INCLUDING THE GNATS, MIDGES, CRANE FLIES, HAWK FLIES, BEE FLIES, BREEZE
FLIES, BOT FLIES, AND THEIR ALLIES.
The _Diptera_, or two-winged flies, are among the most numerous orders
of insects, but they have been so little studied hitherto that we have
but a very imperfect knowledge of them, especially as regards the
tropical species.
[Illustration: Fig. 120.--Midas Giganteus.]
The number recorded in the British Isles is about 3,000, that of the
whole world probably exceeds 30,000, and it is certain that this number
will be increased immensely as time goes on.
The _Diptera_ may be defined as insects with a sucking mouth
(_Haustellate_), and with only two wings, which are membranous, while
the hind wings are represented only by a pair of small knotted organs,
called _halteres_. Their metamorphoses are complete.
The _larvæ_ are footless "grubs," generally with a soft body, but
sometimes leathery or even nearly horny. Many of them possess a
distinctly marked head, which may contain _ocelli_, but in the majority
the head appears as if it were simply one of the neighbouring segments.
[Illustration: Fig. 121.--Craspedia Coriaria.]
Their food generally consists of decayed animal and vegetable matter,
and thus they are very useful as scavengers, particularly in hot
climates.
The _larvæ_ frequently live in the substance upon which they feed,
while others reside in water, and many are parasitic.
For the classification of the _Diptera_ we owe much to that eminent
entomologist, Osten-Sacken, whose arrangement has been here generally
adopted.