25 



of the larval organs by a gradual process of degeneration, and the 

 rebuilding of the new material thus produced by a process of " histo- 

 genesis " into the new organs, the germs of which he showed to exist 

 within the organism. His views have since been applied to the whole 

 of the holometabolous orders by other workers, and shown to be 

 applicable to all of them. The imaginal discs, buds, or folds, are 

 minute cellular masses, which arise from the epidermis ; they are 

 usually present in very young larvae, and it has been shown that 

 imaginal buds exist not only for the appendages, wings, and various 

 organs, but for the different sections of the alimentary canal. 



Preceding the change of the larva to a pupa there is a quiescent 

 period, during which many important internal changes take place ; and 

 although the change in form from larva to pupa is at last sudden, this 

 does not represent by any means an equally sudden development of 

 the pupal structures. During this quiescent period the imaginal 

 discs grow, and at the same time there is a destruction of the larval 

 organs. The process of destruction is due to the leucocytes or blood- 

 corpuscles, the larval organs thus broken up forming a creamy mass. 

 In this mass the buds exist, resisting the influence of the leucocytes, 

 and, on the other hand, utilising the material formed by this process 

 of histolysis for their own growth. It must not be assumed that the 

 destruction of all the larval tissues, or the building up of all the ima- 

 ginal tissues, takes place simultaneously. The histolysis and histo- 

 genesis of a given organ is continuous, so that there is no distinct 

 break between the existence of any given larval organ and the 

 imaginal organ that replaces it morphologically ; but the histolysis and 

 histogenesis of some organs take place in the very earliest stages of 

 pupal life, whilst others are not changed until the very end of the 

 pupal existence, the last steps in the destruction of the larval organs 

 taking place only after the imaginal organs have assumed their definite 

 size and shape. 



It must be quite evident that in some cases the imaginal buds serve 

 for the formation of new organs where such do not exist in the early 

 stages, or are gradually being built up during those stages ; the wings 

 afford an excellent example. In other cases they serve for the trans- 

 formation of organs already in existence— legs, antennae, maxillae, — 

 entirely different structures being formed as a result of their develop- 

 ment, although morphologically equivalent to the organs preceding 

 them. 



The imaginal buds of the wings are not subjected to the disturbing 

 influences engendered by the larval moults. They are at first entirely 

 internal structures, and do not form a cuticle, according to Gonin, 

 until towards the end of the last larval stage. As we have already 

 elsewhere fully dealt with the formation of the lepidopterous wing, we 

 will not enter into the subject here other than to say that, although the 

 wings are present in the embryonic larva before it hatches from the 

 egg, its growth is comparatively slow until towards the end of the last 

 larval instar, when its rapid develcjpment makes it readily observable 



