shape; in the more specialized families, such as the
_Tabanidae_ (horse-flies), _Syrphidae_ (hover-flies) or _Muscidae_
(house-flies, blue-bottles and their allies), the number of antennal
joints is greatly reduced by coalescence, so that the antennae appear to
consist of only three joints. In these forms, however, the third joint
is really a complex, which in many families bears in addition a jointed
bristle (arista) or style, representing the terminal joints of the
primitive antenna. Although in the case of the majority of Diptera the
body is more or less clothed with hair, the hairy covering is usually so
short that to the unaided eye the insects appear almost bare; some
forms, however, such as the bee-flies (_Bombylius_) and certain
robber-flies (_Asilidae_) are conspicuously hairy. Bristles are usually
present on the legs, and in the case of many families on the body also;
those on the head and thorax are of great importance in classification.

Between 40,000 and 50,000 species of Diptera are at present known, but
these are only a fraction of those actually in existence. The species
recognized as British number some 2700, but to this total additions are
constantly being made. As a rule flies are of small or moderate size,
and many, such as certain blood-sucking midges of the genus
_Ceratopogon_, are even minute; as extremes of size may be mentioned a
common British midge, _Ceratopogon varius_, the female of which measures
only 1-1/4 millimetre, and the gigantic _Mydaidae_ of Central and South
America as well as certain Australian robber-flies, which have a body
1-3/4 in. long, with a wing-expanse of 3-1/4 in. In bodily form Diptera
present two main types, either, as in the case of the more primitive and
generalized families, they are gnat- or midge-like in shape, with
slender bodies and long, delicate legs, or else they exhibit a more or
less distinct resemblance to the common house-fly, having compact and
stoutly built bodies and legs of moderate length. Diptera in general are
not remarkable for brilliancy of coloration; as a rule they are dull and
inconspicuous in hue, the prevailing body-tints being browns and greys;
occasionally, however, more especially in species (_Syrphidae_) that
mimic Hymenoptera, the body is conspicuously banded with yellow; a few
are metallic, such as the species of _Formosia_, found in the islands of
the East Indian Archipelago, which are among the most brilliant of all
insects. The sex