332 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1024 



The persistent going upward of the cankerworm moth might ap- 

 pear to the observer to be due to her center of gravity being behind 

 the bases of her legs, which, by a natural equilibrium on a vertical 

 surface, would swing her head upward and induce her to go in this 

 direction as the one easiest to follow. But let her ascend to the top 

 of the block and see what she does there. She very simply goes 

 around the edge of the top surface and then proceeds down on the 

 other side. The descent is made partly in a position with the head 

 down and the abdomen straight up, and partly in a sidewise attitude 

 with the body sustained horizontally. Clearly, then, the moth is not 

 guided in climbing by any mechanical adjustment of her weight 

 with regard to gravity. 



A child watching a canker moth go up the block, encircle the top, 

 and then go down again would say the moth found no way of going 



higher and so did the nat- 

 ural thing in going down 

 again. The biologist, how- 

 ever, who has invoked a 

 tropism to get the moth up 

 to the top of the block, 

 must invoke another to get 

 her down. He, therefore, 

 says that under the altered 

 conditions the insect has 

 become negatively geo- 

 tropic, without explaining 



Fig. 16.— FemaJe moth of fall cankerworm and eggs. VCry definitely jUSt wliat 

 A, moth depositing flat mass of eggs on a twig ; B, J^^S happened to its posi- 

 egg mass nearly surrounding a twig ,. , . i • i •^^ 



tive geotropism, which will 

 become active again when the insect subsequently encounters a 

 vertical surface. 



In this connection it is interesting to note that when the moth is 

 on a vertical surface the light does not attract her. She persists in 

 going up or down regardless of where the electric bulb may be held, 

 though on the horizontal surface she once more follows its lead. 

 Either, then, her geotropism is stronger than her phototropism, or 

 the condition of being on a vertical surface somehow inhibits her 

 response to light. 



Tlie writer is not particularly recommending the theory of tro- 

 pisms as applied to animals. The evidence in favor of it has been 

 presented too exclusively by its devotees, the work of skeptics would 

 be more convincing. But a careful study of all the facts bearing 

 on the habits and behavior of insects, and the physical and physio- 

 logical conditions that modify them, is very much to be desired, re- 

 gardless of what conception we may have of the inner workings of 



