THE ORIENTATION OF PORCELLIO TO LIGHT 113 



During, the day Porcellio is usually found under stones, logs, 

 rubbish, in dark cellars, and various other sorts of cover from 

 the light of the sun. Correlated with this habit is a definite 

 negative phototropism. 



In our first experiments, this phototropism was more or less 

 masked by large individual differences in sensitiveness to light, 

 and the apparent indifference of many individuals to light com- 

 ing from incandescent bulbs placed directly in front of them. 

 Later we discovered that the locomotion of many such indif- 

 ferent individuals could be controlled with great definiteness 

 by holding an incandescent bulb behind them, as they marched 

 over a dead black table top, and moving it to one side or the 

 other. Under these conditions — Mazda bulbs of both 25 w. 

 and 60 w. were used— the organisms would move away from 

 the light with the precision of a boat answering the helm. They 

 could be guided in circles, in spirals, in courses that were di- 

 rected, now to the right, now to the left, at the will of the 

 experimenter. 



That the eyes were the organs responsive to light was demon- 

 strated by blinding them with a mixture of charcoal and glue. 

 Individuals with the right eye blinded reacted to light from the 

 left only; when the left eye was blinded light from the right 

 was alone effective; when both eyes were blinded the individ- 

 uals thus treated were indifferent to light from any direction. 



Porcellio responds not only to changes in the direction of 

 light. Exposure to light stimulates into activity animals that 

 in darkness are quiescent; though sudden changes in intensity 

 of illumination may produce inhibitory effects. Individuals vary 

 considerably in their responses to these and all other types of 

 stimulation. Marked differences may exist between individuals 

 of the same size and apparently the same age; also between the 

 reactions of the same individual at different times. Age differ- 

 ences are frequently connected with different reactions. Very 

 young, unpigmented individuals are more responsive to direc- 

 tive stimulation than old. It is the rare exception for them to 

 fail to respond, although adults are not uncommonly refractory. 

 To sudden changes in intensity of light, however, old react at 

 least as sensitively as young. In this connection the following 

 case may be cited. 



