240 ANIMAL AGGREGATIONS 



tions prevail, and there is decidedly greater survival the more dense 

 the population. In making these observations, due care was taken to 

 consider only those insects located on trees in comparable situations 

 and on comparable parts of the trees. Bliss concludes that death 

 from these sprays is not due to suffocation, as has been supposed, but 

 is due to the taking-up of some poisonous material by the insects; 

 and that when they are present in greater numbers, the toxic mate- 

 rial is divided between more insects, no one of which is so likely to 

 receive a lethal dose as if the population were less dense. 



POPULATION DENSITY AND LONGEVITY IN DROSOPHILA 



The relation between the density of population of laboratory cul- 

 tures of wild-t>pe Drosophila and their life-duration were less ex- 

 pected. Pearl and Parker showed in 1922 that statistical analyses 

 of data accumulated in other studies indicated greatest longevity 

 from bottles originally stocked with from 35 to 45 of the wild 

 stock per bottle. In 1923 the same workers reported the results of 

 an experiment made to test out this question of an optimal popula- 

 tion. The data on the length of life of 12,382 individuals showed 

 that the optimal density of population, when longevity is taken as 

 the criterion, is not found in the minimal populations but lies in the 

 region of initial densities of from 35 to 55 per i-ounce vial, and the 

 increase in length of life from the lowest density is at a much more 

 rapid rate than is the decline of duration of life after the optimal 

 density is passed. 



A more complete analysis of the problem is given by Pearl, Miner, 

 and Parker (1927). In this experimental work the flies were kept 

 in I -ounce vials stoppered with cotton plugs and held at 25° C. 

 The bottles were examined daily, the dead flies were removed and 

 their age recorded, and the living flies were at the same time trans- 

 ferred to fresh bottles of newly prepared food. In the first experi- 

 ments banana agar was used as food, but similar results were ob- 

 tained with a synthetic medium. 



The extent of the experimental data may be visualized from the 

 following statement of the numbers used in one experiment. One 

 hundred and fifty vials were started with an initial population of 



