EXTERNAL GENITALIA OF DIPTEkA 5 



it is then not easy to distinguish ; in this case it is said to be pulvilliform. 

 The pulvilli and empodium have some practical importance, for it is 

 on them that bacteria may be carried from place to place by the fly. 



THE ABDOMEN 



The segmentation of the abdomen is much simpler than that of the 

 other regions of the body, as each metamere is represented in the 

 external wall by a pair of plates, the tergite and the sternite. These 

 are transversely elongate, and in the unfed fly the tergite usually 

 overlaps the sternite, thus concealing the membrane which unites them. 

 Each plate overlaps the one behind it, and is attached to its neigh- 

 bours by a narrow strip of membrane, which permits of movements 

 of the segments on one another. The pleural membrane which con- 

 nects the tergites and the sternites is lax enough to be capable of 

 considerable distension when the fly is gorged with food. The num- 

 ber of segments visible externally varies from eight or nine to four, and 

 progressively diminishes from the more primitive to the more complex 

 forms, the diminution corresponding generally with a modification of 

 the terminal segments into structures connected with the reproductive 

 system. The terminal segments are always reduced in size. The open- 

 ing of the alimentary canal is situated between the last tergite and 

 sternite. In the higher forms, along with a reduction in the number 

 of segments, there is also a great reduction in the extent of the ster- 

 nites, which become restricted to a narrow space in the ventral middle 

 line ; the tergites are increased in extent, but not sufficiently to com- 

 pensate for the reduction of the sternites, *so that there is a wide lateral 

 area which is occupied by membrane. In the Pupipara the division 

 of the abdominal wall into segments, and the separation between the 

 tergites and sternites, are to a large extent lost, and the whole abdo- 

 men is enclosed by a tough but flexible integument. In these forms 

 the anterior end of the abdomen is much contracted, and is joined 

 to the thorax by a narrow ' waist '. 



The external genitalia consist of structures which are designed to 

 assist in copulation, and, in the female, also in oviposition. They are 

 formed from the terminal segments of the abdominal 



wall, and probably also from the remains of true The Extflrnal 



i i i T , i i , Genitalia 



abdominal appendages. It has long been recognized (Plate XVIII) 



that these organs, especially those of the male, present 



an extraordinary degree of variability, and thus provide most useful 



distinctive characters by which forms very similar to one another 



