The author draws attention to the disproportionately large head of the larvae, which, however, he did not describe, as he had ‘brought some of them alive, and some eggs for exhibition.’
Sialis larvae occur in most ponds with muddy bottoms. They may be taken by scooping up some of the mud in a long-handled spoon—a most useful instrument for the collector—and washing it, or by throwing in the drag, and bringing to land a mass of water-weed, roots and all. A few may generally be detected near the roots. They may be picked up with a small pair of forceps, or with a brush, and dropped into a bottle; or, better still, into separate tubes; for they are fierce and voracious, and, failing other food, by no means indisposed to prey on each other.
Their general appearance, and especially their powerful mandibles, give them some resemblance to the larvae of a water beetle, for which a celebrated naturalist not unnaturally took them, when he began to study them. And this would seem to be the opinion of some mounters, for I have a slide of the mouth parts of this larvae, labelled ‘Mouth parts of the larvae of a water beetle.’ It was not till I had broken up a good many Sialis larvae that I found out what the slide really was.
These larvae may be kept alive in small bottles of water, if they are supplied with food. They will eat Chironomus larvae and those of Tanypus. Professor Miall has found that they will eat caddis-worms and May-fly larvae. Probably, no small aquatic creatures that they can overcome are safe from them.
A larva that is full-fed, and ready to change to become a pupa, will measure about an inch in length or a little more (Fig. 85). The general colour is brownish, with dark markings. The legs are powerful, and our lens will show us that they end in two strong curved claws. From each of the first seven segments of the abdomen are given off a pair of jointed tracheal gills or breathing-organs, which are directed upwards and backwards when the larva is at rest—a rare occurrence—but which wave to and fro in the water when the creature is swimming.
From this fact has arisen the statement found in most books that the larva uses these gills not only for respiration, but for locomotion. Professor Miall has come to a contrary conclusion, and, as he has kindly informed me, is confirmed in his opinion by the weakness of the muscles.
Fig. 85.—Larva of Sialis (enlarged).
It will be well to make repeated observations till we are satisfied on the subject. When these larvae are kept, the conditions necessary to provide them with food and to keep the water aërated by means of growing vegetation are unfavourable to close observation. It will, therefore, be necessary to remove one or more of these larvae to a bottle in which there is nothing but pure water.
The work is now rendered much easier. There is nothing to obstruct. As soon as the larvae reach the bottom they will walk round and round, giving us a good opportunity of watching them. In swimming—which may be backwards as well as forwards—the abdomen is waved from side to side. To see the backward motion one need only put a dipping-tube or a pencil, or the like, in front of the larva, so as to bar its progress. The creature will retreat a step or two, and then, with a flourish of the abdomen, dart back through the water. The larva will sometimes wave the abdomen up and down, just as one may see a Chironomus larva do when it has affixed its dwelling to the side of the glass, and this motion probably assists the process of respiration.