220 CHARLES HARLAN ABBOTT 



orientation, which shows the results of sHght differences in 

 effectiveness of the receptive mechanism." If that be so, the 

 abrupt and definite orientation of Oniscus to low intensities 

 indicates that Oniscus is extremely sensitive to light and is 

 definitely oriented by light. 



The reactions of Porcellio to low intensities show as usual 

 more variation than those of Oniscus, but they are in the main 

 negative, and essentially the same as the reactions to higher in- 

 tensities. They show that Porcellio, as well as Oniscus, is re- 

 sponsive to extremely low intensities of light. 



These results are particularly interesting, because Banta ('10) 

 found Asellus communis unresponsive to light of an intensity of 

 1 CM. or less. As will be considered later, Oniscus and Por- 

 cellio appear to be more sensitive to light than is Asellus, the 

 common fresh-water isopod. 



The conclusion to be derived from these experiments is that 

 land isopods are sensitive to a wide range of light intensity and 

 their reaction to all intensities is essentially the same. 



2. Effect of repeated stimulation 



A second condition which might change or reverse the reac- 

 tion is repeated stimulation. For these experiments, Oniscus, 

 since it is definitely negative, was chosen, in order to watch for a 

 possible reversal from negative to positive phototaxis. The 

 animals were placed facing the light, so that every negative re- 

 sponse would necessarily mean a definite turning away from the 

 source of the stimulus. Records were made in each instance 

 of the course taken and of the interval before response. When- 

 ever no response occurred during sixty seconds, the result was 

 recorded as "no response, sixty seconds." 



Sixteen individuals were used in these experiments and were 

 given a varying number of successive stimuli: one animal re- 

 ceived 100 successive stimuli, three received 60, one received 40, 

 five received 30, and six received 20. The longest time used in 

 testing any one animal was one hour. The first individual men- 

 tioned, which gave 100 responses in one hour, was unusually 

 active. 



