REACTIONS OF LAND ISOPODS TO LIGHT 197 



IV. NORMAL BEHAVIOR 

 A. REACTIONS TO LIGHT 



Animals respond to photic stimuli by three methods: 



1. Increased or decreased activity, without orientation. 

 (Photokinesis.) 



2. Definite orientation with respect to the source of Hght, 

 usually accompanied by locomotion toward or away from the 

 light. (Phototaxis.) 



3. In animals with image-forming eyes, response to some sort 

 of image made upon the photoreceptive cells. (Vision.) 



Inasmuch as sowbugs live in concealed places, the part which 

 light normally plays in their activity is not easily observed. It 

 is, however, clear that they can become accustomed to living 

 in the ordinary daily rhythm of hght and dark, because they 

 lived a normal life for weeks in the laboratory under these 

 conditions. 



1. Photokinesis 



Sowbugs appear to respond to sudden increases in intensity 

 of light, both when they are exposed by the overturning of a 

 log under which they are concealed, and, in the laboratory, 

 when a near-by artificial light is turned on. Two tj^jes of re- 

 sponse follow under either of the described conditions: 



1. Increased activity. 



2. Complete cessation of activity. 



If the response is one of increased activity, the animals usu- 

 ally keep moving until they reach conceahnent or, at least, par- 

 tial concealment in a crevice. The individuals which are inac- 

 tive at first later become active and seek a more protected situa- 

 tion. No sowbugs are left in sight a short time after a log which 

 has concealed them is overturned. 



2. Phototaxis 



Under the conditions just described, the light is too diffuse to 

 bring about any definite orientation of the isopods. When, 



