THE ORDER DIPTERA 37 



its pronotum is eifher not seen at all, or shows only as a 

 sort of collar or a pair of shoulder-knots. The metathorax 

 also is small, and its metanotum is more or less overlapped 

 dorsally by the mesonotum. The mesothorax on the other 

 hand is so large that in a dorsal view its mesonotum fills 

 almost the whole field of the thorax : it consists of two parts, 

 namely, a larger anterior part, or scutum (Lat. scutum = 

 buckler), and a smaller posterior lobe, or scutellum (diminutive 

 of scutum), which overhangs the metanotum : the scutum is 

 sometimes divided into two nearly equal parts by a transverse 

 groove. The sides, or pleura, of the thoracic segments are 

 greatly developed ; their somewhat complicated structure is 

 explained in Fig. 4. In the pleura of the prothorax and meta- 

 thorax the large anterior and posterior spiracles are placed. 

 Notice should be taken of the sternopleural and hypopleural 

 plates, as the large bristles borne by them are of systematic 

 importance in dealing with the Muscoidea. 



The legs consist of the usual segments — coxa, trochanter, 

 femur, tibia, and tarsus, the tarsus being composed of five 

 pieces. The coxa may be long or short. The first tarsal 

 segment is sometimes elongate and is often spoken of as the 

 " metatarsus," an ill-chosen term which it is perhaps better 

 to drop : the last tarsal segment carries a pair of claws, and 

 ventral of these there may be a pair of membranous pads 

 or pulvilli (Lat. pulvillus = a little cushion) ; between the 

 claws there often exists a small median appendix — the 

 empodium (eV = on ; -wo^iiov = the foot) — which may be either 

 bristle-like, or broad and membranous like the pulvilli. 



The wings, as of the majority of insects, are simple folds 

 of integument supported by veins or nervures, and they are 

 membranous, though they may be covered with hairs or 

 scales. Only the anterior (mesothoracic) pair is present, and 

 even they are occasionally absent (wingless flies). Near the 

 root of the wing, posteriorly, there is a notch — the axillary 

 notch — which marks off a small lobe — the posterior lobule. 

 Internal to the posterior lobule there are often one or two 

 smaller membranous plates (one lying above the other 

 when there are two) known as the tegula and antitegula (or 

 squama and antisquamd) ; these probably facilitate the 

 closing; and opening of the wings, since they are often very 



