242 ANIMAL AGGREGATIONS 



somewhere in the region of 35-55 flies per one ounce bottle contain- 

 ing 8 cc. of food substrate. At densities of population above and 

 below the optimum, the specific death rates are higher at all ages 

 than they are at the optimum." 



Such results as these make one fairly bristle with questions, of 

 which only a few can as yet be answered. Pearl and his associates 

 have reported on investigations concerning the effect of changes in 

 density during the progress of the experiments. Since an initial 

 population of about 35 flies per xdal was found to lie near the opti- 

 mum under the conditions used, a number of vials were set up 

 with this density and apparently' were followed according to the 

 practice of the preceding experiments until the sixteenth day, at 

 which time the survivors were etherized and the tips of the wings 

 of half of them were clipped, after which they were returned to 

 their proper bottles. Appropriate tests showed that the wing-clip- 

 ping as practiced did not significantly affect life-duration. 



Vials with an initial population of 200 were treated in like manner 

 up to the sixteenth day, when they were etherized and counted. 

 In part of such bottles the population was then brought back to its 

 initial density of 200 by adding marked flies which up to that time 

 had lived under the optimal conditions furnished by an initial popu- 

 lation pressure of 35 flies per ounce vial. Others were brought to 

 the original population density of 200 per ounce bottle by adding 

 flies surviving from stocks started from that density. The results 

 of such treatment are shown graphically in Figure 20, which gives 

 survival curves starting with 100 flies at 16 days of age, and the 

 effect of previous history of crowding upon the duration of life 

 after that age. 



In commenting upon these results, the experimenters state that 

 here, as before, flies subject to an initial density of 35 per ounce 

 bottle survive, on the average, about double the time those live 

 that are subjected to an initial density of 200 for the same size vial 

 and with the same food supply. They continue: 



"When flies which have lived the first 15 days of their fives under 

 the conditions implied by an initial density of 35 are on the i6th 

 day of their age submitted to a density of 200, and live out the 



