500 



been gradually inserted into the direct development; feeding 

 gradually became confined to the earlier part of the life 

 period, further development to the latter. In other words, 

 processes which ran side by side, have gradually become 

 separated. This conception is undoubtedly correct; but no 

 explanation has, so far as I am aware, ever been offered as to 

 why such a complex process should ever have replaced the 

 simpler one, nor have the various types of metamorphosis 

 ever been considered as throwing light on the structural 

 changes through which the insect passes as they developed 

 metamorphoses. It is these questions that I wish to discuss 

 here. 



When we consider the insects as a group and seek to 

 account for their extraordinary success in nature, two char- 

 acters in their structure present themselves to us — the wings, 

 which have enabled them to conquer a new environment; 

 and the hard chitinous body wall, which makes them so secure 

 against attack. For the less specialized insects — cockroaches, 

 silverfishes, grasshoppers, etc. — which live in truly hidden 

 localities (Cryptozoa), these structures, though important, are 

 not constantly essential. A grasshopper may pass much of its 

 life without wings and -may even temporarily cast its cuticle. 

 For the females of such insects, moreover, a large egg mass 

 is no fatal burden, and we find that the eggs are well pro- 

 vided with yolk. The presence of this yolk enables the embryo 

 to undergo a large part of its development within the egg. 



In the earlier Palaeozoic times none but these generalized 

 insects existed. But as the struggle for existence increased, 

 these insects began to adopt more fully the new environment 

 which had become available to them when wings were evolved ; 

 as the pressure of life increased more and more this. specializa- 

 tion became more and more marked, till there was produced 

 the marvellous diversity of form that exists to-day. 



Now this specialization must have had two marked con- 

 sequences : — 



fa) As the insects began to adopt a more active type of 

 existence such as we see in the Diptera, Lepidoptera, and 

 many Coleoptera, it became increasingly hazardous, or even 

 impossible, to moult during this period of active life. It 

 would be clearly impossible for a butter-fly or a blow-fly to 

 undergo a moult at the present day. Moreover, the cuticle of 

 these specialized insects constitutes a considerable proportion 

 of the body weight. Tlie casting and reforming of this, several 

 times, would be an insurmountable strain on the insect's 

 economy. 



fhj With increase in activity it became more and more 

 impossible to carry large masses of yolked eggs. Either the 



