REACTIONS OF LAND ISOPODS TO LIGHT 243 



According to table 12, the length of the intervals before re- 

 sponse and the percentage of response after delay are essentially 

 the same under the two conditions of the experiment. This is, 

 however, probably a matter of photokinesis rather than of 

 phototaxis. 



From this experiment it may be concluded that orientation in 

 land isopods occurs after delayed response only when the stimu- 

 lus is continuous. In other words, if the initial stimulus is not 

 sufficient to bring about immediate response, the stimulus must 

 be continued in order for orientation to occur. Since, then, the 

 response to a continuous constant intensity can occur in cases 

 of delayed response, there appears to be no reason why the same 

 type of stimulus cannot be a cause of the reaction when the 

 response is not delayed. 



3. Conclusions 



From the preceding experiments and discussion the following 

 conclusions may be drawn: 



a. Orientation of land isopods appears to be direct, and not 

 the result of a selection of random movements. 



b. Orientation is at times due to the continuous action of 

 light acting at constant intensity, even though it may also be 

 due to the shock produced by changes of intensity. 



VII. SUMMARY 



A study of the reactions to light of three species of land iso- 

 pods, Oniscus asellus, Porcellio rathkei, and Porcellio scaber, 

 resulted in the following conclusions: 



1. Land isopods respond to light stimuli in three ways: by 

 photokinesis, by phototaxis, and by vision. Vision is, however, 

 only slightly developed. The experiments were confined to 

 phototaxis. 



2. Oniscus is negative to diffuse daylight and to controlled 

 horizontal illumination by artificial fight. The latter was shown 

 in detail by a study of the percentage of negative responses, 

 by a graphical representation of the degree of turning away 



