REACTIONS OF LAND ISOPODS TO LIGHT 235 



b. Reactions to contact. Isopods as a group have the general 

 plan of structure common to contact animals, as is seen best of 

 all in the oval bodies, with flattened ventral surfaces, which 

 characterize the Oniscoidea. Positive thigmotaxis is doubtless 

 an important factor in keeping them in a suitable habitat. The 

 use of the sense of contact as a substitute for the sense of sight 

 has already been referred to (page 198). 



c. Reactions to light. With the recognition that land isopods 

 are contact animals and that further studies will probably show 

 reactions to other classes of stimuli, the part in their normal life 

 played by the reaction to light remains to be considered. Since 

 studies of the reaction, both to artificial light and to daylight, 

 have shown that sowbugs are decidedly negative, even to low in- 

 tensities and since the reaction is constant under most condi- 

 tions, it seems reasonable to conclude that negative phototaxis 

 is a character to be considered in an ecological interpretation 

 of these animals. 



Inasmuch as sowbugs usually live in the dark, it seems prob- 

 able that during the greater part of the time light is not an influ- 

 ential factor in their lives. When, however, they accidentally 

 wander from their places of concealment, their reaction to light 

 may be expected to turn them back again into the dark. Their 

 actual behavior out of doors when exposed to light can be ob- 

 serv^ed when a log or other object concealing them is overturned. 

 Under these conditions, some of the sowbugs become active at 

 once, soon disappearing in crevices or under the log, while others 

 crouch down motionless. In the latter case, when thigmotaxis 

 apparently overcomes phototaxis, the result is only temporary, 

 for in a short time no sowbugs are in sight. Although the light 

 is so diffuse that it cannot direct their course as unmistakably 

 as under experimental conditions, yet its influence seems suffi- 

 cient to carry them to dark places and to keep them there. From 

 this fact it may be concluded that light serves as a stunulus to 

 turn them back whenever they wander into it in the course of 

 their ordinary activities. The same is probably true in the 

 early morning, whenever they leave their places of concealment 

 at night. 



