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. Brood II--occurs along the eastern seaboard from Connecticut and southeastern New York to North Carolina; 1945, 1962. Brood V--a compact brood that occurs in the eastern half of Ohio and throughout West Virginia except in the extreme southern part; 1948, 1965. Brood VIII--occurs in western Pennsylvania, eastern Ohio, and the northern Panhandle of West Virginia; 1951, 1968. Brood IX--occurs in West Virginia, western Virginia, and northwestern North Carolina; 1952, 1969. Brood X--the largest brood; occurs in abundance over much of the northeastern quarter of the United States; 1936, 1953. Brood XIX--occurs over much of the southern part of the United States; extends into the southern part of Illinois and to northern Missouri; 1946, 1959. Brood XXIII--occurs in the Mississippi Valley from southern Indiana, southern Illinois, and Missouri to the Gulf; 1950, 1963. The Cicada's Song A few days after the periodical cicadas appear, their incessant drumming, or singing, is heard. Only the males have sound-producing apparatus, which consists of two shell-like inflated drums on the sides of the abdomen. Two strong muscles set the drums in motion. The chorus begins at dawn. As the temperature rises, the volume swells. Throughout the day a blanket of sound rests over the countryside. In the evening the sound ceases. When heard from a distance, the cicada chorus is a whirring, droning monotone. But if attention is concentrated on an individual insect, several notes can be distinguished. The loudest is represented by the syllables "tsh-ee-EEEE-e-ou." It is sustained 15 to 20 seconds. The middle portion is loud and shrill.