INTRASPECIFIC VARIABILITY. 41 



VIII. INTRASPECIFIC VARIABILITY. 



The species of Drosophilinse, 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 Drosophilinse. Four pairs are present in Blcesochcetophora 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 Acalypterse, scattered through most of the subfamilies. 

 It is especially common in the Sciomyzinse, Geomyzinae, and Mili- 

 chiinse, but in none of the other subfamilies is it as usual as in the 

 Drosophilinae. 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 



