All suctorial insects which in the perfect state possess only two
membranous wings, are called Diptera, from two Greek words--[Greek:
dis], twice, and [Greek: pteron], wing.
The Diptera were known and scientifically described at a very early
date. They are frequently mentioned by Aristotle in his "History of
Animals;" and he applied the term to the same insects as now
constitute the order.
The absence of the second wings, common to other insects, which are in
this case replaced by two appendages, which have received the name of
balancers,[10] because they serve to regulate the action of flight,
constitutes the chief characteristic of the Diptera. Let us, however,
give a glance at their other organs, which have more or less affinity
with those which exist in other classes of insects, preserving,
nevertheless, their own especial characteristics.
[10] Sometimes called _halteres_.--ED.
The mouth, for instance--suited for suction only--is in the form of a
trunk, and is composed of a sheath, a sucker, and two palpi. The
antennæ are generally composed of only three joints. The eyes--usually
two in number--are very large, and sometimes take up nearly the whole
of the head. They are both simple and compound. The wings are
membranous, delicate, and veined; the limbs long and slight. In giving
the history of the principal types of Diptera, we shall explain more
fully the formation of these organs.
The Diptera, by their rapid flight, enliven both the earth and the
air. The different species abound in every climate, and in every
situation, some inhabiting woods, plains, fields, or banks of rivers;
others preferring our houses. They like the neighbourhood of
vegetation, choosing either the flowers, the leaves, or the stems of
the trees of our woods, our gardens, or our plantations. Their food
varies very much; and the formation of the sucker is regulated by it.
Some imbibe blood, others live on the secretions of animals. Their
chief nourishment, however, consists of the juices of flowers, on
whose brilliant corollas the Diptera abound, either plundering from
every species indiscriminately, or attaching themselves to some
particular kind. They display the most wonderful instinct in their
maternal care, and employ the most varied and ingenious precautions to
preserve their progeny.
The Diptera, besides their variety and the number of their species,
are remarkable on account of their profusion. The myriads of flies
which rise from our meadows, which fly in crowds around our plants,
and around every organised substance from which life has departed,
some of which even infest living animals, are Diptera.
The profusion with which they are distributed over the face of the
globe, causes them to fulfil two important duties in the economy of
Nature. On the one hand, they furnish to insectivorous birds an
inexhaustible supply of food; on the other, they contribute to the
removal of all decaying animal and vegetable substances, and thus
serve to purify the air which we breathe. Their fecundity, the
rapidity with which one generation succeeds another, and their great
voracity, added to the extraordinary quickness of their reproduction,
are such that Linnæus tells us that three flies, with the generations
which spring from them, could eat up a dead horse as quickly as a lion
could.
These Diptera, which are worthy of so much attention, and deserve so
much study with regard to the part they play in the general economy of
Nature, are an object of fear and repulsion when one considers their
relations to us and other animals. Gnats and mosquitoes suck our
blood; the gad-fly and the species of Asilus attack our cattle. The
order Diptera is composed of a great number of families, which are
again divided into tribes, each comprising several genera. We shall
only notice the more remarkable genera of Diptera.
M. Macquart, the learned author of "L'Histoire Naturelle des
Diptères,"[11] divides this great class of insects into two principal
groups. In one of these groups, the antennæ are formed of at least six
joints, and the palpi of four or five: these are called Nemocera. In
the other, the antennæ consists only of three joints, and the palpi of
one or two: these are the Brachycera.
[11] "Suites à Buffon." 2 vols. 8vo.
The Nemocera may generally be distinguished from the other Diptera,
independently of the difference in the antennæ and palpi, by the
slenderness of the body, the smallness of the head, the shape of the
thorax, and the length of the feet and wings. The result of this
organisation is a graceful, light, and aerial form.