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 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 cassinit, but the specific differences 
are not sufficiently well defined to entitle it to rank as a 
species. 
