5° 



Development and Metamorphosis 



to the ancestral fish-like ancestors of all vertebrates. Do then the larvae 

 and pup» of insects with complete metamorphosis represent ancestral stages 

 in insect evolutionary history? In some degree the larval stage does, but 



in no degree does the pupal. 

 Insects are certainly not de- 

 scended from an animal that, 

 like a pupa, could neither move 

 nor eat and which had no in- 

 ternal organs except a nervous 

 system, heart, and rudimentary 

 reproductive glands. Biologists 

 recognize that the exigencies of life during adolescence may profoundly 

 modify what might be termed the normal course of development. As 

 long as the developing animal is shielded from the struggle for existence, 

 is provided with a store of food and protected from enemies by lying in an 

 egg-shell or in the body of the mother, it may pursue fairly steadily its reca- 

 pitulatory course of development; but once emerged and forced to shift for 



Flc. 82. — .\ bit of degenerate musrle from tussock- 

 moth, Hemerocampa leucosligmti. Note phago- 

 cytic cells attacking muscle at the margins. 

 (Greatly magnified.) 





■ >fc--. 





Fig. 83. — Degenerating muscle from pupa of giant crane-fly, Holoriisia nihigiiiosa, show- 

 ing phagocytic cells penetrating and disintegrating the muscle-tissue. (Greatly 

 magnified.) 



itself, it must be, at whatever tender age it is turned out, or whatever ancient 

 ancestor it is in stage of simulating, adapted to live successfully under the 

 present-day and immediate conditions of life. If the butterfly gets hatched 

 long before it has reached its definitive butterfly stage, and while it is in 

 a stage roughly corresponding to some worm-like ancestors — and from such 

 ancestors insects have undoubtedly descended — it must be fitted to live 



