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 

| dyn “ 
ss 10 
: — 
11 
12 
OVIPOSITOR PLATES. 
Fig. 9.—Scaptomyza adusta. Fic. 11.—Drosophila melanogaster. 
Fig. 10.—Scaptomyza graminum. Fia. 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. 
Tn 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 strongly 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- 
