In this strange and apparently cruel operation one circumstance is truly remarkable. The larva of the Ichneumon, though every day, perhaps for months, it gnaws the inside of the caterpillar, and though at last it has devoured almost every part of it except the skin and intestines, carefully all this time avoids injuring the vital organs, as if aware that its own existence depends on that of the insect on which it preys! Thus the caterpillar continues to eat, to digest, and to move, apparently little injured, to the last, and only perishes when the parasitic grub within it no longer requires its aid. What would be the impression which a similar instance amongst the race of quadrupeds would make upon us?—If, for example, an animal—such as some impostors have pretended to carry within them—should be found to feed upon the inside of a dog; devouring only those parts not essential to life, while it cautiously left uninjured the heart, arteries, lungs, and intestines,—should we not regard such an instance as a perfect prodigy, as an example of instinctive forbearance almost miraculous?
Some Ichneumons, instead of burying their eggs in the body of the larvæ that are to serve their young for food, content themselves with gluing them to the skin of their prey, which the young grubs pierce as soon as hatched. Another tribe, whose activity and perseverance are equally conspicuous, which includes the beautiful genus Chrysis and many other hymenopterous insects, imitating the insidious cuckoo, contrive to introduce their eggs into the nests in which bees and other insects have deposited theirs. With this view they are constantly on the watch, and, the moment the unsuspecting mother has quitted her cell for the purpose of collecting a store of food or materials, glide into it and leave an egg, the germe of a future assassin of the larva that is to spring from that deposited by its side.
The females of the insects of which we have been speaking, in providing for their offspring, are saved the trouble of furnishing them with any habitation. Either they occupy that of another insect, or find a convenient abode within the body of that on which they feed. But upon the maternal affection of another large hymenopterous tribe, belonging to Latreille's Family of Burrowers (Fossores), whose young in like manner feed on other insects, is imposed the arduous task not merely of collecting a supply of food, but of inclosing it along with their eggs in cells or burrows often of considerable depth, and dug with great labour in sand or the solid earth.
The general economy of these insects is similar. Having first dug a cylindrical cavity of the requisite dimensions, and deposited an egg at the bottom, they inclose along with it one or more caterpillars, spiders, or other insects, each particular species for the most part selecting a distinct kind, as a provision for the young one when hatched, and sufficiently abundant to nourish it until it becomes a pupa. Many thus furnish several cells. This process, however, is varied by different species, some of whose operations are worthy of a more detailed description.
One of the most early histories of the procedure of an insect of this kind, probably the common sand-wasp (Ammophila vulgaris), is left us by the excellent Ray, who observed it along with his friend Willughby. On the 22d of June 1667, he tells us, they noticed this insect dragging a green caterpillar thrice as big as itself, which after thus conveying about fifteen feet, it deposited at the entrance of a hole previously dug in the sand. Then removing a pellet of earth from its mouth, it descended into the cavity, and, presently returning, dragged along with it the caterpillar. After staying awhile it again ascended, then rolled pieces of earth into the hole, at intervals scratching the dust into it like a dog with its fore feet, and entering it as if to press down and consolidate the mass: flying also once or twice to an adjoining fir-tree, possibly to procure resin for agglutinating the whole. Having filled the burrow to a level with the surrounding earth so as to conceal the entrance, it took two fir-leaves lying at hand, and placed them near the orifice as if to mark the place.—Such is the anecdote left on record by our illustrious countryman, of whose accuracy of observation there can be no doubt[661]. Who that reads it can refrain from joining in the reflection which it calls from him, "Quis hæc non mihi miretur et stupeat? Quis hujusmodi opera meræ machinæ possit attribuere[662]?"
I myself, when walking with a friend some months ago, observed nearly similar manœuvres performed by another hymenopterous insect which may be called a spider-wasp (Pompilus), which attracted our attention as it was dragging a spider to its cell. The attitude in which it carried its prey, namely with its feet constantly upwards; its singular mode of walking, which was backwards, except for a foot or two when it went forwards, moving by jerks and making a sort of pause every few steps; and the astonishing agility with which, notwithstanding its heavy burthen, it glided over or between the grass, weeds, and other numerous impediments in the rough path along which it passed—together formed a spectacle which we contemplated with admiration. The distance which we thus observed it to traverse was not less than twenty-seven feet, and great part of its journey had probably been performed before we saw it. Once or twice, when we first noticed it, it laid down the spider, and making a small circuit returned and took it up again. But for the ensuing twenty or twenty-five feet it never stopped, but proceeded in a direct line for its burrow with the utmost speed. When opposite the hole, which was in a sand bank by the way side, it made a sharp turn, as evidently aware of being in the neighbourhood of its abode, but when advanced a little further laid down its burthen and went to reconnoitre. At first it climbed up the bank, but, as if discovering that this was not the direction, soon returned, and, after another survey perceiving the hole, took up the spider and dragged it in after it.
