The 17-Year Secret: How Cicadas Synchronize Their Mass Emergence
A groundbreaking new study reveals how periodical cicadas, famed for their remarkably synchronized life cycles, track time and coordinate their mass emergences after 13 or 17 years underground. Researchers have discovered that these insects appear to make emergence decisions based on achieving a critical body weight, assessed at four-year intervals, with a fascinating shift in eye color signaling their readiness to emerge.
Periodical cicadas (Magicicada spp.) exhibit one of the most unusual life cycles in the animal kingdom, a phenomenon that has captivated evolutionary biologists for decades. What sets them apart is their ability to remain developmentally synchronized, ensuring that all members of a population emerge as adults simultaneously in the same year. This synchronized emergence is particularly striking in the United States, where these insects appear every 13 or 17 years – prime numbers that may serve as an evolutionary strategy to avoid predator exploitation.
There are seven species of Magicicada – three with a 17-year cycle and four with a 13-year cycle. These insects spend the vast majority of their lives as undeveloped nymphs, living underground within roughly two feet of the surface and feeding on plant roots. The synchronized, mass emergence isn’t just a quirk of nature; it’s a powerful adaptation. By overwhelming predators with sheer numbers, cicadas increase their chances of finding mates and successfully reproducing. Similar periodical emergences have been observed globally, with the “World Cup cicada” in northeast India emerging every four years and another species in Fiji appearing every eight.
“The periodical cicadas of the genus Magicicada are an extremely enigmatic group of insects, and how they control their life cycles is a mystery I have been wishing to solve,” explained a senior entomologist at Kyoto University, leading the recent research.
A 17-year lifespan is exceptionally long for an insect. The key to this longevity, researchers believe, lies in the cicadas’ slow metabolism and growth rate during their nymphal stage. “Cicadas have low metabolic and growth rates during the juvenile (nymphal) stage and ageing…occur only in the adult stage,” the entomologist noted. Their underground environment provides a stable, safe, and cool habitat, further contributing to their extended development.
Historically, studying these insects has been challenging due to their long life cycle, making laboratory rearing difficult. However, a team led by the Kyoto University entomologist recently undertook a detailed investigation, digging up cicada nymphs aged between 11 and 16 years from various locations across the eastern United States. These nymphs, representing different yearly cohorts or “broods,” were then analyzed for growth, development, and gene expression patterns.
The research revealed a clear indicator of impending emergence: a change in the nymph’s eye color from white to red. Almost all 16-year-old nymphs exhibited red eyes and had reached a critical body weight. Interestingly, a noticeable proportion of 12-year-old nymphs also displayed red eyes and exceeded the critical weight threshold, suggesting they were preparing to emerge four years ahead of schedule.
Further analysis of gene expression profiles showed that red-eyed nymphs had increased expression of genes involved in responding to external stimuli, particularly light, and those facilitating adult development. Genes related to metamorphosis and molting, however, were only activated after the nymphs had overwintered at 17 years old. This suggests that 17-year cicadas make emergence decisions at roughly four-year “gates,” based on whether they’ve attained the necessary body weight.
But how do cicadas know when they’ve reached these four-year checkpoints? “Although the mechanism is entirely unknown, we speculate that nymphs use some internal clock based on yearly epigenetic changes to cyclically count 4n years of age,” the entomologist stated. They theorize that seasonal changes in soil temperature and host tree physiology provide cues, triggering epigenetic changes that regulate gene expression and inform the emergence decision.
The study also investigated whether 13-year and 17-year cicadas differ in body size. Previous research indicated a size cline, with southern cicadas generally being larger. However, at the boundary between 13- and 17-year species, both exhibited similar body sizes. This suggests that the difference in emergence timing isn’t due to overall size, but rather to variations in growth rate. “This implies that, under the same climatic condition, the critical weight is common and the emergence year difference is based the difference in growth rate, which is faster for 13-year cicadas than 17-year cicadas,” the entomologist explained.
While this study focused on 17-year cicadas, researchers believe the underlying mechanisms governing their life cycle are likely the same in 13-year cicadas. Further research is already planned to explore this connection. The findings, published in Proceedings of the Royal Society B: Biological Sciences in February 2025, offer a significant step toward unraveling the long-standing mystery of periodical cicada synchronization.
