THE PERIODICAL CICADA
ages, but those of the legs are usually inverted during the shedding and disappear from the outside of the slough, though the holes where they were pulled in can be round before the membrane becomes too dry.
The nymph (Figs. 125, 7; 126) usually runs about at first in the groove of the twig containing its egg nest and then goes out on the smooth bark. Here any current of air is likely to carry it off immediately, but many wander about for some time, usually going toward the tips of the twigs, some even getting clear out on the leaves. But only a few nymphs are ever to be round on twigs where piles of embryonic skins show that hundreds have recently hatched; so it is evident that the great majority either fall off or are blown away very shortly after emerging. Many undoubtedly fall before the shedding of the egg membrane, for the inclosed creature has no possible way of holding on, and even the free nymph has but feeble clinging powers. Those observed on twigs kept indoors often fell helplessly from the smooth bark while apparently making real efforts to retain their grasp. Their weak claws could get no grip on the hard surface. Instead, then, of deliberately launching themselves into space in response to some mysterious call from below, the young cicadas simply fall from their birthplace by mere inability to hold on. But the same end is gained—they reach the ground, which is all that matters. Nature is ever careless of the means, so long as the object is attained. Some acts of unreasoning creatures are assured by bestowing an instinct, others are forced by withholding the means of acting otherwise.
The cicada nymphs are at first attracted by the light. Those allowed to hatch on a table in a room will leave the twigs and head straight for the windows ten feet away. This instinct under natural conditions serves to entice the young insects toward the outer parts of the tree, where they have the best chance of a clear drop to earth; but even so, adverse breezes, irregularity of the
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