MORPHOLOGICAL EFFECTS OF CROWDING 333 



developing in a bottle, other conditions being equal, has a pro- 

 nounced effect on the mean bristle number. This effect is summa- 

 rized in Table XXXVII for populations from eggs laid during 4 days 

 for the first 7 groups, and during 8 days for the last 5 groups. The 

 temperature was held at 25° C. throughout. 



In commenting on these results, Plunkett says: "It is evident 

 from the table that, under these conditions, there is no correlation 

 between bristle number and density of population up to about forty 



TABLE XXXVII 



Showing Data tor the Effects of Density of Popul.a- 



TiON UPON Bristle Number in Dichaete Drosophila 



(After Plunkett) 



Flies Mean Number of 



per . Posterior Dorsocentral 



Bottle Bristles per Half-Fly 



11-3 0.234 + 0.030 



24. 2 O. 284+0.017 



33 . 4 O . 284 + O . 01 2 



43-5 0.183 + 0.009 



550 0.136 + 0. on 



65.7 o. 1 20+0. on 



76.0 0.056 + 0.009 



95-5 0.055 + 0.008 



103 .5 0.051 + 0.007 



148.0 0.027 + 0.006 



226.0 0.022 + 0.005 



351..0 0.017 + 0.003 



flies per bottle; but above this the bristle number falls off rapidly 

 with increasing population, reaching almost zero (for these bristles) 

 when the population is much in excess of 100 flies per bottle, as in 

 ordinary 'stock' bottles. This 'crowding effect' makes it unsafe to 

 draw quantitative conclusions as to the effects of other genetic 

 factors or other environmental factors (e.g., temperature) on bristle 

 numbers in flies raised under these conditions; i.e., more than forty 

 offspring per bottle from eggs laid over a period of several days." 

 In later experiments, when parents were kept in the bottles for not 

 over 24 hours, tlj|^ were no obvious effects of crowding up to about 

 80 or 100 offspri^Pflies per bottle. 



Plunkett continues: "Experiments designed to analyze the factors 



