230 CHARLES HARLAN ABBOTT 



The reactions of the two species of Porcellio appeared in various 

 experiments to be essentially alike, but no comparative tests 

 were made. The present paper deals only with generic 

 differences. 



3. Oniscus was more negative than Porcellio under all condi- 

 tions^ in which comparative experiments were made, and, as is 

 shown in figure 13, the range of reaction in Porcellio was much 

 greater. Furthermore, a decided reversal of reaction from 

 negative to positive was observed more frequently in Porcellio 

 than in Oniscus. While in most instances the cause of this re- 

 versal was not determined, one apparent cause in the case of 

 Porcellio was continued exposure to strong artificial light (fig. 

 10, table 9). 



4. To summarize the preceding statements, Oniscus and 

 Porcellio are both negative to directive light, but Porcellio is the 

 less negative. The reaction of Porcellio appears to be modified 

 more readily by changes in both internal and external condi- 

 tions. The possible ecological significance of this difference 

 between the genera will be considered in a later section. 



7. Conclusions 



The reaction of Oniscus is negative under nearly all of the con- 

 ditions tried and is not easily modified. At times in the course 

 of the experiments, individuals of this species have shown a high 

 degree of positiveness, but the cause for this occasional be- 

 havior has not as yet been found in any of the modifying condi- 

 tions tried. The normal negative phototaxis was lost under the 

 unnatural conditions of an aquatic environment. 



1 In the one instance where there is apparently little difference between the 



two genera, when the animals were taken from a maximum of moisture, the real 



contrast is shown by a comparison of figures 9 and 10. The arithmetical mean 



(A.M.), from which the figures for the angle of negativeness were derived, was 



similar in the two instances, but the true distinction is shown by the average 



deviation (A.D.), and by calculating the coefficient of variability (C.V.) by the 



A D 

 formula C.V. = . \ -- ' The results are shown in a table. 

 A.M. 



Genus A.M. A.D. C.V. 



Oniscus 6.1 0.741 0.121 



Porcellio 5.8 1.555 0.268 



