OF MUTANT CHARACTERS. 151 



describing it (Morgan, 1911; Morgan and Lynch, 1912; Morgan, 1912). 

 The name "vestigial" was adopted when it was found that the "scale' ' 

 was the remaining basal portion of the normal wing with the venation 

 characteristic of that region (plate 7, fig. 2). The enormous reduction 

 in the size of the wing is mainly due to the trimming away of the ter- 

 minal and marginal regions of the wing. There is a marked uniformity 

 in the extent of this trimming and in the character of the venation ves- 

 tiges. Most commonly the wing is trimmed away as far as the anterior 

 cross- vein, which in many specimens follows the new margin. The true 

 marginal vein with its characteristic chsetae is entirely removed. The 

 basal parts of all five longitudinal veins are easily recognizable, and have 

 their normal relationship and junctures with one another. Certain 

 small veins at the base of the wing are represented here as in the normal 

 wing. The vestigial wing is ordinarily held out at right angles to the 

 body, probably because of the relative thickness of the posterior margin 

 of the wing. Sometimes, however, the ends of the wings are bent 

 sharply backward. These wings seem to be cut off in a squarer fashion 

 than the normal vestigial wing. It is not known whether this varia- 

 tion has any hereditary significance. The "balancers" of vestigial 

 flies are affected in a way analogous to the wings. The basal segment 

 is little affected, except that it is slightly shorter and smaller. The 

 second segment is much reduced in size and in apparent complexity. 

 The terminal segment shows the greatest reduction, becoming a barely 

 discernible pip (plate 7, fig. 2) instead of the balloon-like segment 

 which is the largest part of the normal balancer (see plate 7, fig. 1). 

 Another constant feature of vestigial flies is that the two rearmost 

 bristles on the scutellum are separated a little wider than normal and 

 are erect (plate 7, fig. 2) instead of turning backwards (plate 7, 

 fig. 3). Occasionally vestigial wings are somewhat longer than is typi- 

 cal and it is probable that this lengthening is more frequent during 

 hot weather. 1 The vestigial flies are sometimes inactive, but at other 

 times run about very actively, appearing much like ants. Special care 

 has to be taken with experimental cultures involving vestigial to see 

 that the vestigial flies are all shaken out, since they cling fast to the 

 food or paper in the culture bottle and are exceptionally slow and 

 difficult to get out. The viability of vestigial flies is fairly good, very 

 close approaches to expectation being obtainable when pairs are used 

 and food conditions are favorable. The earlier work showed consider- 

 able deviations from expectation because of failure to recognize the 

 necessity of these conditions. Vestigial flies tend to hatch two or more 

 days later than the not-vestigial flies, and unless the cultures are run 

 full term will give ratios in which the numbers of vestigial are below 

 expectation. 



1 Since the above was written, Roberts (J. E. Z., 1918) has strikingly confirmed the fact that 

 high temperature favors the production of wings approaching the wild-type in size. 



