204 AN INTRODUCTION TO ENTOMOLOGY 



By comparing diagrams B and C it will be seen that what was the 

 tip of the first segment of the larva and of the young pupa (++) 

 becomes the neck of the insect after the head is evaginated. 



IV. THE TRANSFORMATIONS OF THE INTERNAL 



ORGANS 



Great as are the changes in the external form of the body during 

 the life of insects with a complete metamorphosis, even greater changes 

 take place in the internal organs of some of them. 



In the space that can be devoted to this subject in this work, only 

 the more general features of the transformation of the internal organs 

 can be discussed; there is an extensive and constantly increasing 

 literature on this subject which is available for those who wish to study 

 it more thoroughly. 



In insects with a gradual or with an incomplete metamorphosis 

 there is a continuous transformation of the internal organs, the changes 

 in form taking place gradually; being quite comparable to the gradual 

 development of the external organs; but in insects with a complete 

 metamorphosis, where the manner of life of the larva and the adult 

 are very different, extensive changes take place during the pupal 

 stadium. The life of a butterfly, for example, is very different from 

 that it led as a caterpillar; the organs of the larva are not fitted to 

 perform the functions of the adult ; there is consequently a necessity 

 for the reconstruction of certain of them; hence the need of a pupal 

 stadium. Pupag are of ten referred to as being quiet ; but physiologi- 

 cally the pupal period is the most active one in the post-embryonic 

 life of the insect. 



In those cases where a very marked change takes place in the 

 structure of internal organs, there is a degeneration and dissolution of 

 tissue, this breaking down of tissues is termed histolysis. 



In the course of histolysis some cells, which are frequently leu- 

 cocytes or white blood corpuscles, feed upon the debris of the disin- 

 tegrating tissue ; such a cell is termed a phahgocyte, and the process is 

 termed phagocytosis. It is believed that the products of the digestion 

 of disintegrating tissue by the phagocytes pass by diffusion into the 

 surrounding blood and serve to nourish new tissue. 



After an organ has been more or less broken down by histolysis, 

 the extent of the disintegration differing greatly in different organs 

 and in different insects, there follows a growth of new tissue; this 

 process is termed histogenesis. 



