This is shown not only by the possession of wings by a vast majority of
the class, but by the mode of breathing to which reference has already
been made (p. 2), a system of branching air-tubes carrying atmospheric
air with its combustion-supporting oxygen to all the insect's tissues.
The air gains access to these tubes through a number of paired air-holes
or spiracles, arranged segmentally in series.

It is of great interest to find that, nevertheless, a number of insects
spend much of their time under water. This is true of not a few in the
perfect winged state, as for example aquatic beetles and water-bugs
('boatmen' and 'scorpions') which have some way of protecting their
spiracles when submerged, and, possessing usually the power of flight,
can pass on occasion from pond or stream to upper air. But it is
advisable in connection with our present subject to dwell especially on
some insects that remain continually under water till they are ready to
undergo their final moult and attain the winged state, which they pass
entirely in the air. The preparatory instars of such insects are
aquatic; the adult instar is aerial. All may-flies, dragon-flies, and
caddis-flies, many beetles and two-winged flies, and a few moths thus
divide their life-story between the water and the air. For the present
we confine attention to the Stone-flies, the May-flies, and the
Dragon-flies, three well-known orders of insects respectively called by
systematists the Plecoptera, the Ephemeroptera and the Odonata.

In the case of many insects that have aquatic larvae, the latter are
provided with some arrangement for enabling them to reach atmospheric
air through the surface-film of the water. But the larva of a stone-fly,
a dragon-fly, or a may-fly is adapted more completely than these for
aquatic life; it can, by means of gills of some kind, breathe the air
dissolved in water.