44 THE NORTH AMERICAN SPECIES OF DROSOPHILA. 



SECONDARY SEXUAL CHARACTERS. 



In general, the two sexes of drosophiline flies are quite similar in 

 appearance, but secondary sexual characters can nevertheless often 

 be found. The following is only a partial catalogue of such sexual 

 differences, and includes only the more striking differences and the 

 less conspicuous ones that occur on the commoner forms. 



Sex-combs: A comb-like row of about ten short, stiff, slightly curved 

 black bristles occurs on the inner distal surface of the basal tarsal 

 joint of the front leg in the males of some species. I have never seen 

 such a comb in a female. It is invariably present in the males of 

 Drosophila melanogaster (see plate 3, fig. 2), D. simulans, D. affinis, 

 the European D. confusa Staeger, and an undescribed South American 

 species that is very similar to D. nebulosa. In D. obscura a somewhat 

 smaller comb is present in this position, and a second one occurs on 

 the second tarsal joint of the front leg (see fig. 47). Here also the 

 female has no tarsal combs. 



Other leg characters: The male of D. immigrans has the two basal 

 joints of his front tarsus distinctly shorter and thicker than the cor- 

 responding joints of the other legs. In the female all the tarsi are 

 similar in size and shape. In the genus Chymomyza (all the species 

 known to me) the males have a row of long, stiff bristles on the lower 

 side of the front femur. These bristles are either missing or much 

 smaller in the females. As suggested above, this sexual difference may 

 be correlated with the peculiar mating habits of this genus. Ac- 

 cording to Oldenberg (1914, p. 9) there is a sexual difference in the 

 shape and hairiness of the tibise and tarsi in the European D. nigro- 

 sparsa Strobl. 



Shape of head: In Zygothrica dispar the males have very broad 

 heads, with the eyes conically produced. This character apparently 

 never occurs in the females, and even in the males it is quite variable. 

 I have seen one male in which the head was not broader than is usual 

 for the female. 



According to Oldenberg (1914), some species of Stegana resemble 

 the Calypterse in that the males have a narrower front than do the 

 females. The same is true of Hendel's genus Thaumastophila (see 

 Apsinota). 



Color: In the genus Leucophenga generally the males are paler in 

 color than the females, and have more whitish poUinosity. There is a 

 sexual dimorphism in mesonotal color in the Oriental Drosophila 

 hypocausta Osten Sacken and in the Ethiopian D. aberrans Lamb. 

 According to Oldenberg, sexual differences in thoracic color occur in 

 Acletoxenus and in Phortica. I have observed such differences in 

 Zygothrica. 



In many species of Drosophila the dark abdominal bands are broader 

 in the male than in the female. In such forms as D. melanogaster, 



