502 



It is necessary finally to point out the changes through 

 which the tissues must have passed as the metamorphosis 

 gradually evolved. As the simple "rejuvenation" meta- 

 morphosis evolved, the cells of the imago had to ''learn" to 

 adapt themselves to a period of rapid feeding, perhaps on 

 special foods, in the early part of the life cycle. As the larval 

 condition, which had then become initiated, became more 

 marked greater specialization of cells resulted. For a time 

 these cells were still able to rejuvenate themselves. But as 

 specialization of the imago (and consequently also of the larva, 

 though in a different direction) proceeded, these larval cells 

 must have found it increasingly difficult and finally impossible 

 to transform themselves into the imaginal cells from which 

 they had phylogenetically descended. Death must eventually 

 have resulted in some of the cells. Now as specialization 

 increased still further more and more of these larval cells must 

 have perished; in order that the individual should survive 

 other larval cells would have to undergo a decrease in 

 specialization. And as the present-day specialization of the 

 imago was gradually attained there would thus have had to 

 occur two parallel processes within the larva — an increase of 

 specialization of some cells (the true larval cells) and a 

 decrease in specialization, i.e., a retention of embryonic char- 

 acters in others. There would in this way be formed imaginal 

 discs. That this is the phylogeny of imaginal discs is clearly 

 shown by the fact that these structures are always found in 

 close connection with the structure of the larva to which they 

 correspond; e.g., in Nasonia the antennal nerve of the imago 

 arises from a cluster of cells in the brain lying very close to 

 the antennal ganglion of the larva; or, to take a simpler case, 

 the imaginal disc of the adult mandible lies in very close 

 communication with the mandible of the larva. If the 

 mandible of the larva had been evolved quite independently 

 of the imaginal mandible this would not have been the case. 



To recapitulate, then, in the struggle for existence which 

 has been going on among insects since Palaeozoic times, the 

 possession of wings and a hard cuticle has enabled the insects 

 to undergo marked specialization. Active flight made the 

 carrying of numerous heavily yolked eggs impossible, and as 

 the laying of numerous eggs has remained essential the quan- 

 tity of yolk material has gradually diminished, reaching a 

 minimum in the chalcids. On the other hand, the increased 

 loss to the animal economy sustained by the moulting of 

 imaginal cuticles has necessitated that growth in size of the 

 body should become more and more concentrated at earlier 

 parts of the free living period; ultimately moulting has dis- 

 appeared in the life of the imago. These two processes have 



