Brood X

Brood X (Brood 10), the Great Eastern Brood, is one of 15 broods of periodical cicadas that appear regularly throughout the eastern United States. It has the greatest range and concentration of any of the 17-year cicadas.

Every 17 years, Brood X cicadas tunnel en masse to the surface of the ground, lay eggs, and then die off in several weeks. The combination of long dormancy, simultaneous emergence of vast numbers, and short period before the nymphs' burrowing underground to safety, allows the brood to survive even massive predation.

Brood X's most recent appearance was in the spring and early summer of 2004 throughout an area roughly enclosed by Illinois, Michigan, New York and Georgia. The next appearances will be in 2021 and 2038.

Pehr Kalm, a Swedish naturalist visiting Pennsylvania and New Jersey in 1749 on behalf of his nation's government, observed in late May an emergence of Brood X. When reporting the event in a paper that a Swedish academic journal published in 1756, Kalm wrote:

The general opinion is that these insects appear in these fantastic numbers in every seventeenth year. Meanwhile, except for an occasional one which may appear in the summer, they remain underground.
There is considerable evidence that these insects appear every seventeenth year in Pennsylvania.

Kalm then described documents (including one that he had obtained from Benjamin Franklin) that had recorded in Pennsylvania the emergence from the ground of large numbers of cicadas during May 1715 and May 1732. He noted that the people who had prepared these documents had made no such reports in other years. Kalm further noted that others had informed him that they had seen cicadas only occasionally before the insects appeared in large swarms during 1749. He additionally stated that he had not heard any cicadas in Pennsylvania and New Jersey in 1750 in the same months and areas in which he had heard many in 1749. The 1715 and 1732 reports, when coupled with his own 1749 and 1750 observations, supported the previous "general opinion" that he had cited.

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