32 



THE NORTH AMERICAN SPECIES OF DROSOPHILA. 



ABDOMEN. 



The abdomen consists of seven visible segments in the female, five 

 in the male. The dorsal and lateral surface of each of these is com- 

 posed of a heavily chitinized dorso-lateral plate, and these are telescoped 

 together, so that the posterior portion of each one overlaps the anterior 

 portion of the one behind it. The first segment is probably morpho- 

 logically really two segments fused together. It possesses a deep 

 transverse furrow, but this is probably secondary, and does not corre- 

 spond to the line of fusion of the two segments. Each of the dorso- 

 lateral plates is clothed with hairs and bristles. 



Ventrally the abdomen bears a series of chitinized plates, separated 

 from the dorso-lateral plates by a parchment-like region. These 

 ventral plates are hairy, and quadrilateral in shape. In the female 

 they are six in number. The five anterior ones are much alike in 

 shape, but the posterior one is narrower and has a deep notch in its 



Fig. 9. — Scaptomyza adusta. 

 Fig. 10. — Scaptomyza graminum. 



Ovipositor plates. 



Fig. 11. — Drosophila melanogaster. 

 Fig. 12. — Drosophila virilis. 



posterior side. Only four ventral plates are to be seen in the male. 

 The three anterior ones are like those of the female; the posterior one 

 is much larger and broader than the others. 



In the female the sixth and seventh dorso-lateral plates are smaller 

 than the first to fifth ones. In pinned material they are usually re- 

 tracted, so that there appear to be five segments like those of the male. 

 In life or in cleared specimens the sixth plate can be seen to be quite 

 narrow and weakly chitinized above (in the mid-dorsal region), but 

 relatively broad and strongly chitinized below. The seventh plate, 

 on the other hand, is broad and stronglj^ chitinized above, and narrow 

 and weakly chitinized below. 



Abdominal spiracles: There are seven spiracles on each side of the 

 abdomen. Two of these lie just under the lower edge of the first 

 dorso-lateral plate. The second to fourth dorso-lateral plates, in- 



