Page:Insects - Their Ways and Means of Living.djvu/273

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 spectacular or unexpected about the change, as in the transformation of the tadpole into a frog, the change of the wormlike caterpillar into a moth, or of a maggot into a 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. Metamorphosis, 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.

127. Moths of the fall webworm

At once it becomes evident that, by reason of the very definition we have adopted, our subject is going to become 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 serious 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 [227]