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Insects (No other class of the animal)

No other class of the animal kingdom is of so much interest and importance to those engaged in gardening, or in farming, as are Insects, which, in in their countless forms and modes of life, force themselves upon the notice of even the least observant. The destruction they cause to field and garden produce is great, and at times excessive; and these attacks can be met successfully only when means are used that are based on a knowledge of the habits of the injurious species. But while Insects are more conspicuous because of the injuries they inflict on our fields and gardens, we must not forget that many kinds are beneficial to plants, either by the services they render in conveying pollen from flower to flower, and thereby securing the production of healthy offspring to the plants; or by feeding on, and thus destroying, the injurious kinds of Insects. Were it not for the aid rendered to us by the parasitic species, we must often be helpless to check the ravages of the others, which, despite this aid, are often but too wide-spread and serious.

Space will not here permit of more than a very brief sketch of some of the leading points in the structure and life-history of the class and of its leading orders, with a mere indication, in the most general terms, of the more conspicuous damage done by them, and of the means of remedy generally applicable; but fuller information will be found plentifully scattered through this work under the various headings referred to below. The word "Insect" literally means any animal which has the body so divided as to seem cut into successive parts, usually resembling rings of hard substance, connected by soft skin. The name has been used to include all the animals with bodies resembling a row of joints, even Worms having been at one time included among Insects, along with Spiders, Mites, Crabs, Woodlice, and Centipedes. At the present time, the name is confined to a considerably smaller group of animals--the true Insects--though some authorities still include with these the other groups named above, except Worms. In this restricted sense, the class of Insects is composed of animals that possess a jointed body made up of a number (twenty or twenty-one) of rings of horny substance (chitine), connected by skin, so united as to form three great divisions in the body, viz., the head, the thorax, and the abdomen; that have antennae, or feelers, on the head, and three pairs of horny jointed legs, and usually two pairs of wings, on the thorax; that breathe by tubes (tracheae) all through the body and limbs, which tubes open on the surface of the body by small holes (spiracles); and that, in course of growth, pass through a succession of changes (metamorphoses), beginning with the egg, and going on through the stages of larva and pupa (chrysalis), before assuming the perfect condition (imago), in which alone they present all the characters enumerated above. This series of changes may be of two kinds. In one, the larva, on emerging from the egg, resembles the mature Insect in form, and differs from it only in size, and in the entire absence of wings; while the pupa differs from the mature Insect only in the wings, though indicated, being small and rudimentary. Both larva and pupa are also sexually immature. This kind of metamorphosis is said to be incomplete. Complete metamorphosis differs from this in the larva being quite unlike the mature Insect. Very generally, it resembles a worm in its form. It may have a distinct head, and the rings of the body, though like one another, may differ in the three immediately behind the head each bearing a pair of horny jointed legs, while those of the hinder part of the body may bear skinny suckers (prolegs)--the number varies considerably, and they may be quite absent. Larvae of this type are called Caterpillars, and are met with among the Butterflies, Moths, and Sawflies. Another group of Insects, in the larval state, are footless creatures, frequently scarcely showing even a trace of a head. Such larvae are familiar to everyone under the name of maggots or grubs, e.g. those of Bees, Flies, &c. The pupae of Insects that undergo a complete metamorphosis are not able to move about or to feed, but remain helpless for a longer or shorter period, while changes are going on within, which result in the development of the perfect insect (imago), showing all the distinctive characters already set forth. These quiescent pupae differ much from the larvae as they show the future limbs (antennae, legs, and wings) of the imago, though these parts are still useless, and of very small size. The Butterflies and Moths, while pupae, have the whole body enveloped in a hard crust, which binds down all the parts immovably to the body, leaving only a slight power of movement in the hinder rings. When the pupa is helpless, its safety is generally provided for by the larva forming a cell or cocoon of silk, earth, or other material, compacted with a cement from its mouth, in which the pupa lies concealed till the imago breaks out from it, and crawls to the outer air, with wings at first crumpled and useless, but soon expanding and becoming firm to carry it in flight through the air. Whatever the degree or kind of metamorphosis undergone by it, the Insect, in its progress from the egg to maturity, grows frequently with enormous rapidity. The outer coat of the body is too rigid to allow of the corresponding extension in it; but the difficulty is met by the larva several times (four times or oftener) throwing off its skin. The old skin separates from the body of the larva, it splits along the back in front, and the larva wriggles its way out, frequently with a skin of a very different colour from that in which it had previously shown itself. In like manner, the skin of the pupa bursts to set free the imago. The sudden and mysterious appearance of certain kinds of Insects at one time led to the belief that they must be produced by spontaneous generation from dead and decaying substances; but the progress of accurate researches into their life-histories has proved conclusively that they always originate from parents, though at times, in the case of certain Insects, e.g., several of the true Gall-flies of the Oak-tree, the parents and the immediate offspring may differ considerably in appearance. The females produce eggs, except in a few groups, such as the Aphides, in which the young Insects may be brought forth, not sexually by eggs, but by a process of budding. In the case of a few other Insects, the eggs are hatched while still within the body of the mother, which thus gives birth to living young or larvae. The fact that Insects are always produced from parents, renders it of the utmost importance, when dealing with the injurious species, to become familiar with the habits of the females as well as of the larvae, which latter are usually the destructive agents, and are therefore the more generally observed by gardeners and agriculturists. Such a knowledge frequently enables us to devise means of warding off the damage that would otherwise be unavoidable, and of assailing the destroyers in the most effective way.

