10 Introductory. 



PREVIOUS WORK ON DROSOPHILA VIRILIS. 



In earlier papers on Drosophila virilis (Metz and Metz, 1915; 

 Metz, 1916c, 1916d, 1918, 1920; Weinstein, 1920) it has been shown 

 that the genetic behavior of this species agrees in a general way with 

 that of the well-known D. melanogaster and that some of its mutant 

 characters bear a striking resemblance to those of melanogaster. 

 In genetic behavior, for instance, the two species agree, (1) in that 

 ''crossing-over" occurs only in the female, (2) in that the Y chro- 

 mosome appears to be functionless as far as the ordinary Mendelian 

 characters are concerned, and (3) in that the number of groups of 

 linked characters agrees with, or at least does not exceed, the haploid 

 number of chromosomes. In regard to the resemblance between 

 mutant characters, it has been found that the four characters con- 

 fluent, yellow, forked, and crossveinless are sufficiently similar in 

 the two species to give some ground for believing them to be homolo- 

 gous, and that the likeness between certain others suggests a similar 

 relation. 



PRESENT AIM OF THE WORK. 



With the completion of the preliminary tests, concerning the 

 general genetic behavior of D. virilis, attention was directed partic- 

 ularly to the genetic analysis of the chromosomes by means of mutant 

 characters, and a detailed comparison of the results with those 

 obtained in other species of Drosophila. The latter feature, and the 

 work of the different investigators involved, is considered in detail 

 in later sections. It may be stated at the outset that the studies 

 have not yet progressed to the point of giving final answers to the 

 main questions toward which they are directed. The aim of the 

 present paper, therefore, is primarily to bring together the data 

 and to indicate the trend of the results thus far obtained. For 

 this reason it is largely descriptive. The descriptive part is also 

 emphasized, necessarily, because of the fact that detailed information 

 concerning both the appearance and the genetic behavior of mutant 

 characters is necessary before those of different species can be com- 

 pared satisfactorily. 



DESCRIPTION OF DROSOPHILA VIRILIS. 



The following taxonomic description of D. virilis Sturtevant is 

 taken from Sturtevant's (1921c) "The North American species of 

 Drosophila," p. 97: 



"&, 9 . Arista with about five branches above and two below. AntennjE brown, 

 third joint dark opaque reddish-brown. Front over one-third width of head, wider 

 above; dull coffee-brown, ocellar dot black. Second orbital one-third other two. 

 Second oral bristle three-quarters length of first. Only one long bristle on each 

 palpus. Carina broad, slightly sulcate, nose-like; face somewhat shiny, brown. 

 Cheeks yellowish brown; their greatest width over one-fourth greatest diameter of 

 eyes. Eyes pilose. 



