METAMORPHOSIS
the sure oJ the departure oJ the young and the departure of the adult Jrom what would have been the normal line of devel- opment if neither had become structurally adapted to a special kind oJ lire. We may express this idea graphically by a diagram (Fig. 138) , in which the line nm represents what might have been the straight course of evolution if neither the adult (I) nor the young (L) had departed along special lines of their own. But, when the adult and the young have diverged from some point (a) in their past history, the line LI, which is the sure of nm to L and of nm to I, represents the change which the young is bound to make in reverting to the adult form. The young must, there- fore, prepare itself for this event in proportion as the distance LI is short or long. Where the structural disparity between the young and the adult is not great, or is mostly in the external form of the body, the young insect changes directly into the adult, as we have seen in the case of the grasshopper (Fig. 9) and the cicada (Fig. ?18). But with many in- sects, either because of the degree of difference that has arisen between the young and the adult, or for some other reason, the processes of transformation are not ac- complished so quickly and require a longer period for their completion. In such cases, the creature that issues at the last shedding of the skin by the young insect is in a very unfinished state, and must yet undergo a great amount of reconstruction before it will attain the form and structure of the fully adult insect. This happens in all the groups of the more highly evolved insects, including the beetles; the moths and butterflies; the mosquitoes and files; the wasps, bees, ants; and others. The newly transformed insect must remain in a helpless condition without the use of its legs and wings for a period of rime varying in length with different species, until the adult organs, particularly the muscles, are completely formed. In the meantime, however, the sort cuticular layer of
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INSECTS
