THE HIDDEN TYPE OF WING-GROWTH 151 



broken down by histolysis, and new regions arise from groups 

 of imaginal cells. The degree of replacement necessary 

 increases with the increase of divergence between imago and 

 larva. In wasps and bees the cellular walls of the main air- 

 trunks pass over from the grub to the adult with considerable 

 increase in capacity brought about by the multiplication of 

 cells, and even in the transformation of the maggot into the 

 blue-bottle the great bladder-like tracheal enlargements known 

 as air-sacs have a similar relation to the larval condition. The 

 excessively fine tubular terminations of the respiratory system, 

 which are known as tracheoles and have no spiral thickening 

 to their chitinous wall, the product of intracellular activity of 

 the living tracheal epithelial layer, become greatly multiplied 

 in preparation for the increased oxidation necessary during 

 the aerial life of the adult, for it is through the exquisitely 

 delicate walls of these tracheoles that the gaseous exchanges 

 between the air and the tissues are carried on. 



From the system of air-tubes with their cuticular lining we 

 pass naturally to consider a few special features in the origin 

 of the outer cuticular structures of the adult in relation to 

 the groups of living cells through whose activity they are 

 formed. It has already been explained how in metamorphic 

 insects the cuticle of the pupa is formed beneath the larval 

 cuticle, and later the cuticle of the imago beneath the pupal. 

 As to the main regions of the body, it is found that in cases 

 where these regions are typically represented in the larva, the 

 cuticle of each is formed ready for pupation, from discs derived 

 from the skin of the corresponding part of the larva. These 

 discs may however grow so as to lie beneath other regions of 

 the larval cuticle. For example, in the grub of the midge 

 Chironomus 1 the brain is situated in the first thoracic segment, 

 and the imaginal discs of the head, including those of the eyes 

 and feelers, grow backwards towards the brain. In the maggots 

 of blue-bottles 2 and their allies, the mouth, opening at the 

 narrow front extremity of the body, leads into a spacious 

 chitin-lined cavity usually styled the pharynx (Fig. 86 A H), 

 but probably to be regarded as the inpushed larval head. 



1 L. C. Miall and A. R. Hammond : " The Structure and Life-history of 

 the Harlequin Fly (Chironomus) ". Oxford, 1900. 



2 J. Van Rees : " Beitrage zur Kenntniss der inneren Metamorphose der 

 Musca vomitoria ". Zool. Jahrb. (Anat.), III. 1888. 



