INSECT BEHAVIOR 313 



behavior of organisms. Furthermore, the study of tropic reactions is 

 complicated by the fact that they are due not only to external stimuli, 

 but also to little-understood internal stimuli, arising from unknown 

 conditions of the alimentary canal, muscles, reproductive organs, etc. 



A recognized property of protoplasm is that of adaptation, as mani- 

 fested in the acclimatization of protoplasm to untoward conditions 

 of temperature, light, contact and other stimuli; and this adaptation to 

 unusual conditions may take place without the aid of natural selection. 



A tropic reaction occurs, whether it is to prove useful to the organism 

 or not. Thus a lady-bird beetle walks upward, on a branch, on a fence, 

 on one's finger. It walks upward as far as possible and then flies into 

 the air. If it happens to reach the tip of a twig and finds aphids there, 

 the beetle stops and feeds upon them. This adaptive result is in a sense 

 incidental. Yet, upon the whole, tropic reactions are wonderfully adap- 

 tive in their results. Here natural selection is of special value as afford- 

 ing an explanation of the phenomena. 



As Loeb and Davenport have insisted, the mechanical reactions to 

 gravity, light, heat and other influences determine the behavior of the 

 organism. 



2. INSTINCT 



Insects are eminently instinctive; though their automatic behavior 

 is often so remarkably successful as to appear rational, instead of purely 

 instinctive. 



A satisfactory definition of "instinct" seems to be impossible, 

 though some of the characteristics of instinctive behavior are quite 

 evident. 



Instinct, as distinguished from reason, attains adaptive ends without 

 prevision and without experience. For example, a butterfly selects a 

 particular species of plant upon which to lay her eggs. Caterpillars of 

 the same species construct the same kind of nest, though so isolated from 

 one another as to exclude the possibility of imitation. Every caterpillar 

 that pupates accomplishes the intricate process after the manner of its 

 kind, without the aid of experience. 



Instinctive actions belong to the reflex type they consist of co-or- 

 dinated reflex acts. A complex instinctive action is a chain, each link 

 of which is a simple reflex act. In fact, no sharp line can be drawn 

 between reflexive and instinctive actions. 



Basis of Instinct. Reflex acts, the elements from which instinctive 

 actions are compounded, are the inevitable responses of particular organs 



