d of its life-story
must be wrought out.

Reverting to the first great division of the Diptera, we find varied
adaptations to aquatic life among many grubs that possess a definite
head. The larva of a Gnat (Culex[9]) has projecting from the hind region
of the abdomen a long tubular outgrowth, at the end of which are the
spiracles, guarded by three pointed flaps forming a valve. When closed
these pierce the surface-film of the water in which the larva lives;
when opened a little cup-like depression is formed in the surface-film,
from which the larva hangs. Or having accumulated a supply of air, it
can disengage itself from the surface-film and dive through the water,
its tracheal system safely closed. Another mode of breathing is found in
the 'Blood-worms' and allied larvae of the Harlequin-midges
(Chironomidae) whose transformations are described in detail by Miall
and Hammond (1900). These larvae have two pairs of cylindrical,
spine-bearing pro-legs--one on the prothorax and the other on the
hindmost abdominal segment; the latter structures serve to fix the
larva in the muddy tube which it inhabits at the bottom of its native
pond. The penultimate abdominal segment has four long hollow outgrowths,
which contain blood, and have the function of gills, while the hindmost
segment has four shorter outgrowths of the same nature. Enabled thus to
breathe dissolved air, the Chironomus larva needs not, like the Culex or
the Eristalis, to find contact with the atmosphere beyond the
surface-film.

[9] See _Frontispiece_, A.

Most remarkable, in many respects, of all aquatic larvae are the grubs
of the Sand-midges (Simulium). These live entirely submerged and, having
no special gills, carry out an exchange of gases through the general
surface of the cuticle between the dissolved air in the water and the
cavities of the air-tube system. The body is shaped like a flask swollen
slightly at the hinder end and possesses a median pro-leg just behind
the head, also another at the tail, which serves to attach the larva to
a stone or to the leaf of an aquatic plant. The head has, in addition to
feelers and jaws, a pair of processes with wonderful fringes which by
their motion set up currents in the water, and bring food particles
within reach of the mouth. A number of the larvae usually live in a
community. Their power of spinning silken threads by which they can work
their way back when accidentally dislodged from their resting-place, h