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.