An inquiring friend of Réaumur having found one of these insects floating about in its muff-tent upon water, concluded that they feed upon aquatic plants; but he was soon convinced that it had only been blown down by an accident, which must frequently happen, as willows so often hang over water. May it not be, that the buoyant materials of the tent were intended to furnish the little inhabitant with a life-boat, in which, when it chanced to be blown into the water, it might sail safely ashore and regain its native tree?
Leaf-mining Caterpillars.
The process of mining between the two membranes of a leaf is carried on to a farther extent by minute caterpillars allied to the tent-makers above described. The tent-maker never deserts his house, except when compelled, and therefore can only mine to about half the length of his own body; but the miners now to be considered make the mine itself their dwelling-place, and as they eat their way, they lengthen and enlarge their galleries. A few of these mining caterpillars are the progeny of small weevils (Curculionidæ), some of two-winged flies (Diptera), but the greater number are produced from a genus of minute moth (Œcophora, Latr.), which, when magnified, appear to be amongst the most splendid and brilliant of Nature’s productions, vying even with the humming-birds and diamond-beetles of the tropics in the rich metallic colours which bespangle their wings. Well may Bonnet call them “tiny miracles of Nature,” and regret that they are not en grand.[BY]
There are few plants or trees whose leaves may not, at some season of the year, be found mined by these caterpillars, the track of whose progress appears on the upper surface in winding lines. Let us take one of the most common of these for an example,—that of the rose-leaf, produced by the caterpillar of Ray’s golden-silver spot (Argyromiges Rayella? Curtis), of which we have just gathered above a dozen specimens from one rose-tree. (J. R.)
It may be remarked that the winding line is black, closely resembling the tortuous course of a river on a map,—beginning like a small brook, and gradually increasing in breadth as it proceeds. This representation of a river exhibits, besides, a narrow white valley on each side of it, increasing as it goes, till it terminates in a broad delta. The valley is the portion of the inner leaf from which the caterpillar has eaten the pulp (parenchyma), while the river itself has been formed by the liquid ejectamenta of the insect, the watery part becoming evaporated. In other species of miners, however, the dung is hard and dry, and consequently these only exhibit the valley without the river (see [p. 255]).
Leaf of the Monthly Rose (Rose Indica), mined by Caterpillars of Argyromiges?
On looking at the back of the leaf, where the winding line begins, we uniformly find the shell of the very minute egg from which the caterpillar has been hatched, and hence perceive that it digs into the leaf the moment it escapes from the egg, without wandering a hair’s-breadth from the spot; as if afraid lest the air should visit it too roughly. The egg is, for the most part, placed upon the mid-rib of the rose-leaf, but sometimes on one of the larger nervures. When once it has got within the leaf, it seems to pursue no certain direction, sometimes working to the centre, sometimes to the circumference, sometimes to the point, and sometimes to the base, and even, occasionally, crossing or keeping parallel to its own previous track.
The most marvellous circumstance, however, is the minuteness of its workmanship; for though a rose-leaf is thinner than this paper, the insect finds room to mine a tunnel to live in, and plenty of food, without touching the two external membranes. Let any one try with the nicest dissecting instruments to separate the two plates of a rose-leaf, and he will find it impossible to proceed far without tearing one or other. The caterpillar goes still further in minute nicety; for it may be remarked, that its track can only be seen on the upper, and not on the under surface of the leaf, proving that it eats as it proceeds only half the thickness of the pulp, or that portion of it which belongs to the upper membrane of the leaf.
We have found this little miner on almost every sort of rose-tree, both wild and cultivated, including the sweet-briar, in which, the leaf being very small, it requires nearly the whole parenchyma to feed one caterpillar. They seem, however, to prefer the foreign monthly rose to any of our native species, and there are few trees of this where they may not be discovered.