INSECTS
In the United States there are numerous species of "annual" cicadas, so called because they appear every year, but their life histories are not actually known in most cases. These species are called "locusts," "harvest files," and "dog-day cicadas" (Fig. 112). They are the insects that sit in the trees during the latter hall of summer and make those long shrill sounds that seem to be the natural accompaniment of hot weather. Some give a rising and falling inflection to their song, which resembles zwing, zwing, zwing, zwing, (repeated in a long series); others make a vibratory rattling sound; and still others utter just a continuous whistling buzz.
During the interval between the times of the appearance of the adult cicadas, the insects live underground. The periodical cicada comprises two races, one of which lives in its subterranean abodes for most of seventeen years, the other for most of thirteen years. Both races inhabit the eastern part of the United States, but the longer-lived race is northern, and the other southern, though their territories overlap. Most of out familiar insects complete their life cycle in a single year, and many of them produce two or more generations every season. For this reason we marvel at the long life of the periodical cicada. Yet there are other common insects that normally require two or three years to reach maturity, and certain beetles have been known to live for twenty years or more in an immature stage, though under conditions adverse for transforming to the adult.
Throughout the period of their underground life the cicadas have a form quite different from that which they take on when they leave the earth to spend a brief period in the trees. The form of the young periodical cicada at the time it is ready to emerge from the ground is shown in Plate 5. It will be seen that it suggests one of those familiar shells so often found clinging to the trunk of a tree or the side of a post. These shells, in fact, are the empty skins of young cicadas that have discarded their
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