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 Calypterz 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 pollinosity. 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, 
