EFFECT OF CROWDING ON SURVIVAL 199 



isopods in the field. Neither was it possible to arrange to make 

 respiration tests without disturbing the aggregations as was done in 

 the laboratory. One must use care in interpreting results based 

 upon the oxygen consumption given during the first hour following 

 the considerable disturbance occasioned by the extraction of 10 

 isopods from the midst of an aggregation as compared with that of 

 10 isopods for the first hour that they are placed together in a small 

 respiration chamber after having been previously scattered. 



It must also be noted that the isopods from the aggregations and 

 those collected while still separate cUd not behave similarly in the 

 respiration chamber. The former tended to gather in the restricted 

 space furnished by the necks of the bottles forming the respiration 

 chamber, where they were well shaded by the rubber tubing that 

 held the two bottles together. Those collected while scattered tend- 

 ed to remain so, and were more likely to be found exposed to the 

 brighter light at the ends of the bottles. Since all lots tried behaved 

 uniformly in this regard, and since previous location also made a 

 difference in the oxygen consumption, the differential behavior with- 

 in the respiratory chamber may be taken as one of the factors in 

 determining the respiration relations observed. The exact weight 

 that should be given this factor has not been determined. Such 

 behavior might result in the shaded, more aggregated isopods using 

 less oxygen during the respiration period than would the unshaded 

 scattered individuals, as was found to occur. 



As a result of experience with respiration of laboratory aggrega- 

 tions, it seems probable that the differential respiration observed is 

 due to a difference in muscular tonus between the aggregated and 

 the isolated animals. If this idea is applied, we have the reasonable 

 assumption that isopods taken from the upper swamp far enough 

 away so that they are undisturbed by the currents set up by the 

 overflow are under approximately the same muscular tension as are 

 those taken from the dense aggregations, and that both are under 

 less muscular tension than are the isolated iospods that have re- 

 cently been exposed to the full impact of a current which in many 

 instances has been carrying them hurtling along over a sandy stream 

 bed. The evidence at hand allows no suggestion as to whether the 



