Effects of Temperature on Drosophila 309 



was put successively into seven vials, the entire lot being trans- 

 ferred from one vial to the next each time. After being made 

 up the vials were kept in the incubator until after the final trans- 

 fer, then all were put in the cold for thirty-six days (three times 

 the ordinary developmental period in the incubator). In the 

 first vial the pupae were nearly ready to hatch when placed in 

 the cold and in the others the embryos were successively younger, 

 those in the last being only one day old. From the first three 

 vials many flies hatched while in the cold. From the last none 

 hatched until six days after removal from the cold. No modi- 

 fied flies hatched from the first vial until near the end of the 

 hatch, six days after removal from the cold; and these had nor- 

 mal eyes, (only the bristles affected). In the next vial modi- 

 fied flies appeared earlier and in larger numbers, and so on 

 through the series until in the later vials modified flies appeared 

 in the first counts and were absent from the final counts. The 

 modifications noted were : ( 1 ) speckling or roughening of the 

 eyes, (2) abnormal number of arrangement of sterno-pleural 

 bristles, and (3) abnormal number or arrangement of scutellar 

 bristles. The latter was less frequent than in virilis and the for- 

 mer bristle modification usually involved additional bristles in- 

 stead of fewer as in virilis. Rarely the posterior cross-vein was 

 affected also. 



The effects of cold, then, on the bent race of melanogaster 

 agreed with those on bent virilis in that they involved the eyes, 

 the sternopleural bristles and the scutallar bristles. Likewise 

 cold had no effect at all, unless it was an inhibiting effect, on the 

 leg and wing modifications. The only effect of cold found in 

 virilis and not thus far found in melanogaster is the thickening 

 of the apex of the fifth vein. It is also to be noted that the ef- 

 fective period of the cold is localized in both species, although 

 the exact developmental stage at which it comes has not yet been 

 determined. 



In conclusion it may be observed that in "normal" stocks of 

 both species reared in the cold none of the above effects has been 

 observed. Also matings of specimens of D. virilis showing the 

 scutellar, sternopleural and delta-like modifications, without ex- 

 posure to cold, have given only ordinary bent offspring. These 

 facts, together with the nature of the results as a whole, are be- 

 lieved to eliminate the possibility of modifying factors or other 

 genetic causes (rather than cold) being primarily responsible. It 



