DIPTERA. 739 



The Diptera form an enormous Order with over 40,000 de- 

 scribed species-. They are as a rule unpopular insects, seldom 

 considered attractive in appearance and usually unpleasing in 

 their habits, difficult to catch and in many cases to preserve. 

 Probably the known species bear a smaller proportion to those 

 that exist than is the case with the other Insect Orders. 



Diptera are flies in the restricted sense of the word, and they 

 are easily recognized by the possession of but one pair of func- 

 tional wings, and frequently by the halteres * which replace 

 the hinder pair (Figs. 365, 368). The latter, however, are not 

 always readily observable. 



The head (Fig. 471) is usually a somewhat flattened sphere, a 

 large portion of whose surface is taken up by the facetted eyes, 

 especially in the male. In about half the Diptera there is a 

 curious concealed vesicle on the front of the head, called the 

 ptilinum. During emergence from the pupa it is distended, and 

 may help to rupture the enclosing shell of the pupa. In mature 

 flies it is quite introverted, only a small space (lunula) under an 

 arched suture indicating its existence. The antennae are usually 

 short and exhibit variations useful in classification : usually 

 they consist of but three segments ; the two basal form the 

 " scape," the .terminal is called the " flagellum." It frequently 

 bears a bristle called the " arista." The mouth-parts are 

 complex and subject to great variation. In the gnat or mos- 

 quito we find the labium a soft, deeply grooved structure. In 

 its hollow lie (i) a pair of wonderfully sharp and fine stylets, 

 the mandibles ; (ii) another pair of slightly coarser stylets, the first 

 maxillae, which are provided with palps ; (iii) a median style, the 

 hypopharynx ; and finally (iv) the labrum, with which is fused the 

 epipharynx. The last named compound structure is so deeply 

 grooved as to be almost a tube, and the fluid food of the insect 

 passes up its hollow. The maxillary palps stand free of the 

 proboscis. The labium ends in two diverging labellae which 

 guide the piercing .mandibles, maxillae, hypopharynx and 

 labrum and epipharynx, as a player guides a billiard cue between 

 his two fingers. Whilst these parts are being pressed (Fig. 472) 

 into a yielding substance, the labium is bowed more and more 



* &XT77pes = dumb-bells which the Greek athletes used to give impetus 

 in jumping. 



