Reactiotis of I supods 483 



occurrence of non-caverniculous species outside of rather than 

 withm caves. 



One needs to experiment with the two species for only a short 

 time to be struck with the similarity of their responses to various 

 influences. The minor differences in their reactions, however, are 

 very significant in relation to the habitats of the animals. While 

 Caecidotea is responsive to only fairly high intensities of light 

 (SoC.M.or greater), it is always negative to any intensity to which 

 it responds at all. Hence, if outside a cave, its light reactions 

 alone would tend to lead it into a cave if there were one near, 

 while if it were in a cave and wandered into the light near an 

 outlet, Its negativity to light would prevent its leaving the cave 

 and passing into waters above ground. If Asellus were near a 

 cave, its response to light would at first tend to direct it into the 

 cave. But, after having been in darkness within the cave for a 

 time, it would again become positively phototactic, so that if it 

 came in reach of light from the outside it would escape. 



Again, Caecidotea, while less strongly rheotactic than Asellus, 

 when first subjected to a current, is more persistentlv rheotactic. 

 In experiments combining the effects of a current of water 

 with light stimulation under such conditions that when the 

 animals moved against the current they passed into a darkened 

 region, it was shown that Caecidotea remained in the darkened 

 region more persistently than Asellus did. This, as already stated, 

 repeats in miniature the usual conditions found in cave streams, 

 since they generally flow out of caves rather than into them. 

 In such cases, even if Caecidotea were not rheotactic, its greater 

 tenacity in holding to the substratum would better enable it to 

 make its way into caves and remain there; whereas, if the stream 

 were swift, Asellus could not hold its own against the current. In 

 the experiments with light combined with a current of water 

 under such conditions that when the animals moving against the 

 current passed out of a darkened region into a strongly illuminated 

 one, the other cave stream condition (found in streams which 

 enter caves from above ground) was in a way duplicated. In 

 those experiments Asellus was more vigorously rheotactic at first, 

 but Caecidotea more persistently so, although under these condi- 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. S, NO. 4. 



