ERUCA Aquatica, Water-Caterpillars. It may seem incredible, that there is any such thing as a caterpillar whose habitation is under water; but experience and observation prove, that there are such, and that they feed on the water-plants as regularly as the common
common kinds do on those at land. These are not named at random like many of the aquatic animals of the larger kinds, as the sea-wolf, the sea-horse, &c. which might as well be called any thing else as wolves and horses; but they are properly what they are called, and do not respire in the manner of the fish-tribe, but by their stigmata as other caterpillars. M. Reaumur, in his observations, met with two species of these; the one upon the potamogiton or pond-weed, the other upon the lenticula or duck-meat. These are both very industrious animals; but the first being much the largest, its operations are more easily distinguished.
This, though truly an aquatic animal, swims but badly, and does not at all love to wet itself. The parent butterfly lays her egg on a leaf of the potamogiton; and as soon as the young caterpillar is hatched, it gnaws out a piece of the leaf, of a roundish shape. This it carries to another part of the same leaf, and lays it in such a manner, that there may be a hollow between, in which it may lodge. It then fastens down this piece to the larger leaf with silk of its own spinning; only leaving certain holes at which it can put out its head, and get to gnaw any of the leaves that are near. It only gets out, though the aperture be naturally small, since a little force from its body bends up the upper leaf, and bends down the lower, both being flexible; and when the creature is out, it has a sort of down that defends it from being wetted, and the natural elasticity of the leaves and of the silk joins the aperture up again, so that no water can get in. The leaves of this kind of plant are also naturally very slippery, and not easily wetted by water. It soon happens that this habitation becomes too small for the animal, in which case it makes just such another; and after that, at times, several others; each being only made fit for it at the size it is then of. The changes of this creature into the chrysalis and butterfly states are in the common method. The butterfly gets out of a chrysalis which was placed on the surface of the water; the lightness of the animal easily sustains it on the water till its wings are dried, and then it leaves that element, never to return to it again.