Why do North American cicadas emerge only every 13 or 17 years?

Science · Published 8 October 2026 Swipe more like this →

In short: Every 13 or 17 years, millions of insects materialize from the soil in a massive, synchronized event. These periodical cicadas follow a mathematical rhythm that seems almost impossible to coordinate, leaving scientists to wonder how they track time while living underground for over a decade.

The genus Magicicada, native to the eastern and midwestern United States, consists of seven distinct species. These insects are defined by their highly synchronized, long-duration life cycles. While most insects live and die within a single season, periodical cicada broods remain underground for over a decade, emerging in spring only at specific prime-number intervals: either 13 or 17 years.

The species are split between these two cycles. Three species—M. septendecim, M. cassini, and M. septendecula—operate on a 17-year schedule. The remaining four species, including M. tredecim and M. neotredecim, follow a 13-year cycle. The mechanism behind this precision remains a mystery. One leading theory suggests that nymphs might monitor seasonal changes in the xylem of the trees they feed on to deduce the passing years.

When emergence occurs, the scale is staggering. In 2024, a rare overlap between the 17-year Brood XIII and the 13-year Brood XIX—the largest known brood—resulted in more than one trillion insects appearing across North America. This specific temporal overlap has not been seen since 1803. During their brief adult lives, which last only 20 to 25 days, males use abdominal organs called tymbals to produce incredibly loud mating calls, creating 'chorus centers' that can last up to six days.

The transition from nymph to adult is a physical transformation. After unearthing themselves, wingless nymphs climb trees, where their exoskeleton splits to allow the winged adults to emerge. These adults, known as imagoes, are characterized by red eyes and a black thorax, typically measuring between 2.4 and 3.3 cm in length.

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