Through a pocket lens

by Henry Scherren

When the larvae have been watched under the conditions above described, I have never been able to detect independent motion of the gills. But it is better that every one should observe for himself, and draw his own conclusions from what he sees.

Now let us examine a specimen more closely with the lens, or under the dissecting microscope. The mouth parts may be broken up separately, or a little careful manipulation will enable us to see the chief features without injuring the specimen. The head is strong and massive, and the group of ocelli, or simple eyes, may be clearly made out. The antennae bear a small pencil of hairs, no doubt sensory in function, at the extremity, but careful management of the light will be required to distinguish them. The mandibles are extremely business-like instruments, and each bears two strong teeth on the inner side. Next come the maxillae, with their palps, each of which has an appendage, while each maxilla carries three strong spines. The labium, with its palps, and the mentum, with its saw-like notchings, may be plainly seen.

The three segments of the thorax offer no difficulty. The legs are worth more than a cursory examination from their apparatus of spines and double fringe of hairs. Nine body-segments succeed to the thorax, and behind these is a long tail-like organ, which some authors consider represents a tenth segment.

Fig. 86.—Diagram of Sialis larva, showing arrangement of gills (represented by vertical lines). h, head; t, thorax; 1–9, segments of abdomen.

The gills are seven on each side, and are given off from the spaces in front of the first seven segments of the abdomen. Each gill is five-jointed—an unusual arrangement, for the gills of the larvae of Limnobia and Gyrinus are without joints. With the half-inch Leitz the branching tracheal tube in the gill may be seen, as well as the double fringe of hairs and the long hairs at the extremity. The tail-like organ, though without joints, bears a close resemblance to the gills. It has two tracheal tubes running through it, and it is fringed on both sides with hair. Indeed, Professor Miall, F.R.S., compares it to ‘two ordinary tracheal gills completely fused together.’ The first glance will convince the observer that the comparison is just.

Fig. 87.—Pupa of Sialis.

The pupa (Fig. 87) need not detain us, for the larva undergoes its transformation in the ground, not in the water, where it could be watched. But it is interesting to notice that the legs and wings are enclosed in separate cases, and that the segments of the abdomen bear spines. These spines are extremely serviceable to the pupa when making its way out of its cell to emerge as a perfect insect, which is well known, especially to fishermen, as the Alder-fly. It may be found near streams, and rarely uses its wings.

Ptychoptera paludosa is a small Crane-fly, with an aquatic larva which will repay observation. It is one of the group generally called ‘rat-tailed maggots,’ from the peculiar character of the breathing apparatus, which consists of a retractile tube at the end of the abdomen. It is, I believe, better known to some dealers than the larva of the Drone-fly, the rat-tailed maggot of Réaumur. A few months ago I wanted some Drone-fly larvae, and asked a dealer to supply me. When the larvae arrived and were turned out for examination, they proved to be Ptychoptera larvae—which I did not want. I naturally wrote to point out the mistake; and was told, in reply, that the larvae sent were the only ‘rat-tails’ known to my correspondent.