The following details of structure have reference only to mature Insects: Mouth. In its essential structure, the mouth consists of certain parts, six in number, though some of them bear appendages. These parts may undergo great modification, and may be adapted to very different uses, so as to be hardly recognisable when compared with the mouth of one of the more typical forms. The chief modifications will be treated of under the various groups of Insects; it is here only necessary to describe the parts of the mouth of a Beetle, selecting this as a type because of all the parts of a fully-developed mouth being well seen in Beetles. In them, the mouth is formed by an upper lip (labrum), two pairs of jaws working horizontally like the blades of scissors (called mandibles and maxillae, or upper and lower jaws respectively), and a lower lip (labium). The mandibles are specially adapted for cutting the food to be taken into the mouth. The maxillae are not so strong, and often bear tufts of hair to serve as brushes. Each also bears a jointed body (palpus), which seems to serve as a sense-organ. The lower lip bears a pair of similar jointed bodies (labial palpi).

Limbs. The middle division of the body (thorax) bears the limbs, viz., three pairs of jointed legs on the lower surface or breast, and two pairs of wings, attached to the upper surface of the two hinder of the three rings of which, closely consolidated, the thorax is made up. The legs are generally present, though, in some Insects, one or more pairs may be ill-developed, or even entirely absent. They vary in relative size, and in the number of parts of which they are made up; but they are of less importance in giving characters for the orders than are the wings. The latter organs, in their typical condition, e.g., in Hymenoptera or in Lepidoptera, are two pairs of broad membranes, supported upon nervures traversed by breathing-tubes (tracheae). The upper or front pair almost always exceed the lower or hind wings in size and in complexity of neuration, but resemble them as regards texture and general appearance. In several groups, however, this typical structure is departed from, as will be more fully stated below. Not a few Insects either have the wings small and utterly inefficient (as the female Winter Moth, or altogether absent (as Fleas and many other parasitic Insects). Periods of Destructive Activity. As a general rule, the larvae, during their rapid growth, are far more hurtful to vegetation than are the perfect Insects; in fact, certain of the groups of Insects are unable, in the perfect state, to feed upon plants, save by sucking up the nectar or honey contained in the flowers, e.g., the whole group of Lepidoptera. Yet these same Insects, in the larval stage, were once provided with strong jaws, well fitted to cut their food, and then probably fed voraciously upon their food-plants. But among the groups provided with a mouth suited for chewing (e.g., Beetles and Orthoptera), or for puncturing the tissues of plants and sucking the juices (Hemiptera, particularly Aphides), the perfect Insects may be almost as destructive as the larvae; indeed, some kinds are not injurious to man except in the mature condition.

General methods of prevention and of remedy against damage from Insects, may be treated of here; but details as to these practices must be sought under the special headings. The methods employed to prevent attacks must depend on the habits and modes of life of the Insects to be dreaded. Of course, the destruction of the creatures, either as larvae, pupae, or mature Insects, before they have laid their eggs, is the most certain means. This may be effected either by direct efforts, such as hand-picking, or applications of poisonous solutions or gases to them; or else-and probably with greater, if less apparent, success-by making use of the natural means of checking their undue increase. Among the most efficient of these natural means are birds and parasitic Insects, both of which destroy vast multitudes of the more hurtful kinds in all their stages. Much has been written, and earnest have been the disputes that have been waged, regarding the relation of birds to agriculture and to gardening. While some writers have represented them as frequently most destructive to the crops and to fruit-trees, others have as strenuously upheld their great value as natural guardians of these crops and fruit-trees from the ravages of swarms of Insect foes. In these, as in so many other disputes, the truth probably lies between the extremes: but, while admitting, as we must admit, that birds are, at certain times of the year, more hurtful than beneficial to farmers and gardeners, yet the danger from them at these times may be comparatively easily guarded against; while the benefits conferred by them, during the rest of the year, are so great as far to outweigh any damage done by them. It must be remembered, moreover, that injury is done only by those birds that feed on fruits or seeds, such as blackbirds, sparrows, and many finches; or on roots or tubers, such as rooks; and that even these birds almost all feed largely on Insects also. On the other hand, many species of slender-billed birds may be said to live exclusively on Insects throughout the year, or such part of it as they spend with us. Some birds, like the bullfinch, are in the habit of pulling off the young buds of the fruit-bearing bushes and trees; but this seems to be done in the search for larvae hidden in the buds, and which, if left in them, would destroy them in any case. In like manner, the apparent injury done to trees by woodpeckers boring into the trunks, is not really such, as the holes are bored by the bird to reach and to extract the larvae of Insects hidden away in tunnels in the wood, or between the wood and the bark, and which, if left there, would have done no less injury; while, if they had reached maturity, they would have reproduced their kind, to the further detriment of the trees.

