In order to make out the scheme of the gills, which is somewhat complicated, one should first of all distinguish the spiracles, remembering that they are not functional. And it is best to begin with those on the middle segments of the body. They may be detected as little dark spots, sometimes enclosed in a ring. The head, the first segment of the thorax, and the last segment of the body, bear no gills; the second segment of the thorax has but two pairs on each side; and there is but a single gill on each side of the ninth segment of the body. On the other (nine) body-segments there are the full number of five gills on each side, arranged two above, and three below the spiracle. The upper pair are called supra-stigmals, or gills which lie _above_ the spiracles; the lower pair are called infra-stigmals, or gills which lie _below_ the spiracles; and the single one, the lowest, is known as the pedal or foot gill. These technicalities need not give us any trouble here, in our examination of the larva; nor do they present any real difficulty. But it is worth while to master the arrangement as soon as we get hold of one of these larvae, and then we shall be able to take up and understand technical descriptions of this aquatic caterpillar and its allies, in so far, at least, as they refer to the breathing apparatus. The gills differ in their character: some few are simple, while most of them are more or less branched. In Limnobia the branching of the gills is into a simple fork; in Paraponyx this kind of division also occurs, and in others most of the gill branches are also given off from the main stem below one of the branches of the fork. In Fig. 84 we have a representation of one of these branched gills. It will not be difficult for us to make out the details as there shown. But the vessels that run down into the filaments, constituting them breathing-organs, are smaller than those of Limnobia, and will consequently require a little more care and patience before we can distinguish them. [Illustration: +---------------+ | GILL GILL | | SPIRACLE | | GILL GILL | | GILL | +---------------+ FIG. 83.--Diagram of segment of Paraponyx, showing arrangements of tracheal gills.] [Illustration: FIG. 84.--Gill of Paraponyx larva. _a_, _b_, stem; _c_, _d_, _e_, _f_, branches; _g_, attachment of the air-vessel of the gill to the main tracheae of the body. (After De Geer.)]