THE DIPTERA, OR TWO-WINGED FLIES. 313 



visible without any close examination^ whilst in other 

 cases they are small, and half hidden between the back 

 of the thorax and the base of the abdomen, or entirely 

 concealed beneath a membranous lobe continued from 

 the base of the true wing, and known as the alulet. 



The head^ which is usually attached to the thorax 

 by a narrow neck, so as to have considerable power 

 of motion, has the greater part of its surface occupied 

 by a pair of large compound eyes, which in many 

 cases, especially in the males, meet in the central 

 line, and thus occupy nearly the whole head. The 

 crown of the head is usually furnished with three 

 ocelli, and the face bears a pair of antennae, the form 

 and development of which vary extraordinarily in 

 different groups, as we shall have occasion to see 

 hereafter. The proboscis or sucking-mouth of the 

 Diptera is very different from the elegant spiral trunk 

 of the Butterflies and Moths ; but as its structure 

 has already been described (p. 15), it will be unne- 

 cessary to dwell upon it in this place. 



The thorax forms a compact mass, on the surface 

 of which, the divisions between the segments are 

 generally indicated by tolerably strong furrows. The 

 mesothorax, as might be expected from its bearing 

 the wings, constitutes by far the greater part of this 

 region of the body; the prothorax forms a narrow 

 collar, which is often scarcely perceptible from above, 

 and the metathorax is also of inconsiderable size. 

 The legs are usually long and well formed, and some- 

 times excessively elongated and slender; the tarsi 

 almost invariably consist of five joints, of which the 

 last bears a pair of claws and usually two or three 

 soft pads, by means of which these flies are enabled 

 to cling to and walk upon the smoothest surfaces. 