Even more efficient than the birds, in reducing the numbers of the injurious kinds of Insects, are the parasitic species of Insects; though, from their small size and unobtrusive habits, they are readily overlooked, and the work done by them undervalued. Among these, some devour the Insects or such out their juices as food. As examples of these may be noted the Ground Beetles, and the larvae of the Ladybird Beetles and of the Lacewing Flies, which devour the Green Flies, or Aphides, in myriads. Other insects (e.g., some Solitary Wasps) carry off small Caterpillars, &c., to serve as food for their offspring. But far more important than even these are the parasites that deposit their eggs upon or in the body of some Insect, generally a larva, there to disclose the grubs. The latter live in the interior of their host, eating the fat, but leaving untouched the vital organs, till at last, when growth has been completed by the parasites, they either form cocoons inside its body, or else burrow out through the skin, to become pupae in some concealment, where, though helpless, they may remain safe. True parasites of this latter sort belong, with rare exceptions, to the Diptera or Two-winged Flies (Tachina and allied genera), or to certain divisions of the Hymenoptera. In this latter order, some groups, with very numerous kinds in each, are entirely, or almost entirely, confined to parasitism and many species in the other groups of the order are also parasites. In fact, it may almost be said that there are few Insects which are not preyed on by one or more (and frequently by many) of these minute parasites. It is hardly in our power to do anything directly to increase the numbers of these allies; but it is well to be able to recognise their general appearance, and to refrain from killing them.

In many cases, however, we must not trust to the efforts of birds and Insects to limit the numbers of our Insect foes. When their attacks are severe, it is necessary to remove or to destroy them more rapidly than can be done by natural agencies; and, though the methods employed must be varied to suit the special circumstances that call for them, yet the same methods are largely applicable for the destruction of many kinds of Insects. A few of the more generally useful may be mentioned here. Tobacco smoke is very fatal to many of the hurtful Insects, where these attack plants under cover, e.g., in greenhouses. It is especially efficacious against Aphides. Either strong coarse tobacco, or the paper in which it has been wrapped, is burned in the place to be freed from the foe; the doors are all kept closed during the operation, and for some time afterwards. Infusions of tobacco, of Hellebore, or other insecticides, are sometimes prepared, and are scattered over the plants by means of a watering pot or syringe. These applications are usually very fatal to larvae feeding on such parts of plants as allow of the solution being properly applied to them, and they do not injure the host. Scale, and other Insects that lie closely adherent to leaves or branches, are seldom much affected by tobacco smoke, and they are best removed by washing the parts with water, or with solutions of soft soap or of poisonous substances; and the efficacy of the applications is increased by using a brush to put them on with. Several kinds of injurious larvae are in the habit of living in groups, in or under webs. These are easily cleared away with the webs; and the whole should be destroyed, by burning or otherwise. Larvae living in the interior of branches, or in galls, cannot be reached without removing the injured structures; and this is also usually the case with root-feeders. When a plant is withering without any apparent cause, the removal of the earth from its root will, at times, disclose the hidden larva that has been feeding there. Sometimes, also, larvae feed at night on the leaves, and during the day hide themselves in the earth. Hence, the depredators can be detected only at night, by the use of a lantern. Many Insect larvae roll up a leaf, or spin together two or three leaves, so as to form a protection for themselves against all means of destruction save hand-picking; but, fortunately, such larvae are seldom fatal in their attacks; though, frequently, the plants become very unsightly under them. Trees and shrubs are attacked in this way more often than are herbaceous plants. A severe shaking will often cause a large number of the larvae to drop from their tubes, and to hang suspended in the air by silken threads till the danger is past, when they remount by their threads. If a sheet is spread below the tree, they may be shaken on to it, and then readily gathered to be destroyed. The mere shaking of the plant is not enough. Many larvae (e.g., those of Gooseberry Sawflies, of Winter Moth, &c., fall to the ground when full fed, there to burrow in order to form cocoons, and to become pupae, within a very little distance of the surface of the ground. Soot, gas-lime, and other nauseous materials are often spread on the soil around the stems of trees and shrubs, to prevent the larvae from burrowing in the immediate vicinity of the plants, and to destroy them by the poisonous properties of the substances. The pupae may be greatly lessened in number by paring off and burning an inch or two of the surface soil in the autumn or winter. Opening the soil with a hoe or a rake is also useful, as it destroys some pupae at once, and exposes others to the chance of being eaten by birds, and to unfavourable conditions during the winter. The females of some of the more hurtful Insects are wingless, or have wings so small as to be of little or no use they must, therefore, crawl up the plants on which they lay their eggs; and their path may be barred by placing a ring of any sticky substance on the soil around the base of the stem, or around the stem itself. Tar, or mixtures of tar with materials to prevent its drying quickly, have been used with success to form such barriers. Other methods of treatment, suitable to particular cases, will be found referred to under the names of the various insects. See also Insecticides.

