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As its name implies, this insect generally requires seventeen years to complete its transformations, a fact that was first pointed out many years ago by the botanist Kalm. The late Prof. Riley, who had given this species a great deal of study, was the first to work out the problem of its periodical returns. He found that there are also thirteen-year broods, and that both sometimes occur in the same locality, but that in general terms the thirteen-year brood might be called the southern form, and the seventeen-year the northern form. At the limits of their respective ranges these broods overlap each other. The shorter-lived form he named provisionally Cicada tredecim. It was the existence of this brood that led entomologists to doubt the propriety of Linné’s name, because, in calculating each appearance as occurring in any locality at the end of every seventeen years, they could not make the dates of its periodical returns correct. But it was Prof. Riley that cleared up the matter. It happened in the summer of 1868 that one of the largest seventeen-year broods occurred simultaneously with one of the largest thirteen-year broods. Such an event, so far as these two particular broods are concerned, has not taken place since 1647, nor will it take place again till the year 2089. There are absolutely no specific differences between the two broods other than in the time of maturing. There is, however, a dimorphous form that appears with both these broods. It is smaller, of a much darker color, has an entirely different voice, appears a fortnight sooner, and is never known to pair with the ordinary form. Dr. J. C. Fisher, in 1851, described it as Cicada cassinii, but the specific differences are not sufficiently well defined to entitle it to rank as a species.