on the under surface of a leaf

perament to match, is also a common frequenter of the aphid colonies and levies a toll on the lives of the meek and helpless insects. This marauder is well named the aphis-lion. He is the larva of a gentle, harmless creature with large pale-green lacy wings and brilliant golden eyes (Fig. 104 A). The parent females show a remarkable prescience of the nature of their offspring, for they support their eggs on the tips of long threadlike stalks, usually attached to the under surfaces of leaves (B). The device seems to be a scheme for preventing the first of the greedy brood that will hatch from devouring its own brothers and sisters still in their eggs.

Fig. 150. A larva of a syrphus fly feeding on aphids

Wherever the aphids are crowded there is almost sure to be seen crawling among them soft grayish or green wormlike creatures, mostly less than a quarter of an inch in length. The body is legless and tapers to the forward end, which has no distinct head but from which is protruded and retracted a pair of strong, dark hooks. Watch one of these things as it creeps upon an unsuspecting aphid; with a quick movement of the outstretched forward end of the body it makes a swing at the fated insect, grabs it with the extended hooks, swings it aloft kicking and struggling, and relentlessly sucks the juices from its body (Fig. 105). Then with a toss it flings the shrunken skin aside, and repeats the attack on another aphid. This heartless blood-sucker is a maggot, the larva of a fly (Fig. 106) belonging to a family called the Syrphidae. The adult files of this family are entirely harmless, though

Fig. 106. Two common species of syrphus flies whose larvae feed on aphids.
(Enlarged about 3¼ times)
A, Allograpta obliqua. B, Syrphus americana

some of them look like bees, but the females of those species whose maggots feed on aphids know the habits of their offspring and place their eggs on the leaves where aphids are feeding. One of them may be seen hovering near a well-infested leaf. Suddenly she darts toward the leaf and then as quickly is off again; but in the moment of passing, an egg has been stuck to the surface right in the midst of the feeding insects. Here it hatches where the young maggot will find its prey close at hand.

In addition to these predacious creatures that openly and honestly attack their victims and eat them alive, the aphids have other enemies with more insidious methods of procedure. If you look over the aphid-infested leaves

Fig. 107. A dead potato aphis that has contained a parasite, which when adult escaped through the door cut in the back of the aphis

on almost any plant, you will most likely note here and there a much swollen aphid of a brownish color. Closer examination reveals that such individuals are dead, and many of them have a large round hole in the back, perhaps with a lid standing up from one end like a trap door (Fig. 107). These aphids have not died natural deaths; each has been made the involuntary host of another insect that converted its body into a temporary

Fig. 108. Aphidius, a common small wasplike parasite of aphids

home. The guest that so ravishes its protector is the grub of a small wasplike insect (Fig. 108) with a long, sharp ovipositor by means of which it thrusts an egg into the body of a living aphid

Fig. 109. A female Aphidius inserting an egg into the body of a living aphis, where the egg hatches; the larva grows to maturity by feeding in the tissues of the aphis. (From Webster)

(Fig. 109). Here the egg hatches and the young grub feeds on the juices of the aphis until it is itself full-grown, by which time the aphid is exhausted and dead. Then the grub slits open the lower wall of the hollow corpse and spins a web between the lips of the opening and against the surface of the leaf below, which attaches the aphid shell to the support. Thus secured, the grub proceeds to give its gruesome chamber a lining of silk web; which done, it lies down to rest and soon changes to a pupa. After a short time it again transforms, this time into the adult of its species, and the latter cuts with its jaws the hole in the back of the aphid and emerges.

In other cases, the dead aphid does not rest flat on the leaf but is elevated on a small mound (Fig. 110 A). Such victims have been inhabited by the grub of a related species, which, when full-grown, spins a flat cocoon beneath the dead body of its host, and in this inclosure undergoes its transformation. The adult insect then cuts a door in the side of the cocoon (B), through which it makes its exit.

Fig. 110. Aphids parasitized by a parasite that makes a cocoon beneath the body of the aphis, where it changes to a pupa and, when adult, emerges through a door cut in the side of the cocoon.

Insects that usurp the bodies of other insects for their own purposes are called parasites. Parasites are the worst enemies that insects have to contend against; but really they do not contend against them in most cases, except in the way characteristic of insects, which is to insure themselves against extermination by the number of their offspring. The aphid colonies are often, however, greatly depleted during a season favorable to the predacious and parasitic insects that attack them; but no species is ever annihilated by its enemies, for this would mean starvation to the next year's brood of the latter. The laws of compensation usually maintain a balance in nature between the procreative and the destructive forces.

The insect parasites and predators of other insects in general comprise a class of insects that are most beneficial to us by reason of their large-scale destruction of species injurious to our crops. But, unfortunately, parasites as a class do not respect our classification of other creatures into harmful and useful species. Even as some predator is stalking its prey, another insect is likely to be shadowing it, awaiting the chance to inject into its body the egg which will mean finally death to the destroyer. Immature insects are often found in a sluggish or half-alive condition, and an internal examination of their bodies usually reveals that they are occupied by one or more parasitic larvae. A larva of any of the lady-beetles, for example, is frequently seen attached to a leaf for pupation (Fig. 111), which, instead of transforming to a pupa, remains inert and soon becomes a lifeless form, though still adhering to the leaf and bent in the attitude that the pupa would assume. In a short time there issues through the dried skin a parasite, giving evidence of the fate that has befallen the unfortunate larva; even if the usurper is not seen, the exit hole in the larval skin bears witness to his former occupancy and escape.

And the parasites themselves, do they lead unmolested lives? Are they the final arbiters of life and death in the insect world? if you are fortunate sometime while studying aphids out-of-doors, you may see a tiny black mite, no bigger than the smallest gnat, hovering about an infested plant or darting uncertainly from one leaf to another, with the air of searching for something but not knowing just where to look. You would probably suspect the intruder of being a parasite seeking a chance to place an egg in the body of an aphid; but here she hovers over a group of fat lice without selecting a victim, then perhaps alights and runs about on the leaf nervously and intensely eager, still finding nothing to her choice. Her senses must be dull, indeed, if it is aphids that she wants. Do not lose sight of her, however, for her attitude has changed; now she certainly has her eye upon something that holds her attention, but the object is nothing other than one of those swollen parasitized aphids. Yet she excitedly runs up to it, feels it, grasps it, mounts upon it, examines it all over. Evidently she is satisfied. She dismounts, turns about, backs her abdomen against the inflated mummy; now out comes the swordlike ovipositor, and with a thrust it is sunken into the already parasitized aphid. Two minutes later her business is ended, the ovipositor is withdrawn, once more sheathed, and the insect is off and away.

Fig. 111. A parasitized larva of a ladybird beetle, and one of the parasites The larva of the beetle has attached itself to a leaf preparatory to pupation, but has not changed to a pupa because of the parasites within it. Above, one of the parasites, which escaped from the beetle larva through a hole it cut in the skin of the latter

This tiny creature is a hyperparasite, which is to say, a parasite of a parasite. In the act just witnessed she, too, has thrust an egg into the aphid, but the grub that will hatch from it will devour the parasitic occupant that is already in possession of the aphid's skin. There are also parasites of hyperparasites, but the series does not go on "ad infinitum" as in the old rhyme, for the limitation of size must intervene.