EYE FACET NUMBER IN DROSOPHILA MELANOGASTER. 179 



the camera lucida drawing in i/ioo sq. in. In flies with a very 

 small number of facets the relation does not hold; in very large 

 eyes, the curved surface makes it difficult to obtain accurate areas. 

 Leaving out of consideration these extremes, the change in 

 facet number is accompanied by a corresponding change in the 

 size of the eye, and there is no change in the size of facets. 



DISCUSSION. 



Observations upon the red and white bar-eye show that tem- 

 perature is an important factor in determining facet number of 

 the flies taken at random from a general population. Flies 

 raised at a higher temperature, however, do not have a higher 

 number of facets, but on the contrary, the higher the temperature 

 of development, the lower the facet number. An explanation 

 which may be offered presupposes the existence of a chemical 

 factor or determiner which acts as an inhibitor of facet formation 

 and that, at a higher temperature, the speed of reaction is much 

 greater than at a lower. Then, according to Van't Hoff's law, 

 with an increase of 10 C. the speed of the reaction should be from 

 two to three times as great and the number of facets one half to 

 one third times as large, and vice versa. An examination of the 

 data shows a considerable degree of approximation to this condi- 

 tion. In the males with a 10 decrease the facet average is 2.6 

 times as great; in the females 3.5 times. Whether or not the 

 high value, 3.5, in the females is due to the fact that bar-eye is 

 sex linked and the female receives two chromosomes containing 

 this factor, while the male receives but one, is a matter for 



speculation. 



SUMMARY. 



1. Temperature is an important factor in the determination 

 of facet number in the bar-eye of Drosophila. 



2. A lower developmental temperature results in a higher facet 

 number, and conversely a higher temperature results in a lower 

 facet number. 



3. With each 10 drop in temperature between 29 and 15, the 

 facet number is increased on the average 2.6 times in the males 

 and 3.5 times in the females. 



4. The increase in facet number except at the extremes is 



