2 smithsonian miscellaneous collections vol. 8l 



Introduction 



Entomologists know considerable about the behavior of insects, but 

 are still unable to explain many of their activities. Since insects are 

 cold-blooded animals and their anatomical organization is entirely 

 different from that of higher animals, their responses to environmental 

 conditions are different. For this reason their activities are not so easily 

 understood. Most of their reactions are tropic responses to external 

 stimuli. We know considerable about some of our own external stimuli, 

 such as light, sound, and heat, and how they affect us ; but we know 

 very little about the external stimuli which cause responses in insects, 

 and we know still less about the sensory impressions which are 

 produced in them by these stimuli. 



When it is desired to control an insect, the first step is to study its- 

 life history, which is largely a study of its behavior, and in turn 

 behavior is largely a result of tropic responses. A study of tropisms 

 is, therefore, a basic one, but economic entomologists in their haste to 

 obtain practical results usually overlook this fact. The late Jacques 

 Loeb was our greatest advocate of the study of tropisms, and as a 

 result of his indefatigable efforts there has arisen a much broader 

 and more important subject — general physiology. If entomologists 

 would study tropisms more seriously, using the best equipment obtain- 

 able, they would certainly obtain much information which could be 

 used in insect control. 



The object of this paper is to bring together the available informa- 

 tion on the tropisms and sense organs of Lepidoptera, hoping that this 

 information will encourage a more serious study of tropic responses. 

 At the suggestion of Dr. A. L. Quaintance, Associate Chief of the 

 Bureau of Entomology, the writer began a series of studies dealing 

 with the tropisms of various insects, particularly those of the codling 

 moth. The results herein discussed include those obtained by the 

 writer in his studies on Lepidoptera and a review of the literature, 

 most of which pertains only to butterflies and moths. 



A. Tropisms 



The term tropism comes from the Greek word meaning " turn." 

 According to Mast (56, p. 53) it was first used in 1832 by DeCandolle, 

 who called the bending of plants toward light heliotropism. Later 

 the word heliotropism came to signify both the bending and the ex- 

 planation of the process. \^erworn and Loeb, in 1886 and 1887, as 

 cited by Mast, using tropisms as a basis for investigation, were the 

 first to study animal behavior from the physico-chemical point of 



