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Periodical Cicadas


I neglected to post on these insects earlier this year, when there was a big emergence of them earlier this year. But I shall catch up.

 Periodical cicadas (Magicicada spp) are insects that spend 13 or 17 years sucking the sap of tree roots before they emerge. They emerge in the spring, they do their last molt, becoming adults, wings and all. The male ones climb up into trees, gather together, and make lots of sound in species-specific fashion. This attracts female ones, and a male one then mates with a female one who shows interest in him by flicking her wings. The females then cut notches in young branches and lay their eggs there, and when the baby cicadas hatch out, they fall to the ground and search for tree roots to suck the sap of. Both sexes of adults live only about 4 to 6 weeks in that phase before dying.

Some cicada species stay underground for much less, typically 2 to 5 years, and they don't have synchronized emergence like the 13-year or 17-year ones. They are often called annual cicadas despite their usually spending well over a year underground.

About the 13-year and 17-year cicadas, why their synchronized emergence and large gatherings? The most successful theory I've seen is predator satiation, that so many of them emerge that their eaters get stuffed with them and let many more of them live. They don't have much defense against predation, so they get a *lot* of predators. Small birds, wild turkeys, squirrels, opossums, raccoons, lizards, snakes, frogs, also dogs and cats.

Peter Turchin The Social Life of Periodical Cicadas - Peter Turchin

He notes that this is a case of the "selfish herd", and that there is no way to freeload, to get the benefits of cooperation while avoiding the costs. A cicada that emerges in the wrong year becomes very vulnerable, and is likely to become some bird's meal before it can reproduce. That produces selection for all the cicadas emerging in one year of the cycle and not some other year. The more that emerge in some year, the better their survival will be.

As to why prime numbers, the theory that I've seen is that it's a way of being out of sync with predators' population cycles. If they have cycles of 3 or 4 or 5 years, then they will be in sync with cicadas' cycles if the cicadas' cycle length is a multiple of these lengths like 8 or 12 or 15. Being prime prevents that.

Finally, I note that cicadas emerge in different years of their cycles in different places. In 1907, entomologist Charles Lester Marlatt named the possible broods by year of emergence in the insects' cycles. Brood I - Brood XVII (1893 to 1909) are for 17-year cicadas and Brood XVIII - Brood XXX (1893 to 1905) are for 13-year cycles.

This year's 17-year-cicada brood is Brood X, the largest of them, and this year's 13-year-cicada brood would be Brood XXIX if it existed -- only half of the possible broods are known to exist.
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