216 THE SECOND-CHROMOSOME GROUP 



A similar study has been made by Morgan of the compounds between 

 strap and antlered. In these strap-antlered compounds, both males 

 and females, the antlered allelomorph dominates the strap, so that the 

 flies are in the mass scarcely to be distinguished from antlered (fig. 78, a 

 and 78, 6). 



DACHS id). 



(Plate 10, figures \a to Id.) 



ORIGIN OF DACHS. 



The mutant now called dachs had a double origin. Morgan found 

 in certain experiments that the venation of the wing of many of the 

 flies was aberrant (October 1912). This mutant, which he called 

 "shifty," was characterized by a reduction and shifting of the veins in 

 the basal region of the wing. Often the second longitudinal vein is 

 joined to the third near its base or even distal to the anterior cross- vein 

 (plate 10, figs. \h,\c,ld). This anterior cross- vein is itself often absent 

 (figs. 1 band 1 c), while in other cases there is an extra cross- vein between 

 the second and third longitudinal veins and placed nearer the base of 

 the wing than the anterior cross-vein (fig. 1 h) . The wing as a whole 

 is slightly shorter than normal and occasionally is much more reduced, 

 with an accompanying reduction of the venation. 



Shortly after this (November 22, 1912) Bridges found in the F2 of a 

 cross of sable male to wild female 6 females with short legs among 

 about 400 offspring (C146). The F2 was from a mass-culture, so that 

 there is no significance in finding more than one and less than a quarter 

 of the flies with this character. The fact that all these flies were 

 females also was of no significance, except as an indication that the 

 character was probably autosomal. A pure stock of "dachs," as the 

 mutation was called, was soon extracted from the ofTspring of the dachs 

 females mated to their wild-type brothers. Some further selection 

 was necessary to eliminate sable from the stock. 



DESCRIPTION OF DACHS. 



The characteristic feature of dachs is the fact that the tarsus of each 

 leg has only 4 joints instead of the 5 possessed by nearly all other Diptera 

 (plate 10, fig. 1 a). These joints are themselves perfectly distinct and 

 normal, except for being slightly shortened. The proximal one has, 

 in the male, the sex-comb typical of the proximal joint of the normal 

 male, so that the omitted joint is one of the more distal ones — perhaps 

 the penultimate. The rest of the leg, especially the tibia, is also 

 shortened. The legs are drawn in close, so that they seem much 

 shorter than they are, and the "dachs" appearance is accentuated. 

 It was soon noticed that the venation was aberrant in the same manner 

 as in "shifty' ' flies, and an examination of the shifty stock showed that 

 all of them had four-jointed tarsi. In fact, the two stocks were prob- 

 ably identical. Whether they were of single origin (from the wild 



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