152 



HAROLD H. PLOUGH 



III. NEGATIVE RESULTS 



Moisture 



Before considering the positive results to be recorded, it may 

 be of interest to describe briefly the environmental effects which 

 were tried and found to be ineffective in causing a change in the 

 amount of crossingover. The relative amount of moisture in 

 the food has been observed to have a slight effect on the length 

 of the period between the laying of the eggs and the final hatch- 

 ing of the adults. In a relatively dry culture the adults hatch 

 about two days sooner than those in a very moist culture started 

 at the same time. The flies from the former are also consider- 

 ably smaller in size than those from the latter. It was conceiv- 

 able that such conditions might have some effect on the percent- 

 age of crossingover. Females from the same stock bottle were 

 mated to black-purple- curved males and placed for four or five 

 days successively in the bottles of Series A, B and C. The bot- 

 tles of Series A contain food as it is ordinarily used — moist but 

 with no free liquid in the bottom of the bottle — as a control. 

 Those of Series B receive a small amount of food from which as 

 much moisture as possible had been pressed out, and which was 

 sufficiently dry so that no moisture was taken up from it by the 

 paper in which it was wrapped. In Series C, about a quarter 

 of an inch of free liquid was kept constantly on the bottom of 

 the bottle. The results of the backcross testing the percentage 

 of crossingover are given below: 



TABLE 2 



b — pr — c 



Control 

 Dry.... 

 Wet . . . 



per cent 

 22.2 

 20.9 

 21.5 



The method, admittedly rather crude, shows no significant 

 difference in the amount of crossingover. 



