INSECTS. Insects are essentially creatures of the air and the land; yet a considerable number pass the whole or the greater part of their lives in rivers, lakes and ponds. Among insects aquatic in all stages we can distinguish between those which glide or skate over the surface of the water, diving not at all, or only exceptionally, and those which habitually dive and swim through the water after the manner of fishes. The most typical of the surface-dwellers are the bugs of the family Hydrometrida. See POND-SKATERS.
Among the Coleoptera the whirligig beetles (Gyrinidre) frequent the surface of ponds and brooks where they may be seen in small com panies, performing a whirling, mazy dance over the surface-film. These insects, when they dive, carry down with them a small air-bubble en closed in a film between the tip of the wing covers to the hinder end of the abdomen. They are not, like the pond-skaters, completely envel oped in air while under water. The beetles of a nearly related family (Dyticidce), well called belong to the group of insects which live habitually submerged. Their con tours are admirably adapted for motion through the water, but there is no dense hairy covering to ensure the formation of an air-bubble and the breathing is provided for in quite another way. The abdominal spiracles open on the upper surface of the segments, which are com pletely covered by the wing-cases when the wings are shut. The wing-cases being convex and the upper surface of the abdomen de pressed, a considerable amount of air is en closed, allowing the insett to remain submerged for some time.
Another mode of adaptation to life in the water is shown by the water-scorpions (Nepi da). They are provided with a pair of long grooved appendages at the tail-end of the body; these can he closely pressed together and form a tube, the tip of which pierces the surface-film and conveys a supply of air to the spiracles. These insects, like the allied °water-boatmen" (Notonertids), have well-developed wings, and make excursions by night to new watery dwell ing-places.
Many insects lead an aquatic life only dur ing their larval stage. Naturally enough, how ever, such insects when adult are to be found flying chiefly in the neighborhood of water in which they will lay their eggs — the May-flies and midges for example. The contrast between the conditions of the larval and the imaginal life in such cases is most striking, and can only have been brought about by slow degrees. A certain amount of moisture in the earth is necessary to the well-being of many burrowing larva, while some are found in semi-liquid mud, in decaying refuse, or in animal excre ment. In such surroundings breathing through
the lateral spiracles becomes impossible, and we find that access to the air-tubes takes place only by one or two pairs of spiracles near the head or tail-end of the body, sometimes open ing through respiratory trumpets° whose ex panded mouths can be thrust out of the clog ging surroundings of the mud or refuse into the fresh air, while the grub remains concealed and continues to feed. A similar suppression of most of the spiracles, with the development of a tubular process at the tail end of the body in connection with the tracheal system, is the adaptation by which many aquatic larva breathe — for example, the grub of the mosquito. The families of insects nearly related to these have larva which live in mud and damp earth, and this suggests that it was from the shores that the waters were invaded by these insect-hosts.
But there is another division of aquatic larva still more perfectly adapted to life in the water. The grub of the gnat or the drone-fly needs to rise to the surface at intervals and pierce the film with its air-tube in order to get a fresh supply of oxygen. But the pupa of the sand-midge, with its tubular gill-filaments, or the larva of a May-fly with its tracheal gill plates, can remain in the water throughout its life, drawing, as do the fishes, sufficient oxygen from the dissolved air. It is interesting to notice that within the limits of a single and restricted order — the dragon-flies— we find some larva breathing by means of tracheal ?ill plates, and others taking supplies of water into the hind-gut over whose walls run branching air-tubes; while in the final nymph stage the thoracic spiracles are open, and the insect rais ing the front part of its body above the sur face, breathes through them after the matmer of an imago. These various adaptations to an aquatic life within a single group indicate clearly that the habit of living in water is not primitive among insects, but that it has become acquired by different races at different times in the course of the development. It may be presumed that larva with the more perfect adaptations for breathing when submerged leaf-like or thread-like gills — are older inhab itants of the water than those which have to rise periodically to the surface to take in a supply of air.