INSECTS
out the year before or the year after. This fact has suggested the idea that the various broods established at the present time had their origin from individuals of a primary brood that, as we might say, got their dates mixed, and came out a year too soon or a year too late, the multiplying descendants of these individuals thus founding a new brood dated a year in advance or a year behind the emergence time of the parent stock. In this way, it is conceivable, the seventeen-year race might come to appear on each of seventeen consecutive years, and the thirteen-year race on each of thirteen consecutive years. Individuals emerging on the eighteenth or fourteenth year, according to the race, would be reckoned as a part of the first brood of its race.
The facts known concerning the emergence of the cicadas seem to confirm the above theory, for members of the seventeen-year race appear somewhere every year within the limits of their range, and the emergence of members of the thirteen-year race has been recorded for at least eleven out of the possible thirteen years. All the individuals of a brood are not, of course, descendants of a single group of ancestors, nor do they necessarily occur together in a restricted area—they are simply individuals that coincide in the year of their emergence. However, at least thirteen of the broods of the seventeen-year race are well defined groups, for the most part with definitely circumscribed territories, though overlapping in many cases. The broods of the thirteen-year race are not so well developed.
The broods are conveniently designated by Roman numerals. According to the system of brood numbering proposed by Doctor Marlatt, and now generally adopted, the brood of the seventeen-year race that appeared last in 1927 is Brood I. This is not a large brood, but it has representatives in Pennsylvania, Maryland, District of Columbia, Virginia, West Virginia, North Carolina, Kentucky, Indiana, Illinois, and eastern Kansas. Brood II, 1928, lives in the Middle Atlantic States, with a few
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