In the larval stage, the Cetonia is a most active manufacturer of leaf-mould. When the metamorphosis occurs and I review the results of my insect-rearing for the last time, [[20]]I am shocked by the amount of eating which my gormandizers have done in the course of their lives; it can be measured by the bowlful.
The Cetonia-larva is worth attention from another point of view. It is a corpulent grub, an inch long, with a convex back and a flat belly. The dorsal surface is wrinkled with thick folds, on which the sparse hairs stand erect like the bristles of a brush; the ventral surface is smooth, covered with a fine skin, through which the ample wallet of ordure shows as a brown patch. The legs are very well-shaped, but are small, feeble and out of proportion to the rest of the body.
The creature is given to coiling itself into a closed ring. This is a posture of repose, or rather of anxiety and defence. At such times the living coil contracts so violently that we fear to see it burst open and void its entrails when we seek to unroll it by force. When no longer molested, the grub unrolls itself, straightens out and makes haste to escape.
Then a surprise awaits us. If placed upon the table, the harassed creature travels on its back with its legs in the air, inactive. [[21]]This extravagant method, contrary to the accepted usages of locomotion, appears at first sight an accident, a chance manœuvre of the bewildered animal. Not at all: it is a normal manœuvre; and the grub knows no other. You turn it over on its belly, hoping to see it progress in the customary fashion. Your attempts are useless: obstinately it lies down on its back again, obstinately it crawls along in a reversed position. Nothing will persuade it to walk on its legs. Either it will remain motionless, coiled into a circle, or, straightening itself out, it will travel upside down. This is its way of doing things.
Leave it undisturbed on the table. It sets off, longing to bury itself in the soil and escape from its tormentor. Its progress is by no means slow. The dorsal pads, actuated by a powerful layer of muscle, give it a hold even on a smooth surface, thanks to their brush-like tufts of hair. They are ambulacra which, by their multiplicity, exert a vigorous traction.
The moving mechanism is apt to roll from side to side. By reason of the rounded form of the back, the grub sometimes turns turtle. The accident is not serious. With a heave of its loins, the capsized grub at once [[22]]recovers its balance and resumes its dorsal crawl, accompanied by a gentle swaying to right and left. It also pitches to and fro. The prow of the vessel, the larva’s head, rises and falls in measured oscillations. The mandibles open and bite at space, apparently trying to seize some support which is lacking.
Let us give it this support: not in the leaf-mould, whose opacity would hide what I want to see, but in a transparent medium. I happen to have what I need, a glass tube of some length, open at both ends and of a gradually diminishing calibre. At the large end the grub enters comfortably; at the other end it finds a very tight fit.
As long as the tube is more than wide enough, the grub moves along on its back. Then it enters a part of the tube whose calibre is equal to that of its body. From this moment the locomotion loses its abnormal character. No matter what its position, whether the belly is uppermost, undermost or to one side, the grub advances. I see the muscular waves of the dorsal pads moving with a beautiful regularity, like the ripples spreading over a calm sheet of water which has been disturbed by the fall of a pebble. [[23]]I see the bristles bowing and standing up again like corn waving in the wind.
The head oscillates evenly. The tips of the mandibles are used as a crutch which measures the paces in advance and gives stability by obtaining a purchase of the walls. In all the positions, which I vary at will by turning the tube between my fingers, the legs remain inactive even when they touch the supporting surface. Their part in locomotion is almost nil. What use, then, can they be? We shall see presently.
The transparent channel in which the larva is worming its way tells us what happens in the heart of the heap of garden-mould. Supported on every side at once, close-sheathed in the substance traversed, the grub progresses in the normal position as often as in the reversed position and even oftener. By virtue of its dorsal waves, which come into contact with the surrounding materials in every direction, it moves back or belly uppermost, indifferently. Here are no longer fantastic exceptions; matters return to their habitual order; if we could see the grub ambling through the heap of rotting leaves, we should not regard it as in any way peculiar. [[24]]