46 



ralists. This biologist showed that a process of histolysis was set up 

 in the larval tissues of certain Diptera, that these tissues were entirely- 

 destroyed, and that from the resulting elements of this histolysis the 

 new organs were built up, the growing imaginal buds utilising the 

 histolytic products of the larva for their own nutrition. Although 

 his researches were confined to the MuscidcB, his observations and 

 conclusions were soon found to have a much wider and more general 

 application. 



The imaginal discs of Weismann are, as w^e have already said, 

 separate cellular masses or folds which give rise to the appendages, 

 wings, and other parts of the imago. They arise from the hypo- 

 dermis, are usually present in very young larvae, sometimes even in 

 the later embryonic stages. Such imaginal buds have been shown to 

 exist for each part of the body, not only for the appendages and 

 wings, but also for the different sections of the digestive canal and 

 other internal organs. During the quiescent stage preceding pupation 

 these discs commence to enlarge, whilst at the same time there is a 

 destruction of the larval organs, the latter being due to the activity 

 of the leucocytes or blood-corpuscles. This continues until most of 

 the larval tissues are reduced to a creamy mass, the imaginal buds, 

 however, remaining unchanged in character, but, on the other hand, 

 utilising for their own growth the material that the histolytic process 

 has produced. These two processes of histolysis and histogenesis 

 go on side by side, and of course are not completed until the final 

 formation of the imago within the pupal case. Some of these discs 

 undergo their development in the early, others in the later, stages of 

 the pupal period. 



An explanation of the process of the development of the imaginal 

 buds into the various imaginal organs would involve far too much 

 detail to be included in this paper. We must, therefore, bring to a 

 conclusion our remarks on metamorphosis, and will do so by sum- 

 marising the principal points that relate to its origin. These may be 

 stated as follows : — (t) The Synaptera or apterous insects have no 

 metamorphosis, the winged insects only undergoing the changes 

 already described. This would suggest that metamorphosis per se 

 was not inherited from the piimitive ancestor of all insects. (2) The 

 earliest and most primitive orders of winged insects pass through a 

 slight metamorphosis only ; but as the adults of certain orders 

 became more specially adapted to get their food whilst in the air, 

 and in a manner totally different from that by which they obtain it 

 during their larval existence, the metamorphosis became more com- 

 plete. (3) The advantage accruing from metamorphosis in such 

 orders as Lepidoptera, Hymenoptera, Diptera, and Coleoptera is 

 evident from the vast number of species that have been developed 

 and are now in existence. (4) The fossil remains of insects suggest 

 that in the Palaeozoic period ametabolous and heterometabolous 

 insects alone were in existence. The holometabolous insects are 

 much newer and are much richer in the number of species than 



