COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES. 379 



would sometimes stop outside the runway and go through the same 

 operations. 



As a rule the animals which react most constantly and uniformly to 

 directive light are those that are under more or less unnatural condi- 

 tions, as was the case with the earthworm, Bipalium, Tenebrio, and, to 

 a less extent, Oniscus, when placed on the glass plate or on the table 

 top. These animals usually come to rest under normal surroundings 

 only when a considerable portion of the body, especially the dorsal 

 side, is in contact with something. Cockroaches, on the other hand, may 

 often be found at rest upon the walls of the dimly lighted cellars and 

 basements where they live. They do not by any means spend all 

 their time in cracks and crevices, though they usually retire to such 

 places when more strongly illuminated. In many animals the ordinary 

 reactions appear to have an inhibitory effect on the reaction to light, 

 so long as the animal is not disturbed in any unusual way. Or the 

 animal may simply come to rest, and yet respond immediately to such 

 constant stimuli as directive light or gravity if it be disturbed. 

 Carpenter (^05) has pointed out, for example, that pomace flies 

 (Drosophila) will come to rest in the darker portions of the dish 

 with their heads turned away from the light, but if they be disturbed 

 by turning the dish slightly, they respond at once with their ordinary 

 phototropic reaction. Mechanical agitation has a similar accelerating 

 effect upon their reaction to gravity. As will be described later, a frog 

 may sit for a considerable time with the axis of the body at right 

 angles to the direction of a light, which apparently has no effect upon 

 him ; but upon being stimulated in any non-directive way he will 

 usually turn at once and face the light, or may even hop toward it. 

 Similarly, reactions to food, to contact stimuli, etc., may inhibit 

 entirely the ordinary reactions to light. Certain pycnogonids cease 

 entirely their efforts to go toward the Hght when the feet can grasp 

 the stems of hydroids, among which these animals normally live ; 

 while if they are placed in a dish of water where they are unable to 

 grasp any such familiar objects, they are strongly phototropic (Cole, 

 : 01). In fact, Loeb ('90, p. 21, et seq.) made special mention of the 

 inhibitory effect of contact stimuli in his pioneer work on phototropism. 



Instances need not be multiplied. The point is that the cockroaches, 

 not being under especially unusual conditions, may have been influenced 

 by other stimuli or by physiological conditions which in the case of 

 animals less " at home " would have been overcome by the phototropic 

 response. This may account for the fact that the slight excess of re- 

 actions was in a direction opposite to what one would expect. But the 

 surprising thing is that this excess should have been so small, for in an 



