Fig. 328.—Case, with head of larva and snare of North American Hydropsychid. (After Riley and Howard.)
The Rhyacophilides is another group in which the larval habitations are fixed. Some of these larvae have no respiratory filaments, breathing only by means of the stigmata, but others have tufts of filaments. These Insects have a peculiarity in their metamorphosis inasmuch as the larva, instead of lying free, constructs a cocoon in its case or other habitation in which to change to a nymph. In the larvae that do not make use of a portable case the abdominal hooks are not essential, and are replaced by other organs differing much in structure, being sometimes apparently of a sensitive nature, in other forms possibly respiratory. Müller tells us of a carnivorous larva of this group in which the anterior legs are armed with powerful forceps for predatory purposes.
The Hydroptilides comprise the most minute of the Phryganeidae, and their species will probably prove to be very numerous in well-watered tropical regions, though few have yet been described from there. The perfect Insects (Fig. 320) bear an extreme resemblance to small moths of the group Tineidae. The larvae (Fig. 329) are destitute of respiratory filaments, and construct portable cases of a variety of forms, some resembling seeds. Müller has given particulars of a curious nature as to the cases of some Brazilian Hydroptilides; one species moors its dwelling to a stone by means of a long silken cable, by this artifice combining safety with the power of ranging over a considerable extent of water. In Diaulus there is only a narrow slit at each end of the case, but one side of it is provided with two chimneys to permit the flow of water for respiratory purposes.
Fig. 329.—Hydroptila maclachlani. B, Case with larva magnified; A, larva more magnified. (After Klapálek.)
The larva of Oxyethira (Fig. 330) is a curious form, possessing comparatively long legs, and a head and thorax slender in comparison with the distended hind body. The cases are fastened, for the purposes of pupation, to a leaf of a water-lily.
Some very curious anomalies as regards the development of the wings exist in the Phryganeidae; Anomalopteryx, for instance, has the wings quite short and useless for flight in the male, while in the other sex they are ample; in Enoicyla—the curious Insect figured on p. 481, in which the larvae are of terrestrial habits—we find the females with only rudiments of wings, while in Thamastes the posterior wings are absent in both sexes. These anomalies are at present quite inexplicable; and we may here mention that we are in complete ignorance as to the functional importance of many of the peculiarities of the Phryganeidae. We do not know why the mouth is reduced from the normal state, the maxillary palpi being, on the other hand, extraordinarily developed; we do not know the importance of the numerous spines and of the spurs on the legs, nor of the hairs on the wings, although these are amongst the most characteristic of the special features of this group of Insects.
Fig. 330.—Oxyethira costalis. A, Larva in case; B, cases fastened to leaf for pupation. (After Klapálek.)