INCIDENTAL PHENOMENA RELATING TO METAMORPHOSIS IN LEPIDOPTERA. 33 



The limbs had a fleshy, • structureless appearance. The wings ex- 

 hibited numerous vessels ramifying over the whole surface. 



Further details of metamorphosis are necessarily included in the 

 consideration of the external morphology and internal structure of the 

 lepidopterous pupa in the succeeding chapters. We may now briefly 

 summarise the chief general principles that appear to us to be con- 

 nected with the nature and use of metamorphosis in insects. 



1. 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 primitive ancestor of all insects. 



2. The earliest and most primitive orders of 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 obtained it during their larval existence, the metamorphosis 

 became more complete. 



3. The advantage accruing from metamorphosis in such orders as Lepidop- 

 tera, 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, ameta- 

 bolous and heterometabolous insects alone were in existence. The holometabolous 

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

 older forms. 



5. The great abundance of species in these orders shows that metamorphosis is 

 a great advantage to insects in the struggle for existence. The period of exuvia- 

 tion is, in all Arthropods, a very critical one, and they are at that time more than 

 usually helpless before the attacks of their enemies. The holometabolous insects, 

 by their power of storing up surplus food (fat-body) in the larval stage, which they 

 can use at leisure for their further development in the pupal stage, by their power of 

 hiding within cocoons, &c, and by their being without the necessity of seeking food 

 during this critical period, are able to undergo the necessary changes in their 

 organisation, with a minimum of exposure and risk. 



Briefly then, we may consider metamorphosis to be an adaptive 

 habit which certain insects have adopted in their struggle for existence 

 against those enemies by which they are everywhere surrounded, and 

 against those animals that compete against them for food. The habit 

 of flying, by which they are able to escape from numberless enemies 

 that have not this power, was probably one of the first factors that led 

 to their ultimate success. The additional ability to store up food in 

 the early active (larval) stages of their existence, so as to allow them 

 to adopt a hiding habit and quiescent external form at the most critical 

 period of life, must, however, have been the proximate cause of that 

 success which has culminated in their being numerically the most suc- 

 cessful types of terrestrial life in existence, the number of species being 

 almost incredible. 



CHAPTEK II. 



INCIDENTAL PHENOMENA RELATING TO METAMORPHOSIS IN 

 LEPIDOPTERA. 



A large number of interesting details, though bearing on the 

 phenomena of metamorphosis, are so far unconnected with the main 

 subject as to be better dealt with, perhaps, in a short separate chapter. 

 The following paragraphs, therefore, are on points that have occurred 

 to us whilst writing the preceding chapter, or that have been suggested 



