Marlatt (1923) recognized “Brood XV” on the basis of scattered records. Simon (1988) considered this brood to be spurious. Is “Brood XV” a brood?
A periodical cicada “brood” is generally defined as all the cicadas emerging in a given region on a given schedule. However, exceptions abound. For example, many broods (e.g., I, II, V) have disjunct populations far separated from the main bodies of their broods; these disjuncts likely have separate evolutionary origins and may be only coincidentally synchronized with other parts of their broods. Such examples demonstrate that the brood concept is more a bookkeeping tool for organizing emergence schedules (Fitch 1854) than a statement of evolutionary history or biological reality.
Periodical cicadas do emerge, sometimes in numbers sufficient to attract notice, on a schedule matching Marlatt’s (1902) “Brood XV.” However, these emergences almost certainly should not be considered a “brood” in the same sense as the other recognized broods. At question is whether these “Brood XV” emergences are self-sustaining or whether they require continuous “subsidies” of stragglers from another brood, offset by ±1 or ±4 years. The fact that all Marlatt’s records of “Brood XV” exist within the range of Brood II, which emerges four years later, suggests that “Brood XV” does not have an existence independent of Brood II, and that while some populations may be marginally self-sustaining, they are continually subsidized by new stragglers from Brood II, which functions as a host brood. Thus, “Brood XV” is considered a “shadow brood” of Brood II.
This brood was not recognized by Simon (1988). Blue symbols are from Marlatt (1923).