Reactions of Isopods to Light 263 



The experiments described above serve to illustrate the series 

 in which Asellus after exposure to diffuse dayhght was subjected 

 to illumination with light of various intensities from 2.5 CM. to 

 2855 CM. Many of the reactions were less pronounced than 

 those described above, and sometimes the results were not very 

 definite. Generally such cases were readily explicable as due to 

 the apathy of the animals after they had once become thoroughly 

 settled in the tank; for such indefinite results usually came from 

 testing animals which had been in the tank 24 hours or longer. 



From the experiments with Asellus communis when subjected 

 to light of various intensities after previous exposure to diffuse 

 daylight, the following conclusions are drawn: 



1 Asellus does not respond to light below about 2.5 CM. in- 

 tensity, but responds to light from 2.5 CM. to 2855 CM. inten- 

 sity. 



2 This response consists of two factors, a non-directive pho- 

 tokinetic effect and a directive negative phototactic effect. The 

 former is often the prevailing influence at the start, but the pho- 

 totactic influence becomes the effective one as the photokinetic 

 effect decreases and eventually disappears. 



B. After being in Darkness 



In conducting experiments on Asellus which had been kept in 

 darkness before the beginning of observations, the following 

 method was pursued. The animals were first placed in the tank 

 and left in the darkened room. In addition to darkening the 

 room, the outer tank was covered with light-proof screens. After 

 the animals had been thus kept in the dark for the desired length 

 of time, the screens were carefully removed and the lamp, which 

 had previously been placed in position for the experiment, was 

 suddenly made light. 



Mention has already been made of the fact that Asellus does 

 not respond to light intensities of i CM. or less. The statement 

 holds true whether the Asellus has been previously exposed to 

 light or has been in the dark. Several intensities between 0.001 

 CM. and i CM. were tried, but definite responses were not 