The class Insecta has been divided into certain great groups, called "orders," by means of characters taken mostly from the structure of the mouth and of the wings, and from the kind of metamorphosis they pass through in their growth. These orders are well distinguished from one another, and there is usually not much difficulty in determining the group to which any Insect belongs. Some orders are of far greater importance to gardeners than are others; and to them the following remarks are confined. These orders are as follows: Coleoptera, or Beetles. These have the mouth formed for biting, with all the parts well developed; the body is usually incased in a hard crust; the front wings are modified to form hard coverings (elytra) extending over the hinder part of the body, and serving to protect the hind wings, which remain membranous for flight. The metamorphosis is complete, i.e., the larva is very unlike the adult Insect, and the pupa is helpless. At times, the larvae of Beetles are hurtful (e.g., those of the Click Beetle and of the Colorado Beetle); but the mature Insects are more often to be dreaded. See also Cockchafers, Ladybirds, and Turnip Fly.

Several kinds of Beetles are useful to gardeners, since they feed on hurtful Insects, either as larvae, or in the perfect state. Of these useful forms we may mention the following: The Ground Beetles feed mostly on Insects, though some of them are partly vegetarians, and Harpalus ruficornis has been found eating Strawberries in large numbers. The Tiger Beetles, and the Devil's Coach Horses or Rove Beetles, feed largely upon decaying matter; but many kinds are of much assistance by destroying noxious insects. The Glow-worm (Lampyris noctiluca), in the larval state, feeds on snails, mostly of the genus Helix, following them into their shells to devour them. It removes the slime encountered in this mode of life, by means of a kind of brush specially suited to its needs. The larvae of the Ladybirds are very useful because of the number of Green Flies eaten by them. The modes in which Beetles inflict injury on garden and field produce, are very various. Frequently, the roots are attacked, and much injured, or entirely destroyed, usually by larvae, e.g., of the Cockchafer. The stems are injured, chiefly by those kinds (Bark Beetles) that bore between the bark and the wood, separating the bark, which soon dies. A few make galls, on roots or stems, e.g., Cabbage Weevil, or in seed vessels. Others attack the leaves, either while larvae (Lily Beetle), or as Beetles (Turnip Fly); while others live as larvae in the seed vessels, eating out the contents of the seeds, and thus rendering the crops a failure while in the soil, or after they have been harvested, e.g., Bean Beetle. Orthoptera. In this order are included Insects with mouths fitted for biting, wings net-veined, front wings like parchment in thickness, long and narrow, serving to protect the large membranous hind wings; metamorphosis incomplete, the larvae resembling the parents except in size, and in having no trace of wings. This order includes Locusts, Crickets, Grasshoppers, Cockroaches, and Earwigs; though some place the last-named Insects in a separate order, called Euplexoptera, because of the very neat folding of the hind wings below the front ones. The Orthoptera are destructive during every stage after leaving the egg, and in warm countries they do excessive damage at times. In Britain, they cannot be regarded as of great consequence; though Cockroaches may gnaw the greenhouse plants, and Earwigs are rather hurtful to florists' flowers, and require to be kept in check.(see Cockroaches, Crickets, and Earwig). With the Orthoptera may also be classed the genus Thrips-a genus of very small Insects, which live in flowers, and gnaw the surface of the petals (see Thrips).

