DENSITY OF POPULATION AND INSECT SURVIVAL 241 



I pair each; similarly 80 vials contained originally 2 such pairs; 

 50 vials contained 3; and 40 vials contained 4 pairs each. Thirty 

 vials were started with 5, and 30 more with 6 pairs each, and 20 

 vials contained an initial population of 15 flies, or 7.5 pairs. Ten 

 vials were started with each of the following populations: 20, 25, 

 35. 45^ 55» 65, 75, 85, 95, 105, 125, 150, and 200. In another experi- 

 ment the initial densities per i -ounce vial were: 5, 25, 50, 75, 100, 

 200, 300, 400, and 500. 



The results from the first experiment are given graphically in 

 Fig. 19, which shows the mean duration of life of wild-type Drosophi- 



10 20 30 ^0 SO 60 70 80 90 100 110 120 130 I'M /SO 160 170 ISO 190 200 210 

 /nitial Density 



Fig. 19. — Showing the mean duration of life in Drosophila with relation to the initial 

 density in i-ounce bottles. (From Pearl, Miner, and Parker.) 



la at different densities of population. The second experiment yield- 

 ed similar results and demonstrated that with the higher densities 

 there is relatively slight effect upon longevity of marked increases in 

 original density. 



Pearl, Miner, and Parker sum up their experience with this sort 

 of experiment as follows (1927): 



"The rate of mortality of Drosophila is profoundly influenced by 

 the density of the population, that is, by the number of flies to- 

 gether occupying a limited universe in which volume of air, volume 

 of food, and area of food surface are constant. 



"There is an optimal density of population for Drosophila under 

 the conditions of these experiments. This optimal density falls 



