An Introduction to Entomology: Vol. 2 / or Elements of the Natural History of the Insects

by William Kirby; William Spence

If insects can boast of enjoying a greater variety of food than many other tribes of animals, this advantage seems at first sight more than counterbalanced in our climates, by the temporary nature of their supply. The graminivorous quadrupeds, with few exceptions, however scanty their bill of fare, and their carnivorous brethren, as well as the whole race of birds and fishes, can at all seasons satisfy, in greater or less abundance, their demand for food. But to the great majority of insects, the earth for nearly one half of the year is a barren desert, affording no appropriate nutriment. As soon as winter has stripped the vegetable world of its foliage, the vast hosts of insects that feed on the leaves of plants must necessarily fast until the return of spring: and even the carnivorous tribes, such as the predaceous beetles, parasitic Hymenoptera, Sphecina, &c. would at that period of the year in vain look for their accustomed prey.

How is this difficulty provided for? In what mode has the Universal Parent secured an uninterrupted succession of generations in a class of animals for the most part doomed to a six months' deprivation of the food which they ordinarily devour with such voracity? By a beautiful series of provisions founded on the faculty, common also to some of the larger animals, of passing the winter in a state of torpor—by ordaining that the insect shall live through that period, either in an incomplete state of its existence when its organs of nutrition are undeveloped, or, if the active epoch of its life has commenced, that it shall seek out appropriate hybernacula, or winter quarters, and in them fall into a profound sleep, during which a supply of food is equally unnecessary.

In two of the four states of existence common to insects, in which different tribes pass the winter, namely, the egg and the pupa state, the organs for taking food (except in some cases in the latter) are not developed, and consequently the animal is incapable of eating. The existence of insects in these states during the winter, differs from their existence in the same form in summer only in the greater length of its term. In both seasons food is alike unnecessary, so that their hybernation in these circumstances has little or nothing analogous to that of larger animals. With this, however, strictly accords their hybernation in the larva and imago states, in which their abstinence from food is solely owing to the torpor that pervades them, and the consequent non-expenditure of the vital powers.—I shall attend to the peculiarities of their hybernation in each of these states in the order just laid down; premising that we have yet much to learn on this subject, no observations having been instituted respecting the state in which multitudes of insects pass the winter.

It is probable that some insects of almost every order hybernate in the egg state: though that these must be comparatively few in number, seems proved from two considerations: first, That the majority of insects assume the imago, and deposit their eggs in the summer and early part of autumn, when the heat suffices to hatch them in a short period: and secondly, That the eggs of a very large proportion of insects require for their due exclusion and the nutriment of the larvæ springing from them, conditions only to be fulfilled in summer, as all those which are laid in young fruits and seeds; in the interior and galls of leaves; in insects that exist only in summer, &c. &c. The insects which pass the winter in the egg state are chiefly such as have several broods in the course of the year, the females of the last of which lay eggs that, requiring more heat for their development than then exists, necessarily remain dormant until the return of spring.

The situation in which the female insect places her eggs in order to their remaining there through the winter, is always admirably adapted to the degree of cold which they are capable of sustaining; and to the ensuring a due supply of food for the nascent larvæ. Thus, with the former view, Acrida verrucivora and many other insects whose eggs are of a tender consistence, deposit them deep in the earth out of the reach of frost; and with the latter, Trichoda Neustria, Lasiocampa castrensis, Hypogymna dispar, and some other moths, departing from the ordinary instinct of their congeners, which teaches them to place their eggs upon the leaves of plants, fix theirs to the stem and branches only. That this variation of procedure has reference to the hybernation of the eggs of these particular species, is abundantly obvious. Insects whose eggs are to be hatched in summer, usually fix them slightly to the leaves upon which the larvæ are to feed. But it is evident that, were this plan to be adopted by those whose eggs remain through the winter, their progeny might be blown away along with the leaf to which they are attached, far from their destined food. These, therefore, choose a more stable support, and carefully fasten them, as has just been observed, either to the trunk or branches of the tree, whose young leaves in spring are to be the food of the excluded larvæ. The latter plan is followed by the female of Trichoda Neustria, which curiously gums her eggs in bracelets round the twigs of the hawthorn, &c. But another provision is demanded. Were these eggs of the usual delicate consistence, and to be attached with the ordinary slight gluten, they would have a poor chance of surviving the storms of rain and snow and hail to which for six or eight months they are exposed. They are therefore covered with a shell much more hard and thick than common; packed as closely as possible to each other; and the interstices are filled up with a tenacious gum, which soon hardens the whole into a solid mass almost capable of resisting a penknife. Thus secured, they defy the elements and brave the blasts of winter uninjured.—The female of Hypogymna dispar, whose eggs have a more tender shell, glues them in an oval mass to the stem of a tree (whence the German gardeners call the larvæ Stamm-raupe), and then covers them with a warm non-conducting coat of hairs plucked from her own body, equally impervious to cold and wet.

