STRUCTURE OF THE DIPT ERA. 71 



these insects feed, which are then easily sacked up through the proboscis. These bristles, or lancets, 

 which are the elongated representatives of the organs of the mouth, are somewhat variable in 

 number. When they are most numerous there are five of them, namely, two pairs, representing the 

 mandibles and maxillae, and an unpaired superior bristle, which is attached to the base of the labrum, 

 or close to it above the pharynx, and is therefore called the epipharynx. The number is reduced by 

 the suppression of some of the bristles usually to three, one pair and a single superior one and it is 

 then a question whether the upper unpaired bristle is still to be regarded as the epipharynx, or 

 whether the latter is suppressed and the third bristle composed of the two representing the mandibles 

 united into a single organ. The latter view is the one now generally adopted. The number of 

 bristles may vary even in the two sexes of the same species, AS is the case in the common Gnat, of 

 which the female alone possesses the full complement. The labium itself has no palpi, but the 

 bristles representing the maxillae have these appendages, which are frequently of considerable length 

 and composed of several joints. The palpi project from near the base of the proboscis, from which, 

 indeed, they actually spring in many instances (as, for example, in the common House Fly). This is 

 due to the fact that in these insects the part bf the maxillae bearing the palpi is amalgamated with 

 the base of the labium into a single mass. 



The head in the Diptera is very freely attached to the front of the thorax by a short and usually 

 slender neck, an arrangement which renders it very movable ; it is generally of a rounded form, but 

 not unfrequently flat or even concave behind. The compound eyes are well developed, except in a 

 few forms, in which they are rudimentary or altogether wanting. They are usually large, often 

 covering nearly the whole upper surface of the head, and leaving only a small triangular space on the 

 top for the three ocelli, which are generally present, and another small space in front for the 

 attachment of the antennae. The eyes are generally larger in the males than in the females, and 

 often meet in the middle line, whilst in the females they are separated by a narrow band. 



The antennae, which are generally inserted upon the face between the eyes, vary considerably in 

 size, form, and structure, but we can distinguish two principal types of these organs. In the one 

 (see figure of head of female Gnat on p. 70), the antennae are more or less elongated and composed of a 

 considerable number of joints, when they are either thread-like or beaded, and often, especially in the 

 males, hairy or feathered throughout ; in the other, they consist apparently only of three joints, of 

 which the last is usually a good deal longer than the others, and not uiifrequently shows signs of its 

 being really composed of several joints in the shape of notches or transverse lines. These antennae 

 sometimes project in front of the head, and sometimes hang down close in front of the face. The 

 third joint is very commonly furnished with a long bristle, which may be jointed, and thus indicates 

 that it is a continuation of the ordinary jointed antennae. This bristle, which is often hairy or 

 feathered, may spring either from the extremity or from the back of the third joint (see Vol. V., 

 p. 284, Fig. 5, E). 



As in the Lepidoptera, the three segments of the thorax are soldered together to form a 

 single mass. The mesothorax, as bearing the wings, is most largely developed ; the prothorax is 

 generally reduced to a very small ring ; and the metathorax is usually confined to the hinder sur- 

 face of the thorax. The hinder part of the mesothorax is cut off by an impressed line, and forms 

 a distinct scutellum. The wings, of which, as already stated, only the first pair are developed 

 as organs of flight, although sometimes dai-k-colourecl or spotted, are generally transparent, and 

 either composed of naked membrane or more or less clothed with hairs. The veins for the most 

 part run, branching more or less, through the length of the wing, but in many species there are 

 cross- veins which enclose complete cells, like those 'of the Hymenoptera, although we never find, as 

 in the latter, a stigma with a regular system of cells in its vicinity. Close to the base of the 

 inner margin of the wing, in a great number of Diptera, we find the margin of the membrane 

 cut into one or two small lobes by notches of variable depth. The outer one, which is clearly a 

 portion of the wing, has been called the alulet ; the inner one, which is attached to the thorax, 

 is the scale (squama), and, when most developed, forms a sort of roof-like covering for the 

 halter es, or appendages of the metathorax. The latter, the nature of which has been the subject 

 of considerable discussion, are now generally regarded as representing the hind wings. They are 

 .small organs resembling the clapper of a bell, composed of a thin stalk bearing a knob at its 



