The number of cicadas emerging in successive years is not maintained at anything like an even level. Four of the 17 "possible" 17-year broods are referred to by such terms as "doubtful," "unimportant," and "almost extinct." We have only scattered records concerning them, and one or more may be extinct. Only two of the 13-year broods are large; a few others exist, but the exact number is not known.
In some years two broods emerge—one in the North and one in the South.
Where They Appear
The range of the periodical cicada covers nearly all the United States east of the Missouri Valley, and includes Arkansas and Louisiana.
There is considerable overlapping between the 17- and 13-year races.
Most broods are limited to fairly definite areas, but some consist of small, widely scattered colonies. Moreover, there are scattered colonies that seem to have little connection with large regional broods; nevertheless, when such a colony appears, it is considered to be a part of whatever brood appears the same year.
For convenience of reference the broods have been designated by Roman numerals. The numerals I through XVII are assigned to the 17-year broods, and XVIII through XXX to the 13-year broods. They are assigned as though a brood were to emerge each year, but, as previously mentioned, there may be gaps, especially in the 13-year broods.
Numbering of the 17-year broods under the present system began with the 1893 brood, which was designated brood I. The 1894 brood was brood II, the 1895 brood was brood III, and so on. In 1909 brood XVII appeared, and in 1910 brood I appeared again.
The following listing shows the more important broods, the area in which each occurs, the year of the last emergence (up to 1953), and the year in which the next emergence is due.
Brood I—a small brood that occurs principally in southeastern Pennsylvania, Maryland, northeastern West Virginia, Virginia, and western North Carolina; 1944, 1961.