Making the weight in four years

Kyoto, Japan -- Periodical cicadas have one of the strangest life cycles in the animal kingdom. The 17-year cicadas spend 99.5% of their lives underground in an undeveloped nymph state, which is the longest strictly regulated juvenile period among insects.

Then in the spring of their 17th year, they simultaneously emerge and the males scream above ground for their four to six week-long adult life. Exactly how these insects are able to control when they mature and emerge has remained a mystery.

The long life cycle of periodical cicadas makes rearing nymphs for study extremely difficult. Recently, however, a collaborative team of researchers from both Japan and the United States, including a team from Kyoto University, was motivated to tackle this conundrum.

The research team proposed a theory involving developmental gates of four years based on critical body weight. They hypothesized that nymphs check their body weight at each four-year gate, and if their weight exceeds the critical amount, they decide to emerge the following spring.

"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," says corresponding author Teiji Sota.

The research team was able to study the growth, development, and gene expression of nymphs aged 11 to 16 years old by digging out nymphs belonging to different yearly cohorts, or broods, in different areas of the eastern United States in the autumn. They also analyzed the gene expression of 17-year-old nymphs just before emergence.

According to the group's findings, the decision to emerge is indicated by an eye color change from white to red. Almost all the 16-year-old nymphs observed were red-eyed with a large body weight likely above the critical threshold.

Additionally, a small but substantial proportion of 12-year-old nymphs had red eyes and larger body weights than the white-eyed members of their brood. In contrast, nymphs of other ages were almost always white-eyed, even if they had large body weights.

Furthermore, red-eyed nymphs showed elevated genes for responding to external stimuli and for facilitating adult-morphology development. However, genes for adult metamorphosis and molting, or shedding their exoskeleton, were expressed only after overwintering at 17 years.

These results align with the team's hypothesis, but some mysteries remain.

"The most enigmatic and challenging component of our life-cycle control hypothesis is the clock-like mechanism that counts a four-year cycle," says Sota. "If a four-year counting mechanism exists, it may be based on an epigenetic cycle."

This study focused on 17-year cicadas, but the fundamental components of this life-cycle control mechanism are expected to be the same in 13-year cicadas. An inherent genetic difference in growth rate between 13 and 17-year cicadas may result in the four-year difference in reaching the critical body weight, which the researchers plan to explore in the future.

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The paper "When and how do 17-year periodical cicada nymphs decide to emerge? A field test of the 4-year-gate hypothesis" appeared on 27 August 2025 in Proceedings of the Royal Society B: Biological Sciences, with doi: 10.1098/rspb.2025.1306

About Kyoto University

Kyoto University is one of Japan and Asia's premier research institutions, founded in 1897 and responsible for producing numerous Nobel laureates and winners of other prestigious international prizes. A broad curriculum across the arts and sciences at undergraduate and graduate levels complements several research centers, facilities, and offices around Japan and the world. For more information, please see: http://www.kyoto-u.ac.jp/en