368 INSECT A. 



through a series of gradual internal and external changes, not dift'er- 

 ine; essentially from the ontogenetic processes which occur in the 

 metamorphosis of most other animals. "We can here trace back the 

 development of the wing-rudiments, the external genital apparatus, 

 and all other alterations of form to simple growth of the larval body. 

 The transformation of the internal organs, chief among which are 

 the genital organs, is accomplished in an ecpually simple manner. 

 We may perhaps assume, although this point has not yet been 

 thoroughly investigated, that, simultaneously with the growth of 

 the internal organs, a gradual regeneration takes place in them, as, 

 indeed, is frequently the case with functional organs. We may 

 assume that some of the cells or cell-groups, exhausted through the 

 performance of their vital functions, are absorbed and replaced by 

 fresh elements, so that a constant gradual regeneration of these 

 organs is in progress. 



In the holometabolic orders of Insects, on the contrary, the tran- 

 sition from the last larval stage to the adult form is accomplished 

 through the intercalation of a resting stage {pupal stage), in which 

 the acts of feeding, and usually also of locomotion, are suppressed, 

 while the whole life-activities of the organism seem directed towards 

 the important and complicated ontogenetic processes, which involve 

 a complete destruction of many of the organs of the larva and their 

 renewal from certain rudiments (imaginal discs) already present in 

 the latter. Only a few of the organs found in the larva pass directly 

 over into those of the pupa and the imago. Among these we must 

 reckon the rudiments of the genital system. The heart also and 

 the central part of the nervous system undergo only slight changes. 

 Most of the other organs of the larva, on the contrary (the hypo- 

 dermis, most of the muscles, the whole of the intestinal canal, and 

 the salivary glands) are completely destroyed. Their cells, under 

 the influence of the blood-corpuscles (leucocytes), which here act 

 as phagocytes, break up into pieces which are taken in and digested 

 by the latter, while, simultaneously with these processes of disinte- 

 gration, the reconstruction of the organs from the formative centres 

 (imaginal discs) already present in the embryo is accomplished in 

 such a way as, in most cases, to preserve the continuity of the organ. 

 We shall only be able to understand these processes by regarding 

 them as an extreme case of the regenerative processes, which we 

 assumed must occur in the Homomorpha. We shall then have to 

 assume that at first only a part of the rudiment of an organ develops 

 and functions for the use of the larva ; this part becomes exhausted 



