INSECT METAMORPHOSIS 



appearance; but rendered into English it means simply 

 "change of form." Not every change of form, however, is 

 a metamorphosis. The change of a kitten into a cat, of 

 a child into a grown-up, of a small fish into a large fish 

 are not examples of metamorphosis, at least not of what 

 is called metamorphosis. There must be something spec- 

 tacular or unexpected about the change, as in the trans- 

 formation of the tadpole into a frog, the change of the 

 wormlike caterpillar into a moth, or of a maggot into a 



Fig. 127. Moths of the fall webworm 



fly. This arbitrary limiting of the use of a word that 

 might, from its derivation, have a much more general 

 meaning, is a common practice in science, and for this 

 reason every scientific term must be defined. Meta- 

 morphosis, then, as it is used in biology, signifies not 

 merely a change of form, but a particular kind or degree 

 of change; the kind of change, we might say, that would 

 appear to lie outside the direct line of development from 

 the egg to the adult. 



At once it becomes evident that, by reason of the very 

 definition we have adopted, our subject is going to be- 

 come complicated; for how are we to decide if an observed 

 change during the growth of an animal is in line or out of 

 line with direct development? There, indeed, lies a seri- 

 ous difficulty, and we can only leave it to the biologist to 

 decide in any particularly doubtful case. But there are 

 plenty of cases concerning which there is no doubt. A 



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