Limnophilides.—These Insects have only three, instead of four, joints in the maxillary palpi, but in most other respects agree with the Phryganeides. There is, however, greater variety in the habits of the larvae, though all live in free cases. In the genus Enoicyla (Fig. 326) we meet with the anomaly of a Trichopterous Insect that lives amongst moss and dead leaves, far away, it may be, from water. The cases of the Limnophilides are constructed of a great variety of materials, and are often decorated with shells containing living inmates.
In the genus Apatania the phenomenon of parthenogenesis is thought to occur, there being at least two species in which no male specimen has ever been discovered, though M‘Lachlan has made special efforts to discover the sex of A. muliebris. It should, however, be stated that these species have not been extensively investigated; A. arctica has been detected in the Arctic regions, and A. muliebris has occurred in several localities in Europe, in Britain chiefly near Arundel in a lake of intensely cold water.
Fig. 326.—Metamorphoses of Enoicyla pusilla. (After Ritsema.) A, Case of full-grown larva; B, larva and case magnified; C, larva extracted; D, wingless adult female; E, male.
Sericostomatides, like the Limnophilides, is a group rich in species; the larvae are chiefly found in streams. They form portable cases out of sand and stones (Fig. 325, B, case of Crunoecia irrorata) in preference to vegetable matter. It is here that the genus Helicopsyche, which for long was an enigma to naturalists, is now placed. This genus consists of Insects whose larvae form spiral cases, similar to small snail shells, of sand or minute stones. These objects occur in various parts of the world. Fritz Müller[[406]] has informed us that the larva inhabiting one of them, when it withdraws entirely within its abode to repose, takes the precaution of anchoring its snail-like habitation, fixing it to a rock or stone by spinning some temporary silken threads. The respiratory filaments in this group are filiform.
Fig. 327.—Cases of Helicopsyche shuttleworthi. (After von Siebold.) A, Natural size; B, C, magnified.
Leptocerides.—The first group of the division Aequipalpia; so that there are five-jointed maxillary palpi in both sexes; these organs are frequently developed in a remarkable manner. The antennae are usually extremely long and slender. The case of the larva is portable (Fig. 325, A, case of Odontocerum); the respiratory filaments are not very conspicuous; they form short tufts placed on various parts of the abdomen. Müller[[406]] has called attention to a species whose larva lives in Brazil between the leaves of Bromeliae on trees.
The Oestropsides is a small group, and has recently been reduced by M‘Lachlan to the rank of an inferior division.
Hydropsychides.—An extensive group, in which the larvae are believed to be chiefly of carnivorous habits. They vary, according to species, as to the nature of the respiratory filaments, and live in fixed abodes; these are less tubular than is the rule with the portable cases, and are formed from pieces of sand and stone spun together and fixed to larger stones under water. Sometimes several larvae live together in loosely compacted structures of this kind, and only form true cases when about to undergo their metamorphosis. Müller describes[[406]] a Brazilian species of Rhyacophylax as forming a case in which the mouth-end has a large funnel-shaped verandah, covered by a beautiful silken net. This larva lives in the rapids of various rivulets, and the entrance to the verandah is invariably directed towards the upper part of the rivulet, so as to intercept any edible material brought down by the water. Several of these larvae, moreover, build their cases so that they form a transverse row on the upper side of a stone; as many as thirty cases may be placed in one of these rows, and sometimes several rows are placed parallel with one another. This same larva has the habit of coming out of its case when necessary, and suspending itself in the water—as some caterpillars do in the air—by means of a silken thread. Other members of the Hydropsychides form tubes or covered ways of silk, earth and mud attached to stones, and in which they can move freely about. Some of the Hydropsychidae have been ascertained with certainty to be carnivorous in the larval state. A species of the genus Hydropsyche has been found by Howard[[407]] to help itself in the task of procuring food by spreading a net in the water in connexion with the mouth of its case. This net is woven in wide meshes with extremely strong silk, and supported at the sides and top by bits of twigs and small portions of the stems of water-plants. Small larvae brought down by the current are arrested by this net for the advantage of the larva that lurks in the tube. The breathing organs of the larvae of Hydropsychides are apparently of a varied character, and would well repay a careful study. Mr. Morton informs the writer that some of our British species of Philopotamus and Tinodes have no gills either in the larval or pupal state, and probably respire by means of modified tracts in the integument. In some of the allied genera, e.g. Polycentropus, the larvae are destitute of gills, but the pupae possess them.