A joint team of researchers from Kyoto University and other institutions has partially uncovered why periodical cicadas simultaneously burst out of the ground every 13 or 17 years.
Also known as “prime-number cicadas” in Japan, the various species that inhabit the eastern region of the United States were found to have growth cycles regulated by four-year intervals.
The scientists said once a cicada exceeds a certain weight threshold in the year prior to molting and emerging from underground as an adult, its vision-related genes kick in to prepare it for a fleeting life on the surface.
Those species in the eastern United States spend exactly 13 or 17 years in the nymph state, and their mass appearance comprises more than one species across each geographical area.
Scientists speculate that they molt en masse partly to facilitate breeding and to defend themselves against birds and other predators.
However, how exactly these bugs are able to measure time, synchronize when they molt and other specifics remain unknown.
“It appears a certain mechanism of genetic control is at work to allow the cicadas to measure the four-year cycle accurately,” said Teiji Sota, a Kyoto University professor emeritus of evolutionary ecology, who led the team. “We hope to do more gene analysis to find out more.”
The researchers hypothesized that certain changes take place in cicada nymphs after multiples of four years, or in the 12th year for 13-year cicada species and in the 16th year for 17-year cicada species.
All go topside the following year, making both cycles multiples of four years plus one.
To corroborate their hypothesis, the scientists began extracting nymphs from the ground in 2019 for four years. They chose nymphs from four 17-year cicada broods that would molt in different years.
Those collected were bred in a lab environment to study growth states and when gene expression took place.
One notable instance of this was a shift in eye color from white to red as the cicada nymphs developed sight before making their way aboveground.
Among the nymphs collected, most of the 15-year-olds were white-eyed, but 97 percent of the 16-year-olds had turned red-eyed and had a mean body weight likely above the set threshold.
The scientists also observed that gene expression tied to a nymph’s ability to register light occurred at age 16, indicating it was developing sight.
Twelve percent of the 12-year-old nymphs had exceeded the threshold weight level and their eyes turned red in preparation for molting, even though they belonged to 17-year cicada species.
By contrast, only one 11-year-old, one 13-year-old, two 15-year-olds and none of the 14-year-olds exhibited red eyes.
The team’s research results were published on Aug. 27 in the British science journal Proceedings of the Royal Society B: Biological Sciences.