REACTIONS OF LAND ISOPODS TO LIGHT 223 



3. Co77iparison of reactions following exposure to light and to dark 



Most of the isopods used in the preceding experiments had 

 been living in the dark, but some had been exposed to the daily- 

 changes of an ordinary room. No relation was observed be- 

 tween the degree of negative phototaxis and this difference of 

 exposure. 



The effect of continued illumination by strong artificial light 

 was tested in a few individuals of both Oniscus and Porcellio. 

 They were placed in a dark room and exposed for a few hours, 

 and in some instances for a longer period, to an electric light, 

 with a rectangular glass jar of water to absorb the heat, be- 

 tween the Ught and the terrarium. While most of the experi- 

 ments already described furnish a suitable control on these 

 results, additional tests were made, using animals which had 

 been placed in a dark corner of the room during the time that 

 the other individuals were exposed to light. In a few instances, 

 the same animals were placed alternately in the two habitats. 



A comparison of the reactions of isopods from these two 

 sources is shown by the diagrams in figures 9 and 10, and by the 

 angles of negativeness in table 8. 



TABLE 8 



Comparison of the angles of negativeness, following exposure to artificial light and 

 to darkness. Oniscus and Porcellio rathkei 



The results after exposure to light are compiled from five indi- 

 viduals each of Oniscus and Porcellio rathkei. The control for 

 Porcellio is compiled from records for ten individuals. That 

 for Oniscus is a summary of the daily reaction for fifteen days 

 of Oniscus A 12 (tables 3 and 4, fig. 7), which was a member of 

 this control series. In the charts, the eight degrees of negative 

 reaction are represented by the abscissae, while the ordinates 

 indicate the proportional number of responses, all the curves 

 being drawn to the same scale. 



