Fig. 288.—Sialis lutaria, larva.
It is very unattractive in appearance, being of a blackish colour, with wings of a yellow-brown tinge, and makes but a poor show when flying. The female deposits patches of elongate eggs, placed on end and packed together in a very clever manner (Fig. 287). These patches of eggs, of a stone-gray colour, are common objects on rushes or stems of grass near water, and it is stated that there may be no less than 2000 or 3000 eggs in one of them. Our figure gives some idea of the mode in which the eggs are arranged, and the curious narrow process that exists at the end of each. The eggs are said to be sometimes placed at a considerable distance from water, so that when the tiny larvae are hatched they must begin their lives by finding the way to a suitable pool or stream. The larvae (Fig. 288) are objects of very great interest owing to each of segments 1 to 7 of the hind body being furnished on each side with a jointed filament, while the last segment ends in a still longer, but unjointed process. These filaments are branchiae by means of which the Insect obtains air, being, as we have said, destitute of spiracles. It is an active creature and waves its filaments in a very graceful manner; this process no doubt aids the branchiae in their respiratory work. These larvae are well able to exist out of water if they have a sufficiently damp environment. They live on animal matter, but their life-history has not been followed in much detail and it is not known how many moults they make. The young larva has the head disproportionately large and the branchial filaments longer. When the growth is completed the larva returns to land, seeks a suitable situation in the soil, and after an interval changes to a pupa, in which the characters of the perfect Insect are plainly visible. Subsequently, without becoming again active, it changes to the perfect Insect, and enjoys, for a few days only, an aerial life.
The anatomy of the larva has been treated by Dufour.[[370]] The supra-oesophageal ganglion is remarkably small; nothing is said as to the existence of an infra-oesophageal ganglion; there are three thoracic and eight abdominal ganglia; the first pair of these latter are nearer together than the others, and this is also the case with the last three. The alimentary canal in the adult is provided with a large paunch attached to the crop by a narrow neck,[[371]] but Dufour could find no trace of this in the larva. The structure of the branchiae has also been described by the indefatigable French entomotomist. A tracheal tube sends a branch into one of the appendages (Fig. 289); the branch gives off numerous smaller tracheae, which at their extremities break up into branchlets close to the integument. The tracheal tube that receives each main branchial trachea, sends off from near the point of entry of the latter another trachea, that distributes its branchlets on the alimentary canal. The margins of each appendage are set with swimming hairs, so that the branchiae act as organs of locomotion as well as of respiration, and by their activity in the former capacity increase the efficiency of their primary function.
Fig. 289.—Structure of tracheal gill of Sialis lutaria. (After Dufour.) a, Base of the gill; b, tracheal trunk with which it is connected; c, trachea given off to alimentary canal.
The genus Sialis occurs in a few species only, throughout the whole of the Palaearctic and Nearctic regions, and reappears in Chili,[[372]] though absent in all the intervening area. Several other genera of Insects exhibit the same peculiarity of distribution.
Fig. 290.—Corydalis crassicornis, male, with greater portions of the wings removed. Texas. (After M‘Lachlan.)
Fig. 291.—Raphidia notata, female. Britain. (After Curtis.)