the prothorax, so that the pupa, which is fairly active, hangs from the
surface-film with its abdomen pointing downwards through the water. This
change of position is correlated with the necessity for the imago to
emerge into the air; were the pupa to hang head downwards as the larva
does, the 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.