The following paper is a short description of the metamorphosis of a species of Caddis Fly (Phryganidæ) which I have worked out during the past spring, and which, I believe, differs in several respects from any previously described. The imago is also interesting, as it bears such a close resemblance to a Lepidopterous insect; and, were its preparatory states unknown, it would very probably be catalogued as one of the Tineina.
Many entomologists are inclined to regard the Phryganidæ as a family of the Lepidoptera, and there is no doubt that a very close affinity exists between them; in fact one family of typical Lepidoptera (the Hydrocampidæ) are strictly aquatic in their habits, the larvæ constructing cases of duckweed, which they pull after them, holding on to the case inside by two hooks, exactly like the caddis worms; the pupa state is also passed floating on the surface of the water, and the moths are commonly taken flying over ponds during the summer.
The larva of this present insect (fig. 1) may be found commonly in the green, slimy weed floating in large masses on all stagnant waters. Being very small, it is rather difficult to detect, and is best procured by washing a small quantity of the weed in a saucer of water, when the little insects will be at once seen walking about at the bottom. On examination with the microscope, the case will first arrest attention, being of a most unique structure; its shape is best described as closely resembling that of a minute flask, very much flattened at the lower end, and almost transparent; its surface is slightly corrugated, and the neck of the flask constructed of a much denser material than the rest. It is open at both ends, the posterior end being perforated by a long, shallow slit, which extends for nearly the whole width of the case, thus admitting a free circulation of water round the larva, who is also able to turn round and project his head and anterior segments through the lower aperture, thus occupying the reverse position to that shown in the illustration. He is, however,
prevented from actually leaving the case by his abdomen, which is too large to be withdrawn from either end. The head and thorax of the larva are very strong in comparison with those portions permanently retained inside, the legs being constructed to fold up into the smallest possible compass, a cavity existing in each joint for the preceding one, this being a structure which is almost universal among the caddis worms. The two organs situated on the posterior segments are doubtless respiratory in their function, a large air-tube taking its rise from each, and ramifying through the body in all directions.
When alarmed, these insects retreat into their cases with lightning rapidity, remaining concealed until the danger is passed. Their food probably consists of the green weed, although they are perhaps carnivorous, feeding on the rotifers and other animalculæ, which swarm in the water where they are found.
With regard to the method employed by the young larva in constructing, and subsequently enlarging, his case, I can give no positive information, although it is undoubtedly made of a viscous fluid secreted by the insect, which hardens when exposed to the water; this secretion is no doubt analogous to the silk of caterpillars, which always exists in the form of a gummy fluid before being spun. In a few Lepidopterous larvæ (Cerura, etc.) it is employed as such to construct the cocoon, which is consequently of a much stronger consistence than where the ordinary silk is used. When about to change, the insect fixes his case down by four ligaments, two at each end, the extremities of these being firmly fastened on to a stone; he then closes the small aperture, and constructs a curious arch-shaped partition of dense material inside, a short distance from the broad end (fig. 2). In about a week's time he is transformed into a pupa, having the limbs, etc., free from the body, but incapable of motion. The fixing down of the case prior to the change may be easily performed by the larva from each of the apertures, which are no doubt left open till the last for this purpose. Before the final transformation the pupa breaks through the partition at the broad end of the case, and wriggles to the surface, the imago ascending a blade of grass to dry and expand its wings. The little exuvia of the pupa may be often noticed floating on the water, and the empty cases are very conspicuous on the sides of a glass aquarium, where the insects generally fix them down when in captivity.
This Caddis Fly (fig. 3) must be tolerably common during the summer, but owing to its small size would not be likely to attract attention. I have never observed it in a state of nature, all my specimens having been reared from the larva. It is probably undescribed; but as there are no catalogues of these insects at present published, it is impossible to speak with certainty on this point.