EFFECT OF CROWDING ON SURVIVAL 197 



Such results are statistically significant. If observed differences in 

 weight are considered, the calculated difference in oxygen consump- 

 tion between the bunched and the isolated isopods is still greater. 



A group of tests were run to find the respiration rate of the 

 bunched isopods as compared with those scattered among the vege- 

 tation in the upper swamp. These latter animals had not aggre- 

 gated; nor had they been exposed recently to the stimulus of a strong 

 current. The tests were made as were those preceding, except that 

 females were used because they were so much more numerous than 

 males in the swamp. Since the females are much smaller than the 

 males, 20 were placed in each respiration chamber, where 10 males 

 had been used in the other tests. The rate of oxygen consumption 

 was sHghtly higher in the isopods taken from the aggregations than 

 that of the similar females from the vegetation of the upper swamp, 

 even when the observed figures are corrected for differences in 

 weight; but the results as observed are without statistical signifi- 

 cance. Before calculation for weight differences there are 8 chances 

 in 10 of securing so great a difference by random sampling. More 

 work is needed at this point, but the results indicate that the fe- 

 males of the aggregations are in more nearly the same physiologi- 

 cal state as those scattered in the upper swamp than were the males 

 from the aggregations like those scattered in the current below the 

 swamp but above the aggregations. 



The rate of oxygen consumption of these females taken from the 

 aggregations approaches that of the males run at approximately the 

 same temperatures. The males tested on the afternoon of April 15, 

 at a temperature of 15° C, consumed, on the average, 33.2 cu. mm. 

 of oxygen per 10 males. The 20 females tested 2 days later at 17° C. 

 used, on the average, 30.2 cu. mm. of oxygen. They weighed 1.1853 

 against 1.3765 grams for the males. If they had continued to con- 

 sume oxygen at the same rate per unit weight and had weighed as 

 much as the males, they would have used 35.1 cu. mm. of oxygen 

 in the same time. 



If the comparison is carried a step farther, we find that these 

 females do not have a rate of oxygen consumption comparable with 

 that of males scattered through the stream just above the aggrega- 

 tions. Using values calculated from equal weights, we find that the 



