56 THE BIOLOGY OF THE SEASONS 



inconspicuous. The finest pupa-cases are surely those 

 which are spun of silk (in most moths), and the making of 

 them in a few days is an extraordinary instance of the 

 intensity of chemical processes in the body, and also of 

 great muscular activity on the part of a creature that is 

 about to fall into sleep. The long thread of silk is a secre- 

 tion, that is to say, it is a not-living organic substance 

 manufactured at the expense of the living matter of the 

 cells composing the silk-glands. It is spun into a cocoon 

 by persistent movements of the head, which must imply 

 great exertion ; thus the larva of Polyphemus is said to 

 move its head a quarter of a million times in making its 

 cocoon. 



Within the pupa cuticle a remarkable process occurs, 

 which has excited the wonder of many generations of 

 naturalists, and is still imperfectly understood. The body 

 of the larva is broken down and is built up again on a 

 new architectural plan. Clusters of active embryonic cells 

 become the foci of new formation, and wandering amoeboid 

 phagocytes, working like sappers and miners among the 

 tissues, transport material from place to place. Although 

 the breaking-down (or histolysis) which precedes the re- 

 construction (or histogenesis) is never so thoroughgoing 

 as in flies, where the pupa returns almost to an egg-like 

 state, there is a gradual transformation of almost all the 

 organs. One may compare what occurs to a not unfamiliar 

 sight, the piece-meal pulling down of a large building, 

 such as a railway station, and its pari passu reconstruction, 

 but all so regulated that the essential activity and what 

 else is life! does not come to a standstill. 



When the reconstruction is completed which may 

 take a couple of weeks or as many years the fully formed 

 insect or imago emerges from the pupa-case its last 

 moult. It is often rather soft and flabby on its emergence, 



