Tower- Building Cicada. 103 



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 Linne'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. 



