4 8 
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- 
Fig. 8o.—S tages in development of wing-buds in the larva of the giant crane-fly, 
Holorusia rubiginosa (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; tr., trachea; trl 
tracheoles; 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, tracheae, 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 
