BEHAVIOR OF ANIMAL ORGANISMS 471 



no desire to escape. As the time approaches for the spinning of the 

 cocoon, however, there comes a day when the larva refuses to eat. It 

 is now restless, traveling repeatedly around the box as if seeking some 

 avenue of escape. At this period in the life history the caterpillars of 

 many butterflies and moths show a change in color. The restlessness as 

 well as the change in color indicate a change in physiological state. As 

 if in response to this change the larva soon betrays a tendency to place 

 itself in contact with as much surface as possible. It enters a mass of 

 leaves or, in the absence of them, seeks a place where it can have a maxi- 

 mum of contact, as in the corner formed by three sides of the box. It 

 then begins to spin, drawing the leaves close about its body and holding 

 them in place by silken threads. This spinning continues until a layer of 

 silk is formed which hides the larva from sight. Now the description 

 must be finished from inferences drawn from the finished cocoon. When 

 this outside layer is of a certain thickness something causes a change in 

 the physiological condition of the organism, and, acting like a machine in 

 which a lever has been operated, it suddenly ceases to add to the layer 

 formed and begins to spin looser silk. Soon, however, another shift 

 takes place and the caterpillar stops spinning this loose silk and begins to 

 spin another dense layer of silk within. In this second dense layer the 

 insect apparently uses all of its remaining supply of silk. This inner 

 layer of the cocoon is then coated on the inside with a secretion which 

 when it hardens makes the surface appear as if it had been shellacked. 

 Its task done the caterpillar sheds its skin and becomes a chrysalis, pro- 

 tected by its cocoon until the time comes for it to emerge as a moth. 



Examination of the cocoon brings to light several remarkable features. 

 The outer layer of the cocoon seems to be a weather-resisting layer, 

 though it is not waterproof; the loose layer seems to function in a way as a 

 heat-conserving layer; while the inner dense coat appears as a second 

 protective layer which is waterproofed within. Furthermore, the cater- 

 pillar has made provision for emerging, since at the end of the finished 

 cocoon through which the moth will emerge each layer is loosely woven 

 to provide an easy avenue of escape. 



The spinning of the cecropia cocoon illustrates several of the salient 

 facts in regard to instincts. (1) The instinct is manifested when a certain 

 physiological state appears. (2) It is initiated in response to an outside 

 stimulus. (3) It is an action involving many different reflex activities, 

 all of which contribute to the one end. (4) It involves changes in action 

 due to some internal change in the nervous system or to a changed physio- 

 logical state in some part of the organism. (5) It involves time and space 

 relations. (6) The capacity to exhibit the instinct is inherent and the 

 instinct itself is clearly inherited. 



Other facts about instincts are brought out by the spinning of this 

 cocoon. One is the stereotyped character of the whole action. All 



