48 



Development and Metamorphosis 



take a single example, the case of the blow-fly (admittedly an extreme one), 

 the phenomena of internal change are, put briefly, as follows: The imaginal 

 wings, legs, and head-parts begin to develop as deeply invaginated little 

 buds of the cell-layer of the larval skin early in larval life. This develop- 

 ment is gradual and continuous until pupation, when the wing and leg rudi- 



FlG. 80. — Stages in development of wing-buds in the laira of the giant crane-fly, 

 Holorusia nihiginosa (the wing-buds have been dissected out and sectioned, so 

 as to show their intimate anatomy). A, B, C, D, four stages successively older ch., 

 chitinized cuticle; hyp., hypoderm or cellular layer of skin; Ir., trachea; trl., 

 trachcoles; p.m., peritrophic membrane; w., developing wing; t.v., tracheal branch 

 indicating position of future wing-vein. (Greatly magnified.) 



ments and the new head are pulled out upon the exterior of the body. Just 

 before pupation, when the larva has given up its locomotion and feeding, 

 the larval muscles, trachea;, salivary glands, alimentary canal, and some other 

 tissues begin to disintegrate, and rapidly break wholly down, so that in the 

 pupa there appear to be no internal organs except the nervous system, 

 reproductive glands, and perhaps the heart, but the whole interior of the 



