Cicada

The longest insect childhood

Periodical cicadas have one of the most remarkable life cycles in the animal world. Nymphs live underground for 13 or 17 years, sucking sap from tree roots, before emerging in unison to mate and die. The adult phase — what most people see and hear — lasts only 4–6 weeks.

There are 15 named broods of periodical cicadas, each on a fixed schedule:

• Brood X (17-year) — the largest, emerges across the Mid-Atlantic and Midwest. Last emerged in 2021.
• Brood XIX (13-year) — emerges in the Southeast.
• Brood XIII (17-year) — Illinois area, emerges 2024 (a rare double-emergence year with Brood XIX).

Each brood emerges in a different year, and the schedules don’t overlap by accident — the prime-number cycles (13 and 17) make it nearly impossible for two broods to synchronize, which limits hybridization and competition.

Why prime numbers

The choice of prime-number life cycles (13 and 17, never 12 or 16 or 18) is one of the most elegant examples of evolutionary mathematics. Possible explanations:

• Predator avoidance — predators with shorter life cycles (2, 3, 4 years) can’t synchronize their populations with cicada emergences. Prime numbers minimize the overlap with any potential predator schedule.
• Hybridization avoidance — sister species emerging on different prime-number cycles overlap rarely (every 13×17 = 221 years), preventing genetic mixing.

The mathematics work out neatly enough that cicadas appear in introductory texts on evolutionary number theory.

Mass emergence

When a brood emerges, the densities are staggering — up to 1.5 million cicadas per acre in peak forests. The emergence is synchronized: nymphs respond to soil temperature reaching a critical threshold (about 18 °C at 20 cm depth), so essentially all individuals in a brood emerge over a few days.

The synchrony is a survival strategy called predator satiation. With so many cicadas emerging at once, every bird, raccoon, fox, snake, and small mammal can eat as much as they want — and they can’t possibly eat them all. Most cicadas survive long enough to mate and lay eggs.

A noise like a chainsaw

Male periodical cicadas produce one of the loudest insect sounds on Earth — collective choruses can exceed 100 decibels, comparable to a chainsaw or a power lawnmower. The sound is produced by tymbals — drum-like membranes on the abdomen that the cicada rapidly buckles in and out using muscular contractions.

The chorus serves several functions: attracting females, advertising species identity, and discouraging predators (some birds avoid cicada-saturated areas because of the disorienting volume).

Annual cicadas

The familiar summer cicadas of most of the world — the ones that buzz from trees on hot afternoons across all temperate regions — are annual cicadas, not periodical. They have shorter (2–5 year) life cycles, but emerge in overlapping years, so adults are present every summer. Their broods aren’t synchronized.

The dog-day cicada (Neotibicen canicularis), common in eastern North America, is the most familiar annual cicada — buzzing through every August.