The opening and closing of these loops may be observed at leisure if a larva be put in a long excavated slip, with some water, and then covered with a plain glass slip. The two slips, fastened together with small elastic bands, should then be laid on the stage of the dissecting microscope for examination; or they may be held in the hands, and the movements of the larva watched with the hand lens. In the posterior segments of the body the tracheal tubes run side by side, while in the tail itself they are, so to speak, intertwined. When the tip of the tail pierces the surface-film a fresh supply of air is taken in. At the base of the extensile portion are two processes which diverge, one on each side, at an angle of 45 degrees. These, according to a German observer, are tracheal gills, and they are absorbed just before the larva enters the pupal condition. Réaumur found these larvae plentifully in the Bois de Boulogne, and gives a figure[67]. He was not, however, successful in rearing the fly. Lyonnet not only took the larvae and kept them in an aquarium, but watched their change into the pupal condition, and saw them emerge as perfect insects. An abstract of this description will probably be of interest. [Illustration: FIG. 89.--_A._ Ptychoptera larva (enlarged). _B._ Tail, showing air-vessels (still more enlarged). (After Lyonnet.)] He tells us that the larvae showed signs of changing into pupae in June. The change was made without the larvae leaving the water, and they underwent all their transformations in less than a fortnight. At the approach of the change the larvae became whiter in colour, but less transparent. Then they cast their skin, leaving therein the air-vessels, or rather their external covering. After this last moult was over, he was surprised to find that the tube which formed the tail of the larva, and by which it took in a supply of air, though it serves the same purpose in the pupa, is attached to the thorax, near the top of the head[68]. Lyonnet appears to have overlooked the fact that there was a second and shorter tube given off from the thorax, which most observers consider to be functionless (Fig. 90). [Illustration: FIG. 90.--Pupa of Ptychoptera. (After Lyonnet.)] Strange as is the larva, the pupa is stranger still, and seems even better adapted for existence in the mud. The hinder part of each segment of the abdomen is thickened and studded with chitinous projections. This thickening is more marked, and of greater extent in each succeeding segment, reaching its greatest development in the last segment, which is armed with hooks. The body part of each segment bears rows of smaller spines, so that this pupa should have little or no difficulty in moving through pretty thick mud.