Back in 2004 I was living in New York and commuting between New York and Washington, DC on the Acela. I was working in a fairly rural part of Virginia and was lucky enough to accidentally experience a once in 17 years event: the emergence of Magicicada Brood X.
(Picture from Wikipedia)
Now I realize that most people probably don't think that being in a place where millions of large winged insects appear from the ground and make a deafening noise in search of a mate is fun. But Magicicada is so cool that it's hard to miss their emergence. And Magicicada loves prime numbers.
For that reason, Magicicada Brood X (which lives along the Eastern coast of the US) is place number 127 in The Geek Atlas. Your next chance to meet the little beastie is 2021.
This particular brood spends 17 years underground living off the sap inside the roots of trees. Once ready to become an adult it burrows upwards and climbs its tree. Then when high up in the tree it molts and spreads its wings.
Once ready to fly it makes a humming sound which, given that millions emerge all at the same time, fills the air with an incredible din. The female cicadas make a clicking sound and between the humming and clicking the males and females find each other and mate.
They live only a few weeks above ground and new baby cicadas fall to the grown and burrow to the roots to start another 17 year wait.
To get a feeling for what it's like to be around when Magicicada makes its appearance, here's Sir David Attenborough:
There are other cicadas that live on a 7 and 13 year cycle. All these cycles are based around a prime number of years. The hypothesis is that the cicada uses a combination of emergence en masse and a prime numbered cycle to avoid predators. A prime numbered cycle means that the cicada rarely meets a predator and mass emergence would overwhelm any predator around.
A prime numbered cycle avoids predators because it doesn't have any factors. Suppose that the cicada had an 18 year cycle, any predator that peaked every 2, 3, 6 or 9 years could synchronize with the cicada and always meet it and eat it. By having a prime numbered cycle the cicada rarely meets a predator and predators have a hard time synchronizing with it.
For a more detailed look at the prime numbered cycle, you can read the paper Prime Number Selection of Cycles in a Predator-Prey Model.
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