he gnat would perforce have to dive into the water. With the
beautifully adapted transfer of the functional spiracles, their position
is appropriately arranged for the gnat's emergence at the surface, and
the empty pupal cuticle floats serving the insect as a raft. On this it
rests securely and the crumpled wings have opportunity to expand and
harden before the insect takes to flight.

[10] See _Frontispiece_, B.

The aquatic pupae of other Diptera, many species of the midges
Chironomus and Simulium for example, breathe dissolved air by means of
tufts of thread-like gills, which arise on either side of the prothorax.
The pupae of Simulium rest in their curious little cup-like dwellings,
attached to submerged stones or plants. The Chironomus pupa is usually
found in an elongate gelatinous case adhering to a stone. From this case
the pupa rises to the surface of the water, that the midge may emerge
into the air. Miall and Hammond (1900) describe the arrangement by
which, when the pupal stage ends, and these gills are no longer
required, their connection with the air-tube system is severed 'without
undue violence.' The walls of the fine air-tubes that pass into the
gills are specially strengthened, but just below the pupal cuticle these
walls are exceedingly thin and delicate. Thus when the pupal cuticle is
cast, they are readily broken there, and the cuticle of the midge
forming beneath has a spiracular opening into the main air-trunk, ready
for use during the insect's aerial life.

Among those Diptera whose larva is the headless maggot a most
remarkable arrangement for protecting the pupa is to be found. The last
larval cuticle, instead of being as usual worked off and cast, after
separation from the underlying structures, becomes hard and firm,
forming a protective case (_puparium_) within which by the processes of
histolysis and histogenesis already described the organs of the pupa and
imago are built up. This puparium (fig. 22 _d_) is usually dark in
colour, often brown and barrel-shaped, and a subcircular lid splits off
from it at the head-end to allow the emergence of the fly[11]. While the
maggot breathes by its tail-spiracles, the functional spiracles of the
puparium (connected with the tracheal system of the enclosed pupa) are
far forward, and these may be situated at the tips of long sometimes
branching processes, which recall the thoracic gills of the aquatic
pupae mentioned a few pages above. Adaptations, va