288 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I37 



that systematics and genetics, the descriptive and experimental sci- 

 ences of evoKition, have their counterparts in ethology and neuro- 

 physiology which are concerned respectively with the description of 

 animal behavior and its functional basis. 



The comparison made by Pringle appears to be no more than a 

 striking analogy, there being no obvious causal relation between racial 

 modifiability and individual learning capacity. For instance, the class 

 Insecta shows intense adaptive radiation, variety of form, habit, and 

 habitat being associated with a wide range and complexity of innate 

 behavioral patterns. However, the ability of insects to learn is not 

 generally marked except for some notable examples among the Hy- 

 menoptera which are then possible only under relatively stereotyped 

 circumstances. It could be argued that the ability to learn has little 

 survival value when compared with a set of complex "built-in" reac- 

 tions in an animal as short-lived as are most insects, but it seems 

 worthwhile to consider this question from a neurophysiological angle. 



Although we still lack any real knowledge of the organic basis of 

 learning, it would seem that the capacity of an individual to form 

 novel combinations of stimulus patterns with effector patterns must 

 depend upon the capacity for a very great number of potential combi- 

 nations or interactions between central nervous units. The number of 

 these interactions which is theoretically possible must depend in some 

 degree upon the actual number of central nervous units or neurons 

 available for combination. It seems unlikely that any organism learns 

 to its full capabilities during its lifetime, thereby realizing all potential 

 combinations. However, there must be a great superfluity of nervous 

 units in organisms having a greater learning potential. 



Conversely, if the number of neurons in the nervous system is 

 strictly limited one might expect behavior to be predominated by in- 

 nate or instinctive patterns. In this case the necessary neuron combi- 

 nations would be determined phyletically like body form and need 

 involve no superfluity of nerve units. Since the general behavior of 

 insects falls into this pattern I suggest that an economy or parsimony 

 of nerve units is an important factor in the organization and operation 

 of their nervous systems. In the following pages I shall examine the 

 possible basis of this neural parsimony in relation to neural mecha- 

 nisms of obvious survival value — predator evasion and predation. 



THE PREY 



Speed of operation must be a prim.e requisite in the neural mecha- 

 nisms by which insects detect and evade predators. It seems reason- 

 able to suppose that speed would dictate simplicity of mechanism, as 



