The imagines of certain species possess filaments—or something of the sort—on the abdomen. Palmén, who has examined these organs in Hydropsyche, thinks that they are the remains of gills that existed in the larva and pupa, and that they are functionless in the imaginal instar. M‘Lachlan thinks that in Diplectrona, where the filaments are elongate, they may be functionally active even in the imago.[[403]]
The skin of the nymph is at first very soft, but it soon hardens, and when about fifteen or twenty days have elapsed the nymph opens its case by means of the mandibular processes, and swims through the water with its back downwards till it reaches some solid object by which it can ascend to the air; the nymph skin then swells and splits, and the thorax of the imago protrudes; this is soon followed by the disengagement of the head and other parts, and the imago having thus escaped, the nymph skin remains a complete model of the external structure of the nymph, and contains a considerable number of tracheae. This sketch of the metamorphosis of a caddis-fly does not apply in all its details to all the forms of caddis-flies, there being exceptions, as we shall mention hereafter.
Dewitz has described[[404]] the first appearance and development of the wings in larvae of Phryganeidae. Each one appears at first in the form of a small thickening of the hypodermis, accompanied outwardly by a minute depression of the chitin (Fig. 324, A). He compares the structure in the earliest stage to the entothoracic projections into the interior of the body. The rudiment grows as the larva increases in size, the chitinous portion being duly shed at the ecdyses. When the rudiment is larger and more complex, a mesoderm layer appears in it (Fig. 324, B); this is derived from a nerve-sheath near the rudiment. During the resting state of the larva—after its case has been closed, but before the pupal form has appeared—the wing assumes the form and position shown in C, Fig. 324. Dewitz's description of the process leaves much to be desired, and it is doubtful whether in C the position of the wing on the exterior of the body is due to the stripping off of the chitinous integument, or to a process of eversion, or to both.
Fig. 324.—Development of wings of Phryganeidae. (After Dewitz.) A, Portion of body-wall of young larva of Trichostegia; ch, chitin, forming at r a projection into the hypodermis m; r and d forming thus the first rudiment of the wing. B, The parts in a largely grown larva; a, c, d, b, the much grown hypodermis separated into two parts by r, the penetrating extension of the chitin; v, mesoderm. C, Wing-pad of another Phryganeid freed from its case at its change to the pupa; b, d, outer layer of the hypodermis, m, of the body-wall; v, inner layer without nuclei.
Fig. 325.—Cases of British Trichoptera. A, Of Odontocerum albicorne; A1, its termination; B, quadrangular case of Crunoecia irrorata; B1, mouth of case.
There are about 500 species of this family of Insects known as inhabiting the European region, and about 150 of this number occur in Britain. These are arranged by M‘Lachlan[[405]]—whose zealous and persevering work at this neglected family of Insects is beyond praise—in eight sub-families, on a system in which the structure of the maxillary palpi plays a principal part; they are called Phryganeides, Limnophilides, Sericostomatides, Leptocerides, Oestropsides, Hydropsychides, Rhyacophilides, Hydroptilides. The first three of these form the division "Inaequipalpia."
Phryganeides.—This group includes the largest forms of the family, and appears to be almost confined to the temperate regions of the northern hemisphere, though a few species are already known from the corresponding districts of the southern hemisphere. This feature in their geographical distribution is, however, by no means peculiar to them, for a similar discontinuity of distribution exists in numerous other groups of Insects, and even in other divisions of the Phryganeidae.
The Phryganeides almost without exception inhabit still waters, and it is more specially to them that the brief sketch of metamorphosis given in the preceding pages will be found to apply. The larva always has the respiratory filaments simple and thread-like, though elongate, and lives in a case that it carries about; this case is open at both ends, and the larva is said to occasionally cut off the end having the least diameter and increase the other end, thus accommodating the habitation to its own growth.