Periodical cicadas are grouped into geographic broods based on the calendar year when they emerge.
For example, in 2014, the 13-year brood XXII emerged in Louisiana and the 17-year brood III emerged in western Illinois and eastern Iowa.

In 1898, entomologist C. L. Marlatt assigned Roman numerals to 30 different broods of periodical cicadas: 17 distinct broods with a 17-year lifecycle, to which he assigned brood numbers I through XVII (with emerging years 1893 through 1909); plus 13 broods with a 13-year cycle, to which he assigned brood numbers XVIII through XXX (1893 through 1905).

Many of these hypothetical 30 broods, however, have not been observed.
Furthermore, two of the brood numbers assigned by Marlatt (broods XI and XXI) existed at one time, but have become extinct.
The Marlatt numbering scheme has been retained for convenience (and because it clearly separates 13- and 17-year lifecycles), although today only 15 broods are known to survive.
Periodical cicadas are grouped into geographic broods based on the calendar year when they emerge. For example, in 2014, the 13-year brood XXII emerged in Louisiana and the 17-year brood III emerged in western Illinois and eastern Iowa.
In 1898, entomologist C. L. Marlatt assigned Roman numerals to 30 different broods of periodical cicadas: 17 distinct broods with a 17-year lifecycle, to which he assigned brood numbers I through XVII (with emerging years 1893 through 1909); plus 13 broods with a 13-year cycle, to which he assigned brood numbers XVIII through XXX (1893 through 1905).
Many of these hypothetical 30 broods, however, have not been observed. Furthermore, two of the brood numbers assigned by Marlatt (broods XI and XXI) existed at one time, but have become extinct. The Marlatt numbering scheme has been retained for convenience (and because it clearly separates 13- and 17-year lifecycles), although today only 15 broods are known to survive.
Periodical cicadas