112 PARKER— THE RELATIONS OF THE 



position of negative phototropism. Here, then, is an animal that in 

 flight is positively phototropic, but in its resting posture is negatively 

 so. These two activities, however, are intimately associated with the 

 animal's environment. The flight toward light carries it under nat- 

 ural conditions to sunlit districts, and its negative position when rest- 

 ing in sunlight enables it to display its colors, which in the act of 

 mating is a very important and significant step, as any one can observe 

 in the open field at the appropriate time of year. 



Not only is the phototropism of the mourning-cloak butterfly com- 

 plicated, but the insect exhibits also this peculiarity : that though posi- 

 tively phototropic when in flight, it does not fly toward the sun, the 

 source of strongest light in its natural environment. An experi- 

 mental test of the animal from this standpoint shows that when it is 

 placed midway between two sources of light of equal intensity, one a 

 small point and the other a large surface, it regularly moves toward 

 the large surface. Under like conditions animals without image eyes 

 keep an even course between the two lights. For the butterfly with 

 eidoscopic eyes the large area of less bright light determines the 

 direction of movements rather than the small area of intense light. 

 Hence in nature these animals fly from one patch of sunlight to 

 another rather than toward the source of all light, and thus they may 

 be said to prefer a place on earth to one in the sun. It takes only a 

 moment's consideration to recognize how complicated the light re- 

 sponses of this butterfly are as compared with those of a purely photo- 

 tropic animal. 



That the reactions of insects to light are built up on a background 

 of phototropic activity seems to the writer to be perfectly clear. The 

 pure phototropic responses are often strikingly exhibited in the larval 

 stages where only direction eyes are present, a condition of aft'airs 

 pointed out long ago by Loeb in the caterpillars of the Porthesia moth. 

 But they are also easily disclosed in the adult condition, where they 

 are covered at most by a veneer of instinctive activities which repre- 

 sent in reality modified tropic movements such as have been pointed 

 out in the mourning-cloak butterfly. Thus Dolley (1916) has shown 

 that even in the mourning-cloak butterfly itself circus movements may 

 occur on blackening one eye, and the same is true of the still more 



