II] CLASSIFICATION 23 



The obtectate pupae, as for example in the mosquito, are 

 often actively motile, progressing almost as freely as the larvae. 

 The coarctate pupae, however, are devoid of all movement 

 and are usually to be found buried in the earth, in crannies, 

 or in similar localities. The mature fly, after breaking out of 

 the pupal case, is aided in forcing its way through any covering 

 of earth, or similar material, by the alternate expansions and 

 contractions of the ptilinum, which becomes completely- 

 retracted soon after the fly has emerged. 



Classification. 



The most generally adopted classification is that of Brauer 

 who divides the Diptera into two suborders, the Orthorrhapha 

 and Cyclorrhapha, according to the manner in which the adult 

 fly emerges from the pupal case, either by a T-shaped opening 

 along the back (Orthorrhapha) or by a circular opening at the 

 anterior extremity (Cyclorrhapha). Although this distinction 

 is of great importance it is rather difficult of practical applica- 

 tion when the immature stages are unknown, and the only 

 method of distinguishing the adult fly is the presence or 

 absence of a suture over the insertion of the antennae. The 

 position of the Pupipara is somewhat doubtful, but from 

 Roubaud's researches it seems probable that the Hippoboscidae, 

 at any rate, are merely a highly specialized group of the 

 Muscidae. For the present, however, they will be regarded as 

 a separate division and accordingly, following Sharp's arrange- 

 ment, the Diptera may be divided into five sections : 



Series i. Orthorrhapha Nematocera. Flies in which the 

 antennae are composed of more than six segments of which all 

 except the first two are equal in size. An arista is not present. 

 The palpi are long and flexible, and usually four- or five-jointed. 

 The second longitudinal vein is often forked and with the 

 exception of the Tipulidae and Rhyphidae a discal cell is not 

 present. This section includes the mosquitoes, sand-flies, 

 Chironomus, etc., all of which are what may be termed gnat- 

 like flies. 



Series 2. Orthorrhapha Brachycera. Flies in which the 

 antennae are composed of three dissimilar segments, the last 



