58 W. C. ALLEE. 



clinging than the posterior ones, which act as braces. When thus 

 clamped the isopods withstood a flow as high as 6,600 c.c. per 

 minute under the conditions of the experiment. Sometimes they 

 failed to clamp when headed positively but this occurred less often 

 than in any other position. The speed of the current successfully 

 resisted appeared to fully equal or surpass the most rapid flow 

 at usual stream levels in those streams in which the isopods are 

 abundant. 



When oriented negatively the isopods may clamp and with- 

 stand a strong current (up to 3,959 c.c. per minute as compared 

 with 6,600 c.c. per minute when oriented positively). In this 

 reaction the abdomen is bent downward much as is the head 

 when the animal is turned in the opposite direction, but it is 

 more easily forced up than the head and hence this is a less 

 efficient method for clinging. Isopods fail to clamp more often 

 under these conditions than when headed positively. This may 

 be due to the difference in length of the posterior and anterior 

 legs. The latter are short and can take more active part in the 

 clamping reaction than can the longer posterior ones. The 

 failure to clamp allows the negatively oriented isopods to be more 

 often swept off their feet by a relatively slow current. 



Comparative tests of the average clinging efficiency of isopods 

 oriented positively and negatively support the evidence given 

 by the maximum clinging ability under the two conditions. The 

 results of these tests which were made under identical conditions 

 are given in Table I. The orientation of the isopod was alter- 

 nated to equalize a possible fatigue effect. In every case except 

 isopod No. 170 the strength of the current resisted when the 

 isopods were headed upstream was approximately double that 

 necessary to sweep the negatively headed isopods off their feet. 

 An average of 64 tests on 10 different isopods shows that a flow 

 of 2,534 (7 2 ) c - c - P er minute was required to sweep a positively 

 oriented isopod off its feet while 1,385 (92.6) caused those 

 headed negatively to lose their footing. In other words a 

 positively oriented isopod is approximately twice as efficient 

 in maintaining its position in a stream as is one oriented 

 negatively. 



There are indications that isopods could maintain themselves 



