32 Linkage Group II. 



III. LINKAGE GROUP II. 



Since confluent was the first non-sex-linked character found in 

 D. virilis, the group to which it belongs is designated Group II. 

 It consists of the four characters confluent, concave, double, and 

 broken. The first two are excellent characters and have been used 

 extensively; the third is very poor for linkage work and has been used 

 relatively little; while the last, although a good character, appeared 

 so recently that it has been possible to use it only in preliminary 

 experiments. 



DESCRIPTION, ORIGIN, AND COMPARISON OF 

 CHARACTERS IN GROUP II. 



Confluent (C). (Plate 3, Figure 1.) 



Description. — Confluent derives its name from the fact that the second vein is con- 

 fluent with the costal vein for a short distance near the tip of the latter, as showTi in 

 the figure (also Metz, 1916, fig. 2). The fly appears to be unaffected otherwise, save 

 for a thickening of the outer ends of the two cross-veins, and a slight roughening of 

 the eyes in most cases. Confluent resembles the extreme form of the sex-linked 

 character triangle, and like the latter is dominant. Unlike triangle, however, it is 

 lethal in the homozygous condition and pure stock can not be obtained. In combi- 

 nation with the other dominant wing characters, branched, net, and extra, confluent 

 is exaggerated, the confluence of the second and costal veins being more extended. 

 It is likewise exaggerated by capsule, as noted under the description of the latter. 



Origin. — Confluent was one of the first characters found in D. virilis (see Metz, 

 1916, p. 593). 



Comparison. — Confluent resembles the "confluent" of D. melanogaster in appear- 

 ance and in being a dominant. It is probable that they agree also in being lethal 

 when homozygous (see Metz, 1916, p. 591, and Morgan, Bridges, and Sturtevant, 

 1919, p. 257). 



Concave (cc). (Plate 3, Figures 3 to 7.) 



Description. — The principal diagnostic characters of concave are the curled, instead 

 of straight, hairs on the arista (fig. 4, plate 3), and the heavy, somewhat wa\y bristles 

 on the scutellum. The former is constant and definite and the latter is usually evident. 

 In addition the scutellar bristles frequently stand erect or point in various directions, 

 the dorso-central bristles sometimes are similarly affected, the posterior scutellar bristles 

 extend nearly parallel instead of crossing, and often the wings are abnormal. The 

 latter effect is very irregular. Sometimes both wings are very small, or narrow, or 

 short and nearly circular (a common type), or concave instead of convex on the inner 

 margin. Often only one wing is affected, or the two may be differently affected. 

 Two typical wing forms are shown in an earlier paper (Metz, 1916, figs. 5 and 6). 



Origin. — The origin of concave is given in the paper just cited. 



Comparison. — Concave is paralleled very closely by the third chromosome re- 

 cessive character "crumpled" in D. melanogaster. Both have the curled hairs on the 

 arista, and the same type of abnormal scutellar bristles and both exhibit a series of 

 wing modifications, of which the most frequent types are similar in the two species. 



Double (de). 



Description. — Double is characterized by an occasional doubling of part of the 

 fourth vein or fifth vein, especially near the posterior cross-vein, by the presence of a 

 tubercle on the first vein in some individuals, and frequently by a broadening and 