Another of those beautiful relations between objects at first sight apparently unconnected, which at every step reward the votaries of Entomology, is afforded by the coincidence between the period of the hatching in spring of eggs deposited before winter, and of the leafing of the trees upon which they have been fixed, and on whose foliage the larvæ are to feed: which two events, requiring exactly the same temperature, are always simultaneous. Of this fact I have had a striking exemplification the last spring (1816). On the 20th of February, observing the twigs of the birches in the Hull Botanic Garden to be thickly set, especially about the buds, with minute oval black eggs of some insect with which I was unacquainted, I brought home a small branch and set it in a jar of water in my study, in which is a fire daily, to watch their exclusion. On the 28th of March I observed that a numerous brood of Aphides (not A. Betulæ, as the wings were without the dark bands of that species) had been hatched from them, and that two or three of the lower buds had expanded into leaves, upon the sap of which they were greedily feasting. This was full a month before either a leaf of the birch appeared, or the egg of an Aphis was disclosed in the open air.—To view the relation of which I am speaking with due admiration, you must bear in mind the extremely different periods at which many trees acquire their leaves, and the consequent difference demanded in the constitution of the eggs which hybernate upon dissimilar species, to ensure their exclusion, though acted upon by the same temperature, earlier or later, according to the early or late foliation of these species. There is no visible difference between the conformation of the eggs of the Aphis of the birch and those of the Aphis of the ash; yet in the same exposure those of the former shall be hatched, simultaneously with the expansion of the leaves, nearly a month earlier than those of the latter: thus demonstrably proving that the hybernation of these eggs is not accidental, but has been specially ordained by the Author of nature, who has conferred on those of each species a peculiar and appropriate organization.

A much greater number of insects pass the winter in the pupa than in the egg state; probably nine-tenths of the extensive order Lepidoptera, many in Hymenoptera, and several in other orders. In placing these pupæ in security from the too great cold of winter and the attacks of enemies, the larvæ from which they are to be metamorphosed exhibit an anxiety and ingenuity evidently imparted to them for this express design. A few are suspended without any covering, though usually in a sheltered situation. But by far the larger number are concealed under leaves, in the crevices or in the trunk of trees, &c., or inclosed in cocoons of silk or other materials which will be described to you in a subsequent letter, and often buried deep under ground out of the reach of frost.—One reason why so many lepidopterous insects pass the winter as pupæ, has been plausibly assigned by Rösel, in remarking that this is the case with all the numerous species which feed on annual plants. As these have no local habitation, dying one year and springing up from seed in another quarter the next, it is obvious that eggs deposited upon them in autumn would have no chance of escaping destruction; and that even if the larvæ were to be hatched before winter, and to hybernate in that state, they would have no certainty of being in the neighbourhood of their appropriate food the next spring. By wintering in the pupa state, these accidents are effectually provided against. The perfect insect is not ready to break forth until the food of the young, which are to proceed from its eggs, is sprung up.

To the insects which hybernate in the larva state, of course belong, in the first place, all those which exist under that form more than one year; as many Melolonthæ, Elateres, Cerambyces, Buprestes, and several species of Libellula, Ephemera, &c. There are also many larvæ which, though their term of life is not a year, being hatched from the egg in autumn, necessarily pass the winter in that state, as those of several Anobia and other wood-boring insects; of Semasia Wœberana and others of the same family; of the second broods of several butterflies, &c. Many of these residing in the ground or in the interior of trees need no other hybernacula than the holes which they constantly inhabit; some, as the aquatic larvæ, merely hide themselves in the sides or muddy bottom of their native pools; while others seek for a retreat under moss, dead leaves, stones, and the bark of decaying trees. Most of these can boast of no better winter quarters than a simple unfurnished hole or cavity; but a few, more provident of comfort, prepare themselves an artificial habitation. With this view the larva of Cossus ligniperda, as formerly observed in describing the habitations of insects[717], forms a covering of pieces of wood lined with fine silk; those of Hepiolus Humuli, Xylina radicea, and some other moths, excavate under a stone a cavity exactly the size of their bodies, to which they give all round a coating of silk[718], and the larvæ of Pieris Cratægi inclose themselves in autumn in cases of the same material[719], and thus pass the cold season in small societies of from two to twelve, under a common covering formed of leaves. Bonnet mentions a trait of the cleanliness of these insects which is almost ludicrous. He observed in one of these nests a sort of sack containing nothing but grains of excrement; and a friend assured him that he had seen one of these caterpillars partly protrude itself out of its case, the hind feet first, to eject a similar grain; so that it would seem the society have on their establishment a scavenger, whose business it is to sweep the streets and convey the rejectamenta to one grand repository[720]! This, however singular, is rendered not improbable from the fact that beavers dig in their habitations holes solely destined for a like purpose[721].

A very considerable number of insects hybernate in the perfect state, chiefly of the orders Coleoptera, Hemiptera, Hymenoptera, and Diptera, and especially of the first. Vanessa Urticæ, Io, and a few other lepidopterous species, with a small proportion of the other orders, occasionally survive the winter; but the bulk of these are rarely found to hybernate as perfect insects. Of coleopterous insects, Schmid, to whom we are indebted for some valuable remarks on the present subject[722], says that he never found or heard of any Entomologist finding a hybernating individual of the common cockchafer (Melolontha vulgaris) or of the stag-beetle (Lucanus Cervus); and suggests that it is only those insects which exist but a short period as larvæ, as most of the tribe of weevils, lady-birds, &c., that survive the winter in the perfect state; while those which live more than one year in the larva state, as the species just mentioned, are deprived of this privilege.