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

by William Kirby; William Spence

The variations in the procedures of the larva of a little moth described by Reaumur, whose habitation has been before noticed[769]—one of those which constantly reside in a subcylindrical case—are still more remarkable. This little caterpillar feeds upon the elm, the leaves of which serve it at once for food and clothing. It eats the parenchyma or inner pulp, burrowing between the upper and under membranes, of portions of which cut out, and properly sewed together, it forms its case. Its usual plan is, to insinuate itself between the epidermal membranes of the leaf, close to one of the edges. Parallel with this it excavates a cavity of suitable form and dimensions, gnawing the pulp even out of every projection of the serratures, but carefully avoiding to separate the membranes at the very edge, which with a wise saving of labour it intends should form one of the seams of its coat; and as the little miner is not embarrassed with the removal of the excavated materials, which it swallows as it proceeds, a cavity sufficiently large is but the work of a few hours. It then lines it with silk, at the same time pushing it into a more cylindrical shape; and lastly, cutting it off at the two ends and inner side, it sews up the latter with such nicety that the suture is scarcely discoverable; and is now provided with a case or coat exactly fitting its body, open at the two ends, by one of which it feeds and by the other discharges its excrement, having on one side a nicely-joined seam, and the other—that which is commonly applied to its back—composed of the natural marginal junction of the membranes of the leaf.

Such are the ordinary operations of this insect, which, when it is considered that the case is rather fusiform than cylindrical; that the end through which it eats is circular, and the other curiously three-cornered like a cocked-hat; and that consequently its cloth requires to be very irregularly and artfully cut, to be accommodated to such a figure—it must be admitted, are the result of an instinct of no very simple kind. Complicated, however, as these manœuvres seem, our ingenious workman is not confined to them. By way of putting its resources to the test, Reaumur cut off the serrated edge from the nearly-finished coat of one of them, and exposed the little occupant to the day. He expected that it would have quitted its mutilated garment and commenced another; and so it certainly would, had it been guided by an invariable instinct. But he calculated erroneously. Like one of its brother tailors of the biped race, it knew how "to cut its coat according to its cloth," and immediately setting about repairing the injury sewed up the rent. Nor was this all. The scissors having cut off one of the projections intended to enter into the construction of the triangular end of its case, it entirely changed the original plan, and made that end the head which had been first designed for the tail.

On another occasion Reaumur observed one of these larvæ to cut out its coat from the very centre of a leaf, where it is obvious a series of operations wholly different must be adopted, the two membranes composing it necessarily requiring to be cut and sewed on two sides instead of on one only. But what was most striking in this new procedure was the alteration which the caterpillar made in the period of sewing up its garment. When these larvæ cut out their case from the edge of a leaf, they seem aware that, if they were to detach it entirely from the inner side before the process of sewing, lining, &c., is completed, having no support on the exterior edge, it would be liable to fall down; at the same time they could not sew together the membranes composing it at the inner side, without cutting them in part from the leaf. While, therefore, they divide the major part of their inner side from the leaf, they artfully leave them attached to it by one of the large nerves at each end: and these supports they do not cut asunder until the intermediate space has been sewed up, and they are ready to step, with their house on their back, upon the terra firma of the disk of the leaf. In this instance, therefore, the larvæ do not wholly separate their case from the leaf, until it is sewed. But when the same larvæ cut out their materials from the middle of the leaf, where, though completely cut round, they are retained in their situation secure from all danger of falling by the serratures of the incisions made by the jaws of the larvæ, these little tailors vary their mode, and entirely detach the pieces from the surrounding leaf, before they proceed to set a stitch into them[770].

