CHAPTER VIH 



THE INSECTS 



rm GREAT group of insects contains an enormous range of Afferent forms 

 and any treatment of insect development which can fit into the frame- 

 work of this book must be an extremely summary and simpltfied one 

 There IS not only a very wide range of different types of embryonic 

 developmem to be covered, but our knowledge of the phenomena of 

 insect nietamorphosis is considerable, and, although we have neglected 

 this problem m the araielids and echinoderms, it seems desirable to give 

 some account of it in insects. As is well known, there are aU gradations in 

 the mtemity of the changes involved in metamorphosis. In some primitive 

 msects (the Ametabola) the larval form develops gradually mto the adult, 

 with no sudden or marked change. There are other more complex types 

 m which the wmg buds may be exposed in the larval stages (Exopterygota) 

 or conceded beneath the surface until the time of metamorphosis (Endo- 

 pterygota); and again the larva (m this case often known as a nymph') 

 may be directly transformed hito the adult (Hemimetabola) or there may 

 be a pupal stage intercalated (Holometabola). In the extreme type, the 

 Holometabo la, the adult organism may have almost no immediately 

 obvious similarity to the larval; the life-history comprises two very dis- 

 tmct developmental systems. Since there will be no space to treat all the 

 intermediate conditions, we shall find it convenient to deal first with the 

 embryonic developnient, by which die larva is produced, and dien to pass 

 on to the processes of metamorphosis, particularly in those types m which 

 tiiey are most intense and far-reaching. 



Embryonic development 



There are many types of insect eggs (Review: Johannsen and Butt 1941) 

 but they are all variations on a fundamental plan which is at first sight 

 ra her unlike that of any of the other eggs described in this book (although 

 related to those of other arthropods, which we shall not consider) Their 

 most obvious characteristic is that the yolk, mstead of being concentrated 

 at the vegetative end of the egg, is more or less uniformly distributed 

 throughout the whole central region of it; whence the eggs are often 

 referred to as centrolecithal' in contrast to the 'telolecithal' eggs of odier 



n3' ^T ?' ''T' ""''' °f y°'^' '^"' i^ ^l™y^ ^cortex, or 

 peripheral sheet of cytoplasm, which may be quite thin, but is sometimes 



