of thread-like gills as J.A.
Palmen (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.

The adult dragon-fly (fig. 8 _d_) is specialised in such a way that it
captures its prey--flies and other small insects--on the wing, swooping
through the air like a hawk and feeding voraciously. The head is
remarkable for its large globular compound eyes, its short bristle-like
feelers, and its very strong mandibles which bite up the bodies of the
victims. The thorax bears the two pairs of ample wings, firm and almost
glassy in texture, and its segments are projected forward ventrally, so
that all six legs, which are armed with rows of sharp, slender spines,
can be held in front of the mouth, where they form an effective
fly-trap. The abdomen is very long and usually narrow.

A female dragon-fly after a remarkable mode of pairing, the details of
which are beside our present subject, drops her eggs in the water, or
lays them on water-weeds, perhaps cutting an incision where they can be
the more safely lodged, or even goes down below the surface and deposits
them in the mud at the bottom of a pond. From the eggs are hatched the
aquatic larvae which differ in many respects from the imago. The
dragon-fly larva has the same predaceous mode of life as its parent, but
it is sluggish in habit, lurking for its prey at the bottom of the pond,
among the mud or vegetation, which it resembles in colour. The thoracic
segments have not the specialisation that they show in the imago; th