THE PERIODICAL CICADA
stage, which is about a year in length. In the fourth stage, which lasts perhaps three or four years, the nymph (Fig. 114) shows distinct wing pads on the two wing-bearing segments of the thorax. In the fifth stage the insect, sometimes now called a "pupa," takes on the form it has when it finally emerges from the earth; its front feet are restored and its wing pads are well developed, but it has entirely lost its small nymphal eyes. Once more, before its long underground sentence is up, the nymph molts, and enters the sixth and last stage of its subterranean life. When mature (Plate 5) it is about an inch and a quarter in length, thick-bodied, and brown in color; it appears to have a pair of bright-red eyes on the head, but these are the eyes of the adult inside showing through the nymphal skin.
Fig. 114. Nymph of the periodical cicada in the fourth stage, about 12 years old, enlarged 2¾ times. (From Marlatt)
According to the investigations of Doctor Marlatt, the nymphs of the periodical cicada do not ordinarily burrow into the earth below two feet, and most of them are to be found at depths varying from eight to eighteen inches. However, there are reports of their having been discovered ten feet beneath the surface, and they have been known to emerge from the floors of cellars at the time of transformation to the adult stage. There is no evidence that the insects, even when present in great numbers in the earth, do any appreciable damage to the vegetation on the roots of which they feed.
Some time before the mature nymphs emerge from the ground, probably in April of the last year of their lives, the insects come up from their subterranean burrows and construct a chamber of varying depth just below the
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