ANATOMY. 33 



elusive, have one apiece just beneath their edges. In the female there 

 is one in the same position with respect to the fifth plate, and one on 

 the sixth plate itself, just above its lower edge. In the male there are 

 two just above the lower edge of the fifth plate, suggesting that this 

 plate, like the first, really represents two fused segments. All these 

 spiracles appear as minute round holes, with tracheae connected to 

 them and indistinctly visible through the body-wall. 



Genital region, female: Behind the seventh dorso-lateral plate of 

 the female are situated the anal and genital openings. The anal 

 opening, above, is on a papilla composed of a long-haired dorsal and a 

 similar ventral chitinized plate. Just below this papilla is the ovi- 

 positor. This organ consists of two similar lateral chitinized plates 

 that bear numerous small peg-like hairs or bristles and one or a few 

 longer and more slender hairs. A few characteristic plates are shown 

 in figures 9 to 12. 



Genital region, male: Behind the fifth dorso-lateral plate of the male 

 is situated a small plate that is very difficult to study. It probably 

 represents the dorso-lateral plates of one or more abdominal segments. 

 Behind this lies a definite but small dorso-lateral plate that I have 

 arbitrarily designated the genital arch. Characteristic types are shown 

 in figures 13 to 16. Below and behind this structure lies a small struc- 

 ture that we may call the clasper. It appears in many diverse forms, 

 and has various relations to the genital arch, but almost always bears 

 peg-like bristles. In Drosophila melanogaster (fig. 13) and in D. 

 simulans (fig. 14) it is weakly chitinized at its base, which articulates 

 to the inner surface of the genital arch. In D. busckii (fig. 15) it is 

 separate from the genital arch and lies directly behind it. In D. fune- 

 bris (fig. 16) it is fused to the lower posterior edge of the genital arch. 



The anal plates are upright and lie on each side of the anus, instead 

 of above and below it, as in the female. They are always hairy, and 

 sometimes bear peg-like bristles, as in D. funebris (fig. 16). In D. 

 caribbea there is an especially large bristle of this type on the lower 

 corner of each plate. In D. repleta and some other species the anal 

 plate is connected to the genital arch by a chitinous bridge. 



These same three plates occur, with various modifications but in 

 the same general relations to each other, in Curtonotum gibbum, 

 Zygothrica dispar, Pseudophortica obesa, Leucophenga varia, Myco- 

 drosophila dimidiata, Scaptomyza adusta, S. graminum, and over a 

 dozen species of Drosophila that I have examined. In Curtonotum 

 helva, however, there is an extra clasper. The usual clasper is present 

 as a slender plate tapering to its apex, and inserted as in D. melano- 

 gaster. The other clasper is a slender, club-shaped structure, attached 

 to the outer surface of the genital arch, near its posterior edge and 

 just below the anal plate. This is evidently the structure known as 

 the posterior clasper in the Calypterse; and the other organ, that is 



