Reactions of Isopods to Light 255 



to be of any significance as far as the influence of light was con- 

 cerned. 



I expected to find a zone of fairly low light intensities in which 

 the responses would be neutral, and below this a region of light 

 intensities to which the animals would respond positively. But 

 repeated experiments with low intensities did not reveal a single 

 response in a positive direction. 



Asellus, then, does not respond to light of the intensity of i 

 CM. or less. This is because the animal is not stimulated by light 

 of such intensities or is stimulated so slightly as not to respond in 

 a directive way. 



The eye of Asellus is composed of from 12 to 20 more or less 

 irregularly shaped facets and functionally is probably little more 

 than a direction eye. It is situated somewhat mediad of the lateral 

 margin of the head and slightly behind its anterior margin. Its loca- 

 tion is, therefore, such that light from one side could not strike 

 the eye on the opposite side, whereas light from above or from a 

 strictly anterior or posterior direction would strike both eyes 

 equally. If stimulated by light at all, it would seem as though the 

 stimulus received from a small source of light ought to be directive 

 in its effect. I incline to the opinion, however, that the animal is 

 not at all sensitive to light of so low an intensity as i CM. This 

 opinion is further supported by the fact that Asellus, although pos- 

 itive to moderate and fairly low intensities, after being in the dark 

 for several hours, is not responsive at all to intensities as low as 

 I CM. 



Asellus after previous exposure to diffuse daylight, was nega- 

 tive to a light of 2.5 CM. (19 c.p. incandescent at 2.75 m. from 

 middle of tank), and to all greater intensities. The negative 

 response was o'ften slow in manifesting itself, but it occured with 

 a fair degree of uniformity. Careful observations upon the actions 

 of individual animals, as well as upon numbers of them at the 

 same time, permitted the following analysis: Three different 

 factors operated in producing the tardiness of the directive re- 

 sponse. First, if the animals were once thoroughly settled in the 

 tank before being exposed to the horizontal light, they were very 

 slow to move, particularly if stimulated by light alone. This 



