Straggling and spurious broods
"Brood XII," shown above, is almost certainly a spurious brood. Sometimes periodical cicadas emerge "off-schedule" by one or more years. This phenomenon is often referred to by the general term "straggling," although straggling cicadas can emerge either later or earlier than expected. Straggling makes it difficult to construct accurate maps of periodical cicada brood distributions, and historical reports of emergences often contain little or no information about how many cicadas were seen. Straggling emergences in which one or two cicadas are present are common; larger unexpected emergences of thousands of individuals have been reported (e.g. Dybas 1969). Stragglers are almost certainly responsible for reports of "spurious broods" that are not generally recognized.
This map is intended to portray only an approximate, present-day brood distribution. It is based on previously published maps (Marlatt 1923). A project is currently underway to make new maps of periodical cicada broods. See the Magicicada mapping project homepage.