Neuroptera are at once far less numerous in species and in individuals, and practically less important than are the orders already discussed. None of the Insects in it can be said to be hurtful to plants. On the other hand, they are frequently of much service, since several of them feed on Insects, and destroy large numbers of injurious kinds. The order is characterised by the possession of four wings, all alike membranous, and supported on a complex network of nervures; a mouth fitted for biting. Certain of the sub-orders undergo only an incomplete metamorphosis, while in others the metamorphosis is complete. The larvae are provided with six jointed legs. Neuroptera are mostly carnivorous, alike in the larval state and when mature. Among the better-known forms may be noted Dragon Flies, Hemerobius, and Lacewing Flies.

Hymenoptera have the wings all membranous and naked, and supported on branching nervures, the hind wings being the smaller; the wings bear no scales. The mouth has jaws for biting; but in some of the sub-divisions of the order, certain parts of the mouth are modified to serve for licking up honey from flowers (in Bees, Wasps, &c.). They all pass through a complete metamorphosis. The larvae vary much in form, often being like maggots (Bees, &c.), while the Sawflies have larvae not unlike the caterpillars of Moths. Sawflies are very injurious to plants, many of them being hurtful to garden and field produce (Turnip Sawfly, Gooseberry Sawflies, &c.). Some of the Sawflies make true galls on Willows; and many of the Cynipidae, or Gall-flies, make galls on Oak, Maple, Roses, &c. Most of the Hymenoptera are of much use in limiting the number of injurious Insects; and among these useful species the Ichneumons hold the first place. In the mature state, none of the Hymenoptera can be said to be really injurious to plants; while many are of great value in conveying pollen from flower to flower. However, Humble Bees are found at time to injure the flowers, by boring through the tube of the corolla to gain readier access to the honey contained therein. See also Ants, Galls, Honey Bee, Humble Bee, Ichneumon Flies, and Sawflies. Lepidoptera have the wings usually large, membranous, and covered all over with small scales; the mouth is of no use for biting but has the lower jaws (maxillae) prolonged, so as, when placed together, to form a tube, adapted for sucking honey from flowers. The metamorphosis is complete. The larvae are of the form known as Caterpillars; and, apart from points of minor importance, vary chiefly in the number of sucker feet on the hinder rings of the body. This order includes the Butterflies and Moths. They are not injurious in their mature condition; but all the larvae feed on plants, and many of them are very hurtful in gardens. See also Hybernia, Tortrix, Turnip Moth, Winter Moth, and others referred to above.

Diptera, or Two-winged Flies, have only one pair of wings, which are membranous and naked, with comparatively few nervures. The hind wings are replaced by very small stalked knobs (halters or poisers). The mouth varies, being in some suited for sucking, in others for biting; in many, the jaws are fitted to pierce the skin of animals, and thereafter to form suckers for drawing out the blood. The metamorphosis is complete. The larvae are maggots or footless grubs. They vary exceedingly in mode of life, many of them living in the interior of roots, stems, or leaves of plants; others form galls on plants; others feed on decaying matters; while yet others live as parasites in the bodies of animals, especially in Insects. Others (Syrphidoe), as larvae, feed on the Aphides, and aid in reducing their numbers. The larvae of the Crane Flies are but too well known to farmers under the names of Grubs and Leather Jackets. The Gnats live in water during their larval condition; the pupae are very generally oval brown bodies. The Diptera are seldom of large size. See also Crane Fly, Onion Fly, and Syrphus.

Hemiptera have the mouth in the form of a beak, turned down so as to lie against the breast when not in use, but capable of being driven into their food when desired. This beak usually consists of a sheath, in which lie four bristle-like pieces, the whole serving for a sucker to draw in the juices on which they feed. The metamorphosis is incomplete, except in the male Scale insects. The wings are different in the two great sub-orders, and a great many of the Insects have no trace of wings. The sub-orders are: Heteroptera, or Plant Bugs. The hind wings of these are membranous, and hidden under the front ones. The latter have the half nearer the body leathery, that farther from the body membranous, giving the appearance of the wings being in halves, whence the name Hemiptera, or half-winged insects. The name Heteroptera refers to the dissimilar appearance of the two halves. In this sub-order, many of the species are more or less parasitic; while among those that feed on plants, there are few that can be said to be conspicuously hurtful.

Homoptera have the wings all membranous and naked, with few supporting nervures, and often resemble small Hymenoptera in general aspect. A great many species are wingless, or, at least, have wingless individuals as well as winged. The name Homoptera refers to the front wings being alike throughout. The insects are mostly very small, but include many injurious forms. See also Aphides, Frog Hopper, Grape Louse, and Scale Insects.