332 ANIMAL AGGREGATIONS 



mum environmental conditions in order to eliminate distortion in 

 ratios in experimental work with Drosophila, and lists the effect of 

 overcrowding as one of three main sources of disturbance. In the 

 early Drosophila work, he says, this was the largest source of diih- 

 culty. 



Eigenbrodt (1925) presents evidence that overcrowding the larvae 

 of a homozygous race of bar-eyed Drosophila decreases the weight 

 attained by the adult flies and decreases the size of the whole animal 

 and of such individual parts as the thoracic length and length of 

 wing. The number of eye facets, of hairs on or around the eye, and 

 the number of teeth in the sex combs (found in the male only), are 

 also decreased. Flies reared under overcrowded conditions do not 

 exhibit the correlation of size with the temperature at which they 

 are reared that is characteristic of normal flies. The rate of devel- 

 opment is retarded by the overcrowding; variability tends to be 

 increased, and sexual dimorphisms tend to disappear. In order to 

 obtain standard results, Eigenbrodt concludes that Drosophila 

 should never be reared under overcrowded conditions. He finds 

 that the flies are normal in the foregoing relations if from 8 to 20 

 hatch out in an 8-dram vial which contains 9 grams of standard 

 food. Under these conditions a higher population than 20 flies rep- 

 resents overcrowding. 



Plunkett (1926), in his attack on the problem of how genes pro- 

 duce their effects, undertook to study experimentally" the effects of 

 various combinations of genetic and environmental factors upon the 

 development of Drosophila bristles. These have the advantage for 

 this sort of work of being discrete units that can be counted with a 

 minimum of observational error. Their number is known to be 

 affected by different genes, and also by such environmental factors 

 as temperature and nutrition. The bristles have been plotted and 

 named for the normal wild-type Drosophila and for various other 

 stocks. Plunkett, in his experiments, used a mutant stock known 

 as Dichaete, selected for low bristle number for many generations, 

 and took care to insure uniformity in the different culture bottles, 

 except for the factors under observation. 



Under these conditions it was found that the number of flies 



