The aquatic young of a stone-fly does not differ sufficiently in form
from its parent to warrant us in calling it a larva; the life-history is
like that of a cockroach, all the instars however except the final
one--the winged adult or _imago_--live in the water. The young of one of
our large species, a Perla for example, has well-chitinised cuticle,
broad head, powerful legs, long feelers and cerci like those of the
imago; its wings arise from external rudiments, which are conspicuous in
the later aquatic stages. But it lives completely submerged, usually
clinging or walking beneath the stones that lie in the bed of a clear
stream, and examination of the ventral aspect of the thorax reveals six
pairs of tufted gills, by means of which it is able to breathe the air
dissolved in the water wherein it lives. At the base of the tail-feelers
or cerci also, there are little tufts of thread-like gills as J.A.
Palmén (1877) has shown. An insect that is continually submerged and has
no contact with the upper air cannot breathe through a series of paired
spiracles, and during the aquatic life-period of the stone-fly these
remain closed. Nevertheless, breathing is carried on by means of the
ordinary system of branching air-tubes, the trunks of which are in
connection with the tufted hollow gill-filaments, through whose delicate
cuticle gaseous exchange can take place, though the method of this
exchange is as yet very imperfectly understood. When the stone-fly nymph
is fully grown, it comes out of the water and climbs to some convenient
eminence. The cuticle splits open along the back, and the imago, clothed
in its new cuticle, as yet soft and flexible, creeps out. The spiracles
are now open, and the stone-fly breathes atmospheric air like other
flying insects. But throughout its winged life, the stone-fly bears
memorials of its aquatic past in the little withered vestiges of gills
that can still be distinguished beneath the thorax.