Fig. 321.—Front view of head of Anabolia furcata after removal of labrum. o, Ocellus; an, base of antenna; au, eye; cm, cardo; st, stipes; l, external lobe; pt, support of palpus; pm, palpus of maxilla; g, condyle of articulation of the absent mandible; ha, channel of haustellum; h, haustellum; sp, apex of channel of haustellum (not explained by Lucas); ch, chitinous point of external lobe of second maxilla; pl, labial palp. (After Lucas.)
The structure of the mouth-parts of the Phryganeidae has given rise to much difference of interpretation; it has recently been investigated by R. Lucas[[401]] in connexion with Anabolia furcata (Fig. 321). He agrees with other observers that mandibles are present in the pupa, but states that no rudiment of them exists in the imago. He calls the peculiar structure formed by the combination of the maxillae and labium a haustellum. He looks on the Trichoptera as possessing a mouth intermediate between the biting and sucking types of Insect-mouths. He considers that the Phryganeidae take food of a solid, as well as of a liquid, nature by means of the haustellum, but the solid matter must be in the form of small particles, and then is probably sucked up by the help of saliva added to it. Lucas says also that in the larvae certain parts of the salivary glands serve the function of spinning organs, and it is from these that the salivary glands of the imago are formed; those salivary glands of the larva that are not spinning glands disappearing entirely.
Fig. 322.—Anabolia nervosa. A, Larva extracted from its case; B, one of the dorsal spaces of the abdominal segments more strongly magnified.
The eggs are deposited in a singular manner; they are extruded in a mass surrounded by jelly; there may be as many as one hundred eggs in such a mass. This is sometimes carried about by the female after its extrusion from the interior of the body, but is finally confided to a suitable place in stream, spring, or pool. It is said that the female occasionally descends into the water to affix the egg-mass to some object therein, but this requires confirmation, and it is more probable that the egg-mass is merely dropped in a suitable situation. As soon as the larvae are hatched they begin to provide themselves with cases; they select small pieces of such material as may be at hand in the water, and connect them together by means of silk spun from the mouth. Particulars as to these tubes we will defer till we have considered the larvae themselves. These have the general appearance of caterpillars of moths; in order to move about they must put their head and the three pairs of legs at the front of the body out of their tube or case, and they then look very like case-bearing caterpillars. The part of the body that usually remains under cover is different in texture and colour, and frequently bears outstanding processes, or filaments, containing tracheae for the purpose of extracting air from the water. Some peculiar spaces of a different texture may be seen on certain larvae (Fig. 322, B); these may possibly be also connected with respiration. On each side of the extremity of the body there is a rather large hook by which the creature attaches its dwelling to its body, and there are also frequently present three large bosses on the anterior abdominal segment, which are supposed to assist towards the same end. The hold it thus obtains is so firm that it cannot be dragged out by pulling from the front; fishermen have, however, discovered a way of extracting it by a strategic operation: the cases are, as a rule, partially open behind, and by putting a blunt object in and annoying the larva it is induced to relax the hold of its hooks and advance forwards in the case, or even to leave it altogether. The firm hold of the larva is maintained in spite of the fact that the body does not fill the case. It is necessary that water should pass freely into and out of the case, and that there should be some space for the respiratory filaments to move in. The mouth of the case is open, and the posterior extremity is arranged by the larva in such manner as to allow a passage for the water; various ingenious devices are adopted by different species of larvae with the object of protecting the hind end of the body, and at the same time of permitting water to pass through the case.
The mode of changing the skin, or the frequency with which this occurs in the larval state of the caddis flies has not been recorded. The duration of life in this stage is usually considerable, extending over several months: indeed in our climate many species pass the winter in this stage, completing the metamorphosis in the following spring or summer; and as one generation each year appears to be the rule, it may be assumed that the larval condition in such cases lasts from seven to ten months. During this stage the Insects are chiefly vegetable feeders, some being said to feed on minute algae; animal diet is not, however, entirely avoided, and it is said by Pictet that not only do some of the Phryganeidae eat other Insects, but that they also sometimes devour their companions.
Fig. 323.—A, Pupa of Phryganea pilosa. (After Pictet.) B, Mandibles of pupa of Molanna angustata.
At the end of the larval period of existence the creature closes its case by a light web spun at each end, taking care not to prevent the ingress and egress of the water; it sometimes adds a stone or piece of stick, and having thus protected itself, changes to a nymph. During the first part of this metamorphosis the creature is completely helpless, for there is so great a difference between the external structures of the larva and nymph as to make the latter a new being, so far as these organs are concerned. The changes take place in the interior of the larval skin, and as they are completed this latter is shed piecemeal. The resulting pupa or nymph greatly resembles the perfect Insect, differing consequently very much from the larva. Pictet, who paid special attention to the nymph condition of these Insects, concludes, however, that many of the organs of the nymph are actually formed within the corresponding parts of the larva, and has given a figure that, if trustworthy, shows that the legs of the nymph, notwithstanding the great difference between them as they exist in the larva and in the perfect Insect, are actually formed within the legs of the larva; each nymphal leg being rolled up in the skin of the corresponding larval leg, in a spiral, compressed manner, and the only articulations that can be detected in the leg being those of the tarsus. The head of the nymph is armed in front with two curious projections that are, in fact, enormously developed mandibles (Fig. 323, B); they serve as cutting implements to enable the nymph to effect its escape from its prison; they are cast off with the nymph-skin, the perfect Insect being thus destitute of these organs. The abdomen of the nymph differs from that of the perfect Insect in possessing external respiratory filaments; the nymphs of some species have also the middle legs provided with swimming-hairs, that do not exist in the imago.
As a rule the larvae bring the respiratory filaments into contact with the water by moving the abdomen, but Fritz Müller found[[402]] that those of a Macronema move the gills themselves—after the manner of Ephemeridae—with much rapidity. Many kinds of larvae of Phryganeids possess at the posterior extremity of the body exsertile pouches in the form of finger-like, or even branched, processes into which tracheae do not enter. Müller observed that in the Macronema alluded to these pouches were generally not exserted; when, however, the larva ceased to move the tracheal gills, then these pouches were protruded. He is inclined to consider them blood-gills. Similar structures are found in Eristalis and some other Dipterous larvae that have to breathe under difficulties.