In the preceding instances the variation of instinct takes place in the same individual, but Bonnet mentions a very curious fact in which it occurs in different generations of the same species. There are annually, he informs us, two generations of the Angoumois moth, an insect which has been before mentioned[771], as destructive to wheat: the first appear in May and June, and lay their eggs upon the ears of wheat in the fields; the second appear at the end of the summer or in autumn, and these lay their eggs upon wheat in the granaries. These last pass the winter in the state of larvæ, from which proceeds the first generation of moths. But what is extremely singular as a variation of instinct, those moths which are disclosed in May and June in the granaries, quit them with a rapid flight at sun-set, and betake themselves to the yet unreaped fields, where they lay their eggs; while the moths which are disclosed in the granaries after harvest, stay there, and never attempt to go out, but lay their eggs upon the stored wheat[772].—This is as extraordinary and inexplicable as if a litter of rabbits produced in spring were impelled by instinct to eat vegetables, while another produced in autumn should be as irresistibly directed to choose flesh.

It is, however, into the history of the hive-bee that we must look for the most striking examples of variation of instinct; and here, as in every thing relating to this insect, the work of the elder Huber is an unfailing source of the most novel and interesting facts.

It is the ordinary instinct of bees to lay the foundation of their combs at the top of the hive, building them perpendicularly downwards; and they pursue this plan so constantly, that you might examine a thousand (probably ten thousand) hives, without finding any material deviation from it. Yet Huber in the course of his experiments forced them to build their combs perpendicularly upward[773]; and, what seems even more remarkable, in an horizontal direction[774].

The combs of bees are always at an uniform distance from each other, namely about one third of an inch, which is just wide enough to allow them to pass easily and have access to the young brood. On the approach of winter, when their honey-cells are not sufficient in number to contain all the stock, they elongate them considerably, and thus increase their capacity. By this extension the intervals between the combs are unavoidably contracted; but in winter well-stored magazines are essential, while from their state of comparative inactivity spacious communications are less necessary. On the return of spring, however, when the cells are wanted for the reception of eggs, the bees contract the elongated cells to their former dimensions, and thus re-establish the just distances between the combs which the care of their brood requires[775]. But this is not all. Not only do they elongate the cells of the old combs when there is an extraordinary harvest of honey, but they actually give to the new cells which they construct on this emergency a much greater diameter as well as a greater depth[776].

The queen-bee in ordinary circumstances places each egg in the centre of the pyramidal bottom of the cell, where it remains fixed by its natural gluten: but in an experiment of Huber, one whose fecundation had been retarded, had the first segments of her abdomen so swelled that she was unable to reach the bottom of the cells. She therefore attached her eggs (which were those of males) to their lower side, two lines from the mouth. As the larvæ always pass that state in the place where they are deposited, those hatched from the eggs in question remained in the situation assigned them. But the working-bees, as if aware that in these circumstances the cells would be too short to contain the larvæ when fully grown, added to their length, even before the eggs were hatched[777].

Bees close up the cells of the grubs, previously to their transformation, with a cover or lid of wax: and in hanging its abode with a silken tapestry before it assumes the pupa state, the grub requires that the cell should not be too short for its movements. Bonnet having placed a swarm in a very flat glass hive, the bees constructed one of the combs parallel to one of the principal sides, where it was so straight that they could not give to the cells their ordinary depth. The queen, however, laid eggs in them, and the workers daily nourished the grubs, and closed the cells at the period of transformation. A few days afterwards he was surprised to perceive in the lids, holes more or less large, out of which the grubs partly projected, the cells having been too short to admit of their usual movements. He was curious to know how the bees would proceed. He expected that they would pull all the grubs out of the cells, as they commonly do when great disorders in the combs take place. But he did not sufficiently give credit to the resources of their instinct. They did not displace a single grub—they left them in their cells: but as they saw that these cells were not deep enough, they closed them afresh with lids much more convex them ordinary, so as to give to them a sufficient depth; and from that time no more holes were made in the lids.

The working bees, in closing up the cells containing larvæ, invariably give a convex lid to the large cells of drones, and one nearly flat to the smaller cells of workers: but in an experiment instituted by Huber to ascertain the influence of the size of the cells on that of the included larvæ, he transferred the larvæ of workers to the cells of drones. What was the result? Did the bees still continue blindly to exercise their ordinary instinct? On the contrary, they now placed a nearly flat lid upon these large cells, as if well aware of their being occupied by a different race of inhabitants[778].