Many aquatic insects and certain of our favorite fresh water fishes can not survive in waters with reduced oxygen levels. The presence or absence of these animals that require higher levels of disolved oxygen can be used to assess the relative environmental health of a water system. Such species are referred to as indicator organisms. More germane to our topic, however, is understanding that insects and other aquatic animals have potential problems of low oxygen availability. One solution to the problem of obtaining oxygen while living in aquatic environments is to directly breathe atmospheric air.
Many insects have respiratory siphons.
In insects with this sort of anatomical arrangement only the posterior spiracles are functional, and they are located at the end of a siphon such that only the posterior portion of the insect penetrates the surface film of water, while the rest of the body is submerged, often suspended from the surface film. The respiratory siphon on larvae of drone flies, Eristalis , is telescopic, not meant to suggest one can see through it. Mosquito larvae also have respiratory siphons, but they are not so long as in drone flies. One constraint on the function of respiratory siphons is that the tips of siphon tubes must be arranged such that air can move into the siphon while water is excluded. Commonly, siphons have hydrofuge hairs associated with their spiracles.