498 



GENERAL CONSIDERATIONS 



generally speaking, is the result of direct response, since though the 

 receptor and effector elements are distinct they are in direct connection. 

 Coordination is through a nerve net and some localization of responses 

 exists. In direct response there is to a considerable degree a correspond- 

 ence between the strength of the stimulus and the vigor of the response, 

 though this is subject to the effect of different physiological states — as, 

 indeed, are all modes of behavior. Direct response is modifiable, the 

 modifications depending upon the physiological state, antecedent stimula- 

 tion, and attendant environmental conditions. 



521. Simple Reflexes. — Simple reflexes may not be different in the 

 general character of the action from a direct response but the mechanism 



Outer dense layer 



Mic/cf/e, /oose 

 layer 



Inner dense 

 layer 



Chrysalis 



Cast-off larval 

 '4 Skin 



A B 



Fig. 358. — Cocoon of the silkworm, Samia cecropia (Linnaeus). A, external appearance. 

 B, section to show construction. From a specimen. 



involved is different, since receptors, adjusters, and effectors are all 

 involved. The results differ in that the responses are distinctly more 

 localized, and this adds definiteness and variety to the actions. This 

 mode of behavior is first developed among the flatworms. 



522. Instincts. — Instincts can best be defined as made up of associated 

 and coordinated reflexes. It is clear from this definition that it is difficult 

 to draw a sharp line between the simplest instinct and a simple reflex act, 

 but in the most complex of instincts we have a very characteristic type 

 of action. 



The nature of an instinct may be well illustrated by a description of 

 one which is very comphcated, such as the spinning of a cocoon by a 

 cecropia silkworm (Fig. 358). If one keeps a caterpillar of this species in 

 a box, giving it fresh food daily, it is a very docile prisoner, eating vora- 

 ciously and, so long as fresh food is regularly supplied, seeming to have 



