Popular Science Monthly/Volume 26/March 1885/The Parental Foresight of Insects

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IN no manner is the mysterious influence of instinct over the insect world more remarkably manifested than by the care taken by parent insects for the future welfare of offspring which they are destined never to behold. As the human parent upon his death-bed makes the best provision he can for the sustenance and prosperity of his infant children, whom death has decreed that he may not in person watch over, so those insects which Nature has decreed shall be always the parents of orphan children, led by an unerring influence within, do their best to provide for the wants of the coming generation.

The butterfly, after flitting through her short life, seeks out a spot whereon to deposit her numerous eggs, not—as one might expect of a creature devoid of mind—upon any chance plant, or even upon the plant or flower from which she herself has been wont to draw her sustenance, but upon the particular plant which forms the invariable food of the larva? of her species. The various kinds of clothes-moths penetrate into our cupboards, drawers, and everywhere where furs, woolen garments, etc., are stored, that they may there lay their eggs, to hatch into the burrowing: scrubs which are the terror of our house-keepers. The ichneumon tribe, one of Nature's greatest counterpoises to keep down the too rapid increase of the insect world, lay their eggs in the larvæ of other insects, which eggs when hatched develop into a devouring brood, which ungratefully turn upon and devour the helpless creature that sheltered them as a nest. The female ichneumon having discovered a caterpillar or grub which her instinct informs her has not been previously attacked, at once proceeds to thrust her ovipositor into the writhing body of her victim, depositing one or more eggs, according to the size of the living food-supply. When hatched, the larvae devour and live upon their foster-parent, avoiding in a marvelous way the vital parts of their victim, whose life is most accurately timed to last until its young tormentors are full grown, and not beyond. At one time, we were led to believe in occasional instances of the instinct of female ichneumons being at fault, by observing them apparently ovipositing upon the dry shells of pupæ from which the butterflies had escaped. This, however, we subsequently found to be an erroneous idea, the fact of the matter being that the caterpillar upon which the parent ichneumon had laid her fatal egg had had time, before the full development of the young ichneumon grub, to turn to the pupal stage. What, then, we saw was the young ichneumon-fly just emerged from the dry pupal case, the contents of which it had first devoured in its own larval stage, then, itself turning to a pupa, it had lain, thus doubly incased, until, having broken forth a perfect fly, it rested upon its late prison, awaiting sufficient strength to come to its wings. What a wooden horse of Troy such a chrysalis would prove, if introduced into the breeding establishment of a collector!

Other members of the ichneumon tribe do not actually insert their eggs into the destined food-supply of their young; but, as it were, going deeper into calculation of future events, content themselves with laying them in close proximity to the eggs of some member of the tribe upon which it is their mission to prey.

There is an old saying:

"Big fleas have little fleas
Upon their backs to bite 'em;
Little fleas have smaller fleas,
So on ad infinitum"—

which is very true, inasmuch as from the great humble-bee down to the tiniest corn-thrips—a mere speck of dust to the naked eye—all insects have their parasites, and generally their own special species of ichneumon, to prevent their over-increase and to preserve the due balance of nature. There is a species of longicorn beetle, found in Pennsylvania, which feeds upon the tender bark of young hickory-shoots. When laying-time arrives, the female, having deposited her eggs in cavities perforated in the bark, carefully cuts a groove, about onetenth of an inch wide and deep, round the shoot just below where her treasures lie. The object, or rather we suppose we ought to say the consequence, of this act is the withering and decay of the shoot, a provision for the sustenance of her young, which, when in their larval state, live upon dead wood! This remarkable insect is called the hickory-girder from the above-mentioned habit, which, we think, is one of the most extraordinary instances of foresight, through a mere blind instinct, that have ever come under observation.

The gadfly (Eustrus equi), whose larvæ are the bots which inhabit the intestines of the horse, gains for her progeny that comfortable position by entrapping the animal itself into introducing her eggs within its stomach. For this purpose, she lays her eggs upon such portions of the horse's body as he is in the habit of frequently licking, such as knees, shoulders, etc. The unerring nature of her instinct is shown by the fact that she never chooses as a nidus any portion of the body which the horse is unable to reach with its tongue. Having thus been introduced into their natural feeding-grounds, the bots there pass their larval existence, until, it becoming time for them to assume the pupal form, they go forth with the animal's dung to reach the earth, burrow into it, and therein pass the insects' purgatory.

Again, one of the grain-moths (Gelechia cerealella) shows remarkable instinct in adapting itself to circumstances according to the time of year when it has to deposit its eggs. The first generation of these moths, emerging in May from pupæ which have lain in the granaries through the winter, lay their countless eggs upon the as yet ungathered corn, upon which their young play havoc until, having passed through the necessary stages, they come out in the autumn as the second generation amid the now stored-up grain. Now, however, their instinct prompts them, not, like the first generation, to go forth to the fields to seek the proper nest and future nourishment of their young, but bids them deposit their eggs upon the store of wheat ready at hand. Thus, two following generations of the same insect are led by their instincts to different habits to suit the altered and, in the last case, unnatural position of their infants' destined food-supply.

The interesting mason-wasp, having with great care and skill bored out a cylindrical hole in some sunny sand-bank, deposits at the bottom of this refuge her eggs. Next, provident mother as she is, she seeks out about a dozen small caterpillars, always of the same species, and immures them alive in the pit, as food for her cruel children. In making her selection of grubs to be thus buried alive, she rejects any that may not have reached maturity; not, we imagine, upon the score of their not being so full-flavored, but because, when not full grown, they require food to keep them alive; whereas, when of mature age, they will live a long time without nourishment, ready to turn to chrysalides when opportunity occurs.

These are but a few of the instances which might be adduced in illustration of this foresight in insects, which compensates for their not being allowed in person to superintend the welfare of their offspring. In many cases, it would be better for human progeny were their parents thus endowed with an unerring instinct, rather than with an uncertain will.—Chambers's Journal.