REACTIONS OF LAND ISOPODS TO LIGHT 



217 



The test was made by three methods: 1) testing individual ani- 

 mals for a series of days, each day at a different intensity; 2) 

 testing several animals from the same habitat on the same day, 

 each animal at a different intensity, and 3) trying the different 

 intensities in turn on each animal. 



The experiments showed that the response is in general the 

 same to all intensities which were tried. If an individual isopod 

 is negative it is negative to all intensities, while if it is indif- 

 ferent, it is alike indifferent to all intensities. This result was 

 obtained by all three of the methods described, so that the 

 different methods furnish a check on each other. 



Gra^M of 



Ne^aTive. 



Positive 



10.5 



20,5- 



30,5 



40.5" 



50.5 



60S fvo'm 



Fig. 8 Reactions to light of different intensities. Two individuals of Oniscus 

 (23 and 25) and two of Porcellio rathkei (24 and 26). Numbers on abscissa indi- 

 cate distances in centimeters from slit in diaphragm through which light passes. 

 Intensities at each of these points are given in the table on page 216. Numbers 

 on ordinate indicate the eight degrees of negativeness shown in figure 2. Shows 

 no consistent variation corresponding to the intensity. 



The third method is perhaps the most satisfactory, because 

 it eliminates errors due to differences between individuals and 

 to variations in 'physiological state' on successive days. It has 

 the disadvantage that the result is based on four responses only 

 (one in each position) at each intensity, rather than on twenty 

 responses. 



The reactions of four isopods, classified according to this 

 method, are shown graphically in figure 8. The numbers on the 

 abscissae indicate the distances from the light, which correspond 



