INTRASPECIFIC VARIABILITY. 41 
VII. INTRASPECIFIC VARIABILITY. 
The species of Drosophiline, as they occur in the wild state, are 
as a rule not strikingly variable. They do vary in size or in intensity 
of color as a result of the amount of food obtained in the larval stage. 
Especially striking examples of this sort of modification are furnished 
by D. cardini and D. funebris, in both of which breeding experiments 
have shown that the variations are not inherited. But real inherited 
variations seem not to be common. Slight variations in the abdominal 
pattern of such forms as D. busckii, D. transversa, and Mycodrosophila 
dimidiata are probably of a genetic nature, and the variations in the 
intensity of the slight “‘trident’’ mark that is often present on the 
mesonotum of D. melanogaster have been shown to be inherited. I 
have studied an inherited variation in mesonotal pattern in D. repleta 
that occurs in wild flies (Sturtevant 1915). A number of investigators 
have shown that the occasional extra dorsocentral bristles found in 
wild stocks are in some cases heritable. Miss Hoge (1915) found 
inherited differences in the number of teeth in the tarsal combs of wild 
D. melanogaster males. Although it is not uncommon to find indi- 
vidual wild specimens that contain mutated genes, I know of no 
cases in the group other than those just mentioned in which a variation 
is established in nature and can be found persisting side by side with 
the parent stock. 
There are certain measurable characters in which the various species 
show variability, which may be heritable or not. Since most of these 
characters are used taxonomically, it becomes of interest to know just 
how variable they are, and what are the limits for different species. 
Unless this information is at hand it is not possible to judge as to their 
value as diagnostic characters. The following data bear on this point: 
The possession of two pairs of dorsocentral bristles is characteristic 
of the Drosophiline. Four pairs are present in Blesochetophora and 
Dettopsomyia; one in Acletoxenus and Drosophila superba, and only 
one large pair in Mycodrosophila; aside from these forms two pairs 
occur in all the forms known to me. The same number is to be found 
in many other Acalypterz, scattered through most of the subfamilies. 
It is especially common in the Sciomyzinez, Geomyzine, and Muili- 
chiine, but in none of the other subfamilies is it as usual as in the 
Drosophilinez. In species where two pairs are the rule, however, 
exceptional individuals can often be found. I have examined a large 
number of specimens of several species for this character, with the 
result shown in table 3. The numbers given refer to the total number 
of dorsocentrals present. Two pairs, for example, equal “4.” The 
flies recorded in these tables were all bred on banana agar under 
laboratory conditions. The numbers from each stock are based on 
examinations of individuals from several cultures made at different 
