EFFECT OF CROWDING ON SURVIVAL 185 



tested in manometer respirometers such as were described by Kraj- 

 nik (1922). The results obtained support the conclusions reached in 

 earher studies with carbon-dioxide production, that is, that with 

 land isopods the recently bunched individuals are carrying on respi- 

 ration at a less rapid rate than when recently isolated. In exact 

 ratios, the rate of oxygen consumption with recently bunched and 

 recently isolated Armadillidium in two tests were 1:1.486 and 

 1 : 1.368, while with Tracheonisciis the same ratios were i : 1.214 and 

 1:1.344. 



The Armadillidium tests allow other comparisons. Since groups of 

 isolated or bunched individuals were set away in the dark for ap- 

 proximately 24 hours, one can compare their physiological condition 

 near the beginning of this period with that at the end and also make 

 cross-comparisons. 



In general, the determinations show that, under the conditions of 

 the experiments, there is a very marked decrease in oxygen consump- 

 tion after 24 hours' isolation and starvation — 70 and 65 per cent, 

 respectively, in two sets of experiments. There is a similar but less 

 pronounced reduction when the animals are bunched and starved — 

 in that case 31 and 29 per cent, respectively. 



At the end of 24 hours the bunched isopods are uniformly using 

 more oxygen per unit weight than are the isolated individuals. The 

 observed ratios were 1:1.64 a-^d 1:1.48. All the differences men- 

 tioned are statistically significant, since the least difference in means 

 is still over eleven and a half times their combined probable error. 



Experiments extending over 50 hours showed that the bunched 

 isopods kept their higher rate of respiration during this period as 

 compared with similar isolated individuals. The aggregated individ- 

 uals exhibited a greater range in rate of oxygen consumption during 

 this time, probably due to the greater difference between repose and 

 occasional activity. They also were much less likely to move about 

 than were the isolated individuals. In all these experiments neither 

 set of isopods was fed, but the w^ater content was manipulated so 

 that there was no essential difference in weight, as shown by random 

 weighings. 



Calibrations show that oxygen consumption for the Armadillidium 



