EXTERNAL ANATOMY. 23 



convex in many genera of Steatiomyid.e ; globular and almost 

 transparent in Cyrtid.e ; oval or conical, as in most Syhphid.e, 

 and other families of Brachycera, and moat of tlie higher 

 MusciDiE ; elongate and cylindrical in AsilidyE, Dolichopid.e ; 

 subtriangular in many Anthomyid.e and Acalyptraia, 



In vestiture it varies also ; thick long spines are present 

 in many genera of Tachinin.e ; in others, bristles, ordinary 

 pubescence, a poUen-like dust, or scales may form the covering. 



The male genitalia in the Diptera exhibit the most diverse 

 modifications, and in many groups afford reliable specific characters. 

 Their taxonomic importance, however, must not be overrated, 

 and it must be recognised that they exhibit great variability, even 

 in the same genus. They reach probably their greatest develop- 

 ment in the DolichopidjB, but are quite conspicuous in many 

 TiPULiD^, Mycetophilid^e, AsiLiDiE, and some smaller groups. 



The female genitalia are much more uniform than those of the 

 male, consisting in the priucipai families of the jN'ematocera of a 

 pair of oval terminal lamellae often withdrawn into the body-cavity, 

 and in the Tipulid.e of a pair of exterior pointed valves ; 

 whilst in nearly all of the Brachycera and the Muscid.e no 

 oi'gans are visible exteriorly. 



The Legs. 



These organs vary throughout the order to an extraordinary 

 degree, from the exceptionally long and delicate legs found in 

 Dolichopeza, Cylhidrotoma, and other Tipulid.e, to the short 

 incrassated ones in some of the Ephydrin.^e and other groups 

 of Acalyptrate Muscid.e. They may be conspicuously dentate, 

 strongly spinose, bristly or practically devoid of hairs, densely 

 pubescent, or ciliate. Frequently the femur, tibia, or tarsus, or 

 one or more joints of the latter may be incrassate, or occasionally 

 fantastically formed, such modifications being at most generic, 

 often not of even that taxonomic value. 



The Wings. 



So far as the perfect insect is concerned, the venation of the 

 wings is, for purposes of classification, the soundest aud most 

 reliable structural character in Diptera. The exceptions, though 

 admittedly numerous, are of such a nature that although doubt 

 as to their exact systematic jwsition may at first arise, they can 

 never be actually identified with a wrong family. Many families 

 or groups of families in the Diptera possess strikingly peculiar 

 types of venation of their own. Outside of the Nematocera, one 

 soon learns to recoguise, for example, the wing of a Stratiomyid 

 from the faintness of the veins near the posterior margin of the 

 uing and the equally unusual " crowding-up "' of the veins near 

 the anterior margin ; aud a Dolichopid may be known by the 

 peculiar " kink ", that is present in so many of the genera in that 

 family, placed about the middle of the 3rd and 4th longitudinal 

 veins ; while the Syephid.t may be distinguished by the upturned 