In the two instances above given, one dead caterpillar or spider only was deposited in each hole. But an insect described by Reaumur under the name of the mason-wasp (Epipone spinipes), very common in some parts of England, after having excavated a burrow, with an ingenuity to which on a future occasion I shall draw your attention, places along with its egg as food for the future young, about twelve little green grubs without feet, which it has carefully selected full grown and conveyed without injuring them. You will inquire, Why this difference of procedure? With regard to the choice of a number of small grubs rather than of one large caterpillar, what I have said in a former letter on the subject of different species of this tribe being appointed to prey upon and thus keep within due limits the larvæ of different kinds of insects, will be a sufficient answer. But one circumstance creditable to the talents of the mason-wasp as a skilful purveyor should not be omitted, namely, that the number of grubs laid up is not always the same, but is exactly proportioned to their size, eleven or twelve being stored when they are small, but only eight or nine when larger. With respect however to the caution of the wasp in selecting full grown grubs and conveying them uninjured to her hole, a satisfactory explanation may be given. If those that are but partly grown were chosen, they would die in a short time for want of food, and putrefying would destroy the inclosed egg, or the young one which springs from it. But when larvæ of any kind have attained their full size, and are about to pass into the pupa state, they can exist for a long period without any further supply. By selecting these, therefore, and placing them uninjured in the hole, however long the interval before the egg hatches, the disclosed larva is sure of a sufficiency of fresh and wholesome nutriment.—To prevent the possibility of any injury to its egg from the motions or voracity of this living prey, the wasp is careful to pack the whole so closely, each grub being coiled above the other in a series of rings, and to consolidate the earth so firmly above them, that they have not the slightest power of motion[663].—Those which select more powerful caterpillars, or revenge the injuries of their insect brethren by devoting spiders to the destruction they have so often caused, take care to sting them in such a manner as, without killing them outright, will incapacitate them from doing any injury.
Zeal and activity in providing for the well-being of their future progeny, not inferior to what are exhibited by the tribe of Ichneumons, Sphecina[664], and mason-wasps, though less cruelly exerted, are also shown by various species of wild bees, of which we have in this country a vast number. Having first excavated a proper cell with a dexterity and persevering labour never enough to be admired, they next deposit in it an egg, which they cover with a mass of pollen or honey collected with unwearied assiduity from a thousand flowers. As soon as the grub is hatched, it finds itself enveloped in this delicious banquet provided for it by the cares of a mother it is doomed never to behold; and so accurately is the repast proportioned to its appetite and its wants, that as soon as the whole is consumed it has no longer need of food; it clothes itself in a silken cocoon, becomes a pupa, and after a deep sleep of a few days bursts from its cell an active bee.
No circumstance connected with the storgé of insects, is more striking than the herculean and incessant labour which it leads them cheerfully to undergo. Some of these exertions are so disproportionate to the size of the insect, that nothing short of ocular conviction could attribute them to such an agent. A wild bee or a Sphex, for instance, will dig a hole in a hard bank of earth some inches deep and five or six times its own size, and labour unremittingly at this arduous undertaking for several days, scarcely allowing itself a moment for eating or repose. It will then occupy as much time in searching for a store of food; and no sooner is this task finished, than it will set about repeating the process, and before it dies will have completed five or six similar cells or even more. If you would estimate this industry at its proper value, you should reflect what kind of exertion it would require in a man to dig in a few days out of hard clay or sand, with no other tools than his nails and teeth, five or six caverns twenty feet deep and four or five wide—for such an undertaking would not be comparatively greater than that of the insects in question.
Similar laborious exertions are not confined to the bee or Sphex tribe. Several beetles in depositing their eggs exhibit examples of industry equally extraordinary. The common dor or clock (Geotrupes stercorarius), which may be found beneath every heap of dung, digs a deep cylindrical hole, and, carrying down a mass of the dung to the bottom, in it deposits its eggs. And many of the species of the Scarabæidæ[665] roll together wet dung into round pellets, deposit an egg in the midst of each, and when dry push them backwards by their hind feet into holes of the surprising depth of three feet, which they have previously dug for their reception, and which are often several yards distant. Frequently the road lies across a depression in the surface, and the pellet when nearly pushed to the summit rolls back again. But our patient Sisyphi are not easily discouraged. They repeat their efforts again and again, and in the end their perseverance is rewarded by success. The attention of these insects to their eggs is so remarkable, that it was observed in the earliest ages, and is mentioned by ancient writers, but with the addition of many fables, as that they were all of the male sex, that they became young again every year, that they rolled the pellets containing their eggs from sun-rise to sun-set every day, for twenty-eight days without intermission[666], &c. It is one of this tribe of beetles (S. sacer) whose image is so often met with amongst the hieroglyphics of the Egyptians, with whom it was a symbol of the world, of the sun, and of a courageous warrior. Of the world, as P. Valerianus supposes, on account of the orbicular form of its pellets of dung, and the notion of their being rolled from sun-rise to sun-set; of the sun, because of the angular projections from its head resembling rays, and the thirty joints of the six tarsi of its feet answering to the days of the month; and of a warrior, from the idea of manly courage being connected with its supposed birth from a male only[667]. It was as symbolical of this last that its image was worn upon the signets of the Roman soldiers; and as typical of the sun, the source of fertility, it is yet, as Dr. Clarke informs us, eaten by the women to render them prolific[668].