Social Life in the Insect World

by Jean-Henri Fabre

It was well that I called in helpers to lend me their eyes and their attention. After eight hours—eight interminable hours, when it was nearly night, the sentinel on the watch calls me. The insect appears to have finished. She does, in fact, very cautiously withdraw her beak, as though fearing to slip. Once the tool is withdrawn she holds it pointing directly in front of her.

The moment has come.... Alas, no! Once more I am cheated; my eight hours of observation have been fruitless. The Balaninus decamps; abandons her acorn without laying her eggs. I was certainly right to distrust the result of observation in the open woods. Such concentration among the oaks, exposed to the sun, wind, and rain would have been an intolerable task.

During the whole of October, with the aid of such helpers as are needed, I remark a number of borings, not followed by the laying of eggs. The duration of the observer's task varies greatly. It usually amounts to a couple of hours; sometimes it exceeds half the day.

With what object are these perforations made, so laborious and yet so often unused? Let us first of all discover the position of the egg, and the first mouthfuls taken by the grub, and perhaps the reply will be found.

The peopled acorns remain on the oak, held in their cups as though nothing had occurred to the detriment of the cotyledons. With a little attention they may be readily recognised. Not far from the cup, on the smooth, still green envelope of the acorn a little point is visible; a tiny needle-prick. A narrow brown aureole, the product of mortification, is not long in appearing. This marks the opening of the hole. Sometimes, but more rarely, the hole is drilled through the cup itself.

Let us select those acorns which have been recently perforated: that is to say, those in which the perforation is not yet surrounded by the brown ring which appears in course of time. Let us shell them. Many contain nothing out of the way; the Balaninus has bored them but has not laid her eggs in them. They resemble the acorns which for hours and hours were drilled in my laboratory but not utilised. Many, on the contrary, contain an egg.

Now however distant the entrance of the bore may be, this egg is always at the bottom of the acorn, within the cup, at the base of the cotyledonary matter. The cup furnishes a thin film like swan-skin which imbibes the sapid exudations from the stem, the source of nourishment. I have seen a young grub, hatched under my eyes, eat as his first mouthfuls this tender cottony layer, which is moist and flavoured with tannin.

Such nutriment, juicy and easy of digestion, like all nascent organic matter, is only found in this particular spot; and it is only there, between the cup and the base of the cotyledons, that the elephant-beetle establishes her egg. The insect knows to a nicety the position of the portions best adapted to the feeble stomach of the newly hatched larva.

Above this is the tougher nutriment of the cotyledons. Refreshed by its first meal, the grub proceeds to attack this; not directly, but in the tunnel bored by the mother, which is littered with tiny crumbs and half-masticated shavings. With this light mealy diet the strength of the grub increases, and it then plunges directly into the substance of the acorn.

These data explain the tactics of the gravid mother. What is her object when, before proceeding to sink her hole, she inspects her acorn, from above, below, before and behind, with such meticulous care? She is making sure that the acorn is not already occupied. The larder is amply stored, but it does not contain enough for two. Never in fact, have I found two larvæ in the same acorn. One only, always only one, digests the copious meal and converts it into a greenish dust before leaving it and descending to the ground. Only an insignificant shell remains uneaten. The rule is, to each grub one acorn.