How do 17-year cicadas, which live underground as nymphs, track the passage of time so they all emerge synchronously as adults?
Magicicada septendecula male (Brood IX). (Credit: C. Simon / doi:10.1371/journal.pone.0000892)
doi:10.1371/journal.pone.0000892
Periodical cicadas (Magicicada spp.) have one of the strangest life cycles in the animal kingdom, and this has long fascinated evolutionary biologists. There are seven periodical cicada species and they get their “periodical” designation because, in any given location, all members of the population are developmentally synchronized so they emerge as adults at the same time in the same year.
In the United States, periodical cicadas emerge every 13 or 17 years – which is curious, because these are both prime numbers – possibly to avoid exploitation by predators with shorter life spans. The periodical cicadas’ mass emergence is also an adaptation to their predators, where they flood out their predators’ ability to consume them all, thereby allowing them to find mates and to breed.
There are seven species of Magicicada – three 17-year species and four 13-year species. Periodical cicadas live underground as undeveloped nymphs, usually within 61 cm (2 feet) of the surface, feeding on the juices of plant roots. After the passage of either 13 or 17 years, the entire population emerges simultaneously in springtime.
It’s notable that periodical emergences of cicadas are also reported from other locations. For example, the “World Cup cicada” emerges every 4 years in northeast India and another cicada species emerges every 8 years in Fiji. This raises the question: how do periodical cicadas regulate their life cycle? How do they know when it is time to emerge and transform into their adult form?
“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,” the study’s senior author, entomologist Teiji Sota, an emeritus professor at Kyoto University, told me in email.
A seventeen year life span is a remarkably long life for an insect. How do periodical cicadas live so long?
“Cicadas have low metabolic and growth rates during the juvenile (nymphal) stage and ageing (a rise of mortality risk) occur only in the adult stage,” Professor Sota explained in email. “The marked extension of nymphal life may have been possible since they live in the stable, safe (enemy-free), and cooler environment of the soil habitats and low metabolic rate owing to the xylem fluid feeding.”
Unfortunately for curious scientists, this long life span makes it extremely difficult to raise periodical cicadas in the lab for study. But recently, Professor Sota and collaborators decided it was time to take a closer look at these fascinating insects.
In autumn, Professor Sota and collaborators dug up cicada nymphs between 11 and 16 years of age in different areas of the eastern United States (Figure 1b). These nymphs were part of different yearly cohorts, or broods. After unearthing the insects, Professor Sota and collaborators studied their growth, development, and gene expression, and also examined the gene expression patterns of 17-year-old nymphs just before emergence.
Figure 1 | (a) Three species groups and seven species of Magicicada with pictures of the three 17-year species (males). (b) Distribution ranges of 13- and 17-year periodical cicadas and sampling sites for the latter in eastern USA.
doi:10.1098/rspb.2025.1306
Professor Sota and collaborators found that the decision to emerge is reliably indicated by the nymph’s eye color: it changes from white to red. They found that almost all 16-year-old nymphs had red eyes and their body weights exceeded the critical weight. Interestingly, Professor Sota and collaborators found that a small but noticeable number of 12-year-old nymphs had body weights that also exceeded the critical weight and thus, they too made the decision to emerge, as indicated by their red eyes (Figure 1d).
Figure 1. (c) Hypothetical mechanism of life-cycle control and growth pattern for 17-year cicada nymphs. Reverse triangles indicate 4-year gates for checking body weight of the last instar against critical body weight (CBW) in 4n-th years. Nymphs that have reached CBW by the 16th year’s gate will emerge as adults in the 17th year (majority); fast-growing nymphs reaching CBW by the 12th year’s gate will emerge in the 13th year (4-year early stragglers). The thick broken line shows changes in median instar number of M. septendecim in Connecticut 1979−1995. (d) Hypothetical changes in nymphal body-weight distribution with age, which is affected by genetic and environmental variation, and the occurrence of 4-year early and ordinary emergence in 17-year cicadas. Nymphs with body weight exceeding CBW in the 12th or 16th year’s gate will have red eyes and emerge in the following year. Inset pictures: fifth instar nymphs of Magicicada sp. with white eyes (left) and red eyes (right) in Chicago and Cincinnati, November 2019 (photographs by T. Sota).
doi:10.1098/rspb.2025.1306
After examining changes in their gene expression profiles, Professor Sota and collaborators discovered that red eyed nymphs showed elevated expression for genes that are involved in responses to external stimuli, especially light, and for genes that facilitate adult morphological development, whereas genes for adult metamorphosis and moulting were expressed only after the nymphs had overwintered at 17 years old. Thus, 17-year cicadas appear to have made the decision to emerge principally at 4-year check points, or “gates”, based on whether they achieved a critical body weight, just as Professor Sota and collaborators hypothesized. And yet, some mysteries remain. For example, how do cicadas know when they’ve reached the four year check point?
“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,” Professor Sota told me in email. “The passing of one year is precisely detected by the seasonal changes in soil temperature and host tree physiology, and some epigenetic changes may occur yearly from state 1, 2, 3, to 4 to regulate the expression of genes involved in the decision making for emergence at every 4n-th year.”
Since this study indicates that cicadas make their decision to emerge as adults based on their body weight, do 13-year periodical cicadas have the same body size as 17-year periodical cicadas?
“Our previous study [ref] showed that adult cicadas of each species group had a size cline where southern cicadas were larger (the converse Bergmann’s cline); however, at the boundary of 13- and 17-year cicadas, both had similar body sizes,” Professor Sota replied in email, showing me this graph.
Adult body size of Magicicada along the average annual temperature gradient.
Teiji Sota
Thus, an inherent genetic difference in growth rate between 13- and 17-year cicadas may underlie in the four-year difference in reaching the critical body weight for emergence of the adults.
“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,” Professor Sota explained email.
Although this study focused on 17-year cicadas, the fundamentals of this life-cycle control mechanism are expected to be the same in 13-year cicadas. Already, Professor Sota and collaborators are planning to examine this further.
Source:
Namiho Saito, Satoshi Yamamoto, Satoshi Kakishima, Yutaka Okuzaki, Andrew Rasmussen, Diler Haji, Shota Nomura, Hiroyuki Tanaka, Takehiko Itoh, Jin Yoshimura, Chris Simon, John R. Cooley, Gene Kritsky and Teiji Sota (2025). When and how do 17-year periodical cicada nymphs decide to emerge? A field test of the 4-year-gate hypothesis, Proceedings of the Royal Society B: Biological Sciences 292(2053) | doi:10.1098/rspb.2025.1306
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