A fine oak-trunk, seamed with scars and gashed with wounds, contains many treasures for my studies. The mallet drives home, the wedges bite, the wood splits; and within, in the dry and hollow parts, are revealed groups of various insects who are capable of living through the cold season, and have here taken up their winter quarters. In the low-roofed galleries built by some Beetle the Osmia Bee has piled her cells one above the other. In the deserted chambers and vestibules Megachiles have arranged their leafy jars. In the live wood, filled with juicy sap, the larva of the Capricorn, the chief author of the oak’s undoing, has set up its home.
Truly they are strange creatures, these grubs: bits of intestines crawling about! In the middle of Autumn I find them of two different ages. The older are almost as thick as one’s finger; the others hardly attain the diameter of a pencil. I find, in addition, the pupa or nymph more or less fully coloured, and the perfect insect ready to leave the trunk when the hot weather comes again. Life inside the wood, therefore, lasts for three years.
How is this long period of solitude and captivity spent? [[211]]In wandering lazily through the thickness of the oak, in making roads whose rubbish serves as food. The horse in the book of Job “swallows the ground” in a figure of speech: the Capricorn’s grub eats its way literally. With its carpenter’s-gouge—a strong black mandible, short and without notches, but scooped into a sharp-edged spoon—it digs the opening of its tunnel. From the piece cut out the grub extracts the scanty juices, while the refuse accumulates behind him in heaps. The path is devoured as it is made; it is blocked behind as it makes way ahead.
Since this harsh work is done with the two gouges, the two curved chisels of the mandibles, the Capricorn-grub requires much strength in the front part of its body, which therefore swells into a sort of pestle. The Buprestis-grub, that other industrious carpenter, adopts a similar form, and even exaggerates its pestle. The part that toils and carves hard wood requires to be robust; the rest of the body, which has but to follow after, continues slim. The essential thing is that the implement of the jaws should possess a solid support and powerful machinery. The Capricorn larva strengthens its chisels with a stout, black, horny armour that surrounds the mouth; yet, apart from its skull and its equipment of tools, this grub has a skin as fine as satin and as white as ivory. This dead white is caused by a thick layer of grease, which one would not expect a diet of wood to [[212]]produce in the animal. True, it has nothing to do, at every hour of the day and night, but gnaw. The quantity of wood that passes into its stomach makes up for the lack of nourishing qualities.
The grub’s legs can hardly be called legs at all; they are mere suggestions of the legs the full-grown insect will have by and by. They are infinitesimal in size, and of no use whatever for walking. They do not even touch the supporting surface, being kept off it by the plumpness of the chest. The organs by means of which the animal progresses are something altogether different.
The grub of the Rose-chafer, with the aid of the hairs and pad-like projections upon its spine, manages to reverse the usual method of walking, and to wriggle along on its back. The grub of the Capricorn is even more ingenious: it moves at the same time on its back and its stomach. To take the place of its useless legs it has a walking apparatus almost like feet, which appear, contrary to every rule, on the surface of its back.
On the middle part of its body, both above and below, there is a row of seven four-sided pads, which the grub can either expand or contract, making them stick out or lie flat at will. It is by means of these pads that it walks. When it wishes to move forwards it expands the hinder pads, those on the back as well as those on the stomach, and contracts its front pads. The swelling of the hind pads in the narrow gallery fills up the space, and [[213]]gives the grub something to push against. At the same time the flattening of the front pads, by decreasing the size of the grub, allows it to slip forward and take half a step. Then, to complete the step, the hind-quarters must be brought up the same distance. With this object the front pads fill out and provide support, while those behind shrink and leave room for the grub to draw up its hind-quarters.
With the double support of its back and stomach, with alternate swellings and shrinkings, the animal easily advances or retreats along its gallery, a sort of mould which the contents fill without a gap. But if the pads grip only on one side progress becomes impossible. When placed on the smooth wood of my table the animal wriggles slowly; it lengthens and shortens without progressing by a hair’s breadth. Laid on the surface of a piece of split oak, a rough, uneven surface due to the gash made by the wedge, it twists and writhes, moves the front part of its body very slowly from left to right and right to left, lifts it a little, lowers it, and begins again. This is all it can do. The rudimentary legs remain inert and absolutely useless.