Insect Architecture

by James Rennie

Bark mined in rays by beetle-grubs.

We have frequently observed a very remarkable instinct in the grubs of a species of beetle (Scolytus destructor, Geoffroy), which lives under the dead bark of trees. The mother insect, as is usual with beetles, deposits her eggs in a patch or cluster in a chink or hole in the bark; and when the brood is hatched, they begin feeding on the bark which had formed their cradle. There is, of course, nothing wonderful in their eating the food selected by their mother; but it appears that, like the caterpillars of the clothes-moth, and the tent insects, they cannot feed except under cover. They dig, therefore, long tubular galleries between the bark and the wood; and, in order not to interfere with the runs of their brethren, they branch off from the place of hatching like rays from the centre of a circle: though these are not always in a right line, yet, however near they may approach to the contiguous ones, none of them ever break into each other’s premises. We cannot but admire the remarkable instinct implanted in these grubs by their Creator; which guides them thus in lines diverging farther and farther as they increase in size, so that they are prevented from interfering with the comforts of one another.

[We now come to one or two of the beetles which bore deeply into the very wood of the trees. As a rule, the musk-beetle keeps rather towards the exterior of the tree, but there are many that are not so cautious, and which besides damage the tree additionally by nibbling a quantity of chips, wherewith they strengthen their cocoons. We will first take the two insects which are shown in the accompanying illustration. That on the right hand is a species of weevil, or Curculio, and is an undescribed species belonging to the genus Rhyncophorus. It is a native of Australia. The insect and its cocoon are drawn one third less than their real size. The colour of the beetle is warm chestnut brown, and the bold marks on the thorax are jet black. In its larval state it burrows into the palm-trees, and when about to assume the pupal condition it makes the remarkable cocoon which is figured. Generally, these wooden cocoons are made of little chips which are bitten from the wood, and woven together with silk. This cocoon, however, is made of long fibres, which are torn rather than bitten, and are so long that one of them will sometimes encircle the cell three times, making an average length of nine inches. It is tolerably compact in structure, and the colour is pale brown.

On the left hand is an opened cocoon of an English beetle belonging to the genus Rhagium. Like the last-mentioned insect, the Rhagium prefers long fibres to short chips, though it does not use them of such a length as the Rhyncophorus. The cocoon is generally made between the bark and the wood, from the latter of which the fibres are torn. In consequence of the mode of structure, the cocoon is pale straw colour, while the hollow in which it rests is quite dark.

Cocoon of Rhagium. Rhyncophorus and cocoon.

All entomologists are familiar with the pretty little wasp-beetle (Clytus arietis), which derives its popular name from the wasp-like colours of its body. In the larval state it is one of the carpenter-grubs, and may be found in posts, fir-trees, and similar localities. In this country, although plentiful, it is not numerous enough to do much harm; but in Ceylon, a closely-allied species is one of the pests of the island. It is popularly known by the title of coffee-borer, from its habit of boring into the stems of the coffee-plant. The landowner looks with absolute horror on this pretty but destructive insect, and would pay a heavy sum annually to any one who would undertake to extirpate the tiny foe. Whole plantations have been swept off by it, and up to the present time no remedy has had more than a temporary and partial success.]

Another capricorn beetle of this family is no less destructive to bark in its perfect state than the above are when grubs, as from its habit of eating round a tree, it cuts the course of the returning sap, and destroys it.

[The late Mr. Waterton once showed me a stout branch which had fallen on his head while he was standing under a tree, the branch having been cut completely through by the jaws of some large longicorn beetle. The mode in which the insect had severed the branch was exactly like that which is practised by the beaver when it cuts down a tree.