es in Diptera are usually alike, though in a number of
families with short antennae the males are distinguished by the fact
that their eyes meet together (or nearly so) on the forehead.
Metamorphosis in Diptera is complete; the larvae are utterly different
from the perfect insects in appearance, and, although varying greatly in
outward form, are usually footless grubs; those of the _Muscidae_ are
generally known as maggots. The pupa either shows the appendages of the
perfect insect, though these are encased in a sheath and adherent to the
body, or else it is entirely concealed within the hardened and
contracted larval integument, which forms a barrel-shaped protecting
capsule or puparium.

Diptera are divided into some sixty families, the exact classification
of which has not yet been finally settled. The majority of authors,
however, follow Brauer in dividing the order into two sections,
Orthorrhapha and Cyclorrhapha, according to the manner in which the
pupa-case splits to admit of the escape of the perfect insect. The
general characteristics of the pupae in these two sections have already
been described.

In the Orthorrhapha, in the pupae of which the appendages of the perfect
insect are usually visible, the pupa-case generally splits in a straight
line down the back near the cephalic end; in front of this longitudinal
cleft there may be a small transverse one, the two together forming a
T-shaped fissure. In the Cyclorrhapha on the other hand, in which the
actual pupa is concealed within the hardened larval skin, the imago
escapes through a circular orifice formed by pushing off or through the
head end of the puparium. The Diptera Orthorrhapha include the more
primitive and less specialized families such as the _Tipulidae_
(daddy-long-legs), _Culicidae_ (gnats or mosquitoes), _Chironomidae_
(midges), _Mycetophilidae_ (fungus-midges), _Tabanidae_ (horse-flies),
_Asilidae_ (robber-flies), &c. The Diptera Cyclorrhapha on the other
hand consist of the most highly specialized families, such as the
_Syrphidae_ (hover-flies), _Oestridae_ (bot and warble flies), and
_Muscidae_ (_sensu latiore_--the house-fly and its allies, including
tsetse-flies, flesh-flies, _Tachininae_, or flies the larvae of which
are internal parasites of caterpillars, &c). It is customary to divide
the Orthorrhapha into the two divisions Nematocera and Brachycera, in
the former of which the antennae are elongate and in a more or less
primitive condi