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. [Illustration: 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. This larva is a mud-dweller, and is best captured by scooping up surface mud near the banks of pools and ditches, just where the water shallows on to the shore. This should be washed in a small dish or saucer, so as to carry away the mud and leave the larvae wriggling on the bottom. They may be picked up with a brush and dropped into a bottle for transport home. There is not the slightest difficulty about keeping them for observation. A bottle of the capacity of six or eight fluid ounces will make a good aquarium for a dozen or even twenty. The bottom should be covered to the depth of about an inch with mud fairly rich in organic matter. My own plan has been to use the accumulation from the bottom of a large aquarium. In this the larvae will bury the body, and feed, the tail protruding and thrust up to the surface of the water, of which there should be about two inches above the mud.