live; and the carnivorous in like manner feed on dead or living
flesh, or its products, many larvae being parasitic on living animals of
various classes (in Australia the larva of a species of _Muscidae_ is
even a parasite of frogs), especially the caterpillars of Lepidoptera,
which are destroyed in great numbers by _Tachininae_. The recent
discovery of a bloodsucking maggot, which is found in native huts
throughout the greater part of tropical and subtropical Africa, and
attacks the inmates when asleep, is of great interest.

It may confidently be asserted that, of insects which directly or
indirectly affect the welfare of man, Diptera form the vast majority,
and it is a moot point whether the good effected by many species in the
rapid clearing away of animal and vegetable impurities, and in keeping
other insect enemies in check, counterbalances the evil and annoyance
wrought by a large section of the Order. The part played by certain
blood-sucking Diptera in the dissemination of disease is now well known
(see MOSQUITO and TSETSE-FLY), and under the term _myiasis_ medical
literature includes a lengthy recital of instances of the presence of
Dipterous larvae in various parts of the living human body, and the
injuries caused thereby. That Diptera of the type of the common
house-fly are often in large measure responsible for the spread of such
diseases as cholera and enteric fever is undeniable, and as regards
blood-sucking forms, in addition to those to which reference has already
been made, it is sufficient to mention the vast army of pests
constituted by the midges, sand-flies, horse-flies, &c., from the
attacks of which domestic animals suffer equally with man, in addition
to being frequently infested with the larvae of the bot and warble flies
(_Gastrophilus_, _Oestrus_ and _Hypoderma_). Lastly, as regards the
phytophagous forms, there can be no doubt that the destruction of
grass-lands by "leather-jackets" (the larvae of crane-flies, or
daddy-long-legs,--_Tipula oleracea_ and _T. paludosa_), of divers fruits
by _Ceratitis capitata_ and species of _Dacus_, and of wheat and other
crops by the Hessian-fly (_Mayetiola destructor_) and species of
_Oscinis_, _Chlorops_, &c., is of very serious consequence.

With many writers it is customary to treat the fleas as a sub-order of
Diptera, under the title Aphaniptera or Siphonaptera. Since, however,
although undoubtedly allied to the Diptera, they must have diverged from
the