Insect Architecture

by James Rennie

The operations of an insect in boring into a leaf or a bud to form a lodgment for its eggs appear very simple. The tools, however, by which these effects are performed are very complicated and curious. In the case of gall-flies (Cynips), the operation itself is not so remarkable as its subsequent chemical effects. These effects are so different from any others that may be classed under the head of Insect Architecture, that we shall reserve them for the latter part of this volume, although, with reference to the use of galls, the protection of eggs and larvæ, they ought to find a place here. We shall, however, at present confine ourselves to those which simply excavate a nest, without producing a tumour.

The first of these insects which we shall mention is celebrated for its song, by the ancient Greek poets, under the name of Tettix. The Romans called it Cicada, which we sometimes, but erroneously, translate “grasshopper;” for the grasshoppers belong to an entirely different order of insects. We shall, therefore, take the liberty of calling the Cicadæ Tree-hoppers, to which the cuckoo-spit insect (Tettigonia spumaria, Oliv.) is allied; but there is only one of the true Cicadæ hitherto ascertained to be British, namely, the Cicada hæmatodes (Linn.), which was discovered in the New Forest, Hampshire, by Mr. Daniel Bydder.

M. Réaumur was exceedingly anxious to study the economy of these insects; but they not being indigenous in the neighbourhood of Paris, he commissioned his friends to send him some from more southern latitudes, and he procured in this way specimens not only from the South of France and from Italy, but also from Egypt. From these specimens he has given the best account of them yet published; for though, as he tells us, he had never had the pleasure of seeing one of them alive, the most interesting parts of their structure can be studied as well in dead as in living specimens. We ourselves possess several specimens from New Holland, upon which we have verified some of the more interesting observations of Réaumur.

Virgil tells us that in his time “the cicadæ burst the very shrubs with their querulous music;”[BH] but we may well suppose that he was altogether unacquainted with the singular instrument by means of which they can, not poetically, but actually, cut grooves in the branches they select for depositing their eggs. It is the male, as in the case of birds, which fills the woods with his song; while the female, though mute, is no less interesting to the naturalist on account of her curious ovipositor. This instrument, like all those with which insects are furnished by nature for cutting, notching, or piercing, is composed of a horny substance, and is also considerably larger than the size of the tree-hopper would proportionally indicate. It can on this account be partially examined without a microscope, being, in some of the larger species, no less than five lines[BI] in length.

The ovipositor, or auger (tarière), as Réaumur calls it, is lodged in a sheath which lies in a groove of the terminating ring of the belly. It requires only a very slight pressure to cause the instrument to protrude from its sheath, when it appears to the naked eye to be of equal thickness throughout, except at the point, where it is somewhat enlarged and angular, and on both sides finely indented with teeth. A more minute examination of the sheath demonstrates that it is composed of two horny pieces slightly curved, and ending in the form of an elongated spoon, the concave part of which is adapted to receive the convex end of the ovipositor.

When the protruded instrument is further examined with a microscope, the denticulations, nine in number on each side, appear strong, and arranged with great symmetry, increasing in fineness towards the point, where there are three or four very small ones, beside the nine that are more obvious. The magnifier also shows that the instrument itself, which appeared simple to the naked eye, is, in fact, composed of three different pieces; two exterior armed with the teeth before mentioned, denominated by Réaumur files (limes), and another pointed like a lancet, and not denticulated. The denticulated pieces, moreover, are capable of being moved forwards and backwards, while the centre one remains stationary; and as this motion is effected by pressing a pin or the blade of a knife over the muscles on either side at the origin of the ovipositor, it may be presumed that those muscles are destined for producing similar movements when the insect requires them. By means of a finely-pointed pin carefully introduced between the pieces, and pushed very gently downwards, they may be, with no great difficulty, separated in their whole extent.

The contrivance by which those three pieces are held united, while at the same time the two files can be easily put in motion, is similar to those of our own mechanical inventions, with this difference, that no human workman could construct an instrument of this description so small, fine, exquisitely polished, and fitting so exactly. We should have been apt to form the grooves in the central piece, whereas they are scooped out in the handles of the files, and play upon two projecting ridges in the central piece, by which means this is rendered stronger. M. Réaumur discovered that the best manner of showing the play of this extraordinary instrument is to cut it off with a pair of scissors near its origin, and then, taking it between the thumb and the finger at the point of section, work it gently to put the files in motion.

Beside the muscles necessary for the movement of the files, the handle of each is terminated by a curve of the same hard horny substance as itself, which not only furnishes the muscles with a sort of lever, but serves to press, as with a spring, the two files close to the central piece, as is shown in the lower figure.

M. Pontedera, who studied the economy of the tree-hoppers with some care, was anxious to see the insect itself make use of the ovipositor in forming grooves in wood, but found that it was so shy and easily alarmed, that it took to flight whenever he approached; a circumstance of which Réaumur takes advantage, to soothe his regret that the insects were not indigenous in his neighbourhood. But of their workmanship, when completed, he had several specimens sent to him from Provence and Languedoc by the Marquis de Caumont.