236 CHARLES HAELAN ABBOTT 



The fact that the reaction of PorceUio appeared from the ex- 

 periments to be less negative than that of Oniscus is correlated 

 with other generic differences. Porcellio, because of the structm-e 

 of its respiratory appendages, is better provided for living in dry 

 situations than is Oniscus, and hence has perhaps less need of 

 a definite negative reaction to keep it in its proper environment. 



Furthermore, the phototaxis of Porcellio seems to be reversed 

 from negative to indifferent or positive more easily than that of 

 Oniscus. This has been observed under natural as well as 

 experimental conditions. A good example was noted at Cold 

 Spring Harbor, Long Island, in July, 1916, when Porcelho was 

 found commonly for three or four days on green plants at a 

 height of four or five feet from the ground. Among the possible 

 causes for this unusual behavior was a partial or complete flood- 

 ing of the ground habitat, but, whatever the cause, it was appar- 

 ently accompanied by a reversal from the usual negative photo- 

 taxis, with perhaps also a reversal of geotaxis. 



Table 10 also suggests, although in this case it does not offer 

 a suitable basis for comparison, that possibly Porcellio rathkei is 

 less negative than P. scaber. If so, this furnishes a further cor- 

 relation, since, as has been mentioned, the breathing appendages 

 of P. rathkei are, of the two species, the better adapted to air 

 respiration, and its habitat is probably slightly less restricted. 



Whether or not it is possible to find any evolutionary signifi- 

 cance in this behavior, it is, at any rate, worth while to com- 

 pare the phototaxis of land and of water isopods. The most 

 complete study of the fight reactions of any aquatic isopods 

 which has yet been made is that by Banta ('10). Table 11 com- 

 pares the results for Oniscus, recorded in the present paper, 

 with Banta's summary ('10, p. 268) of the reactions of Asellus 

 communis. 



This comparison indicates that, of the two genera, Oniscus is 

 the more definitely negative. The negative reaction helps to 

 prevent Oniscus from leaving its habitat in the daytime, while 

 Asellus, with no such restricted habitat, is at definite times 

 positive to light. 



