Cicadas are
winged insects of the superfamily Cicadoidea.
There are over 2,500 species of cicada, native to temperate and tropical
regions across the globe. While most cicadas are an inch or two long, members
of the Pomponia and Tacua genera can be over half a foot
with an 8inch wingspan. Cicadas live on tree sap, sucking up the sweet stuff by
puncturing the bark with their straw-like proboscis;
as they have no defenses, the only way a cicada might hurt a human is by
mistaking them for a tree and going in for drink.
To better attract mates, males’ synch their songs to
amplify the effect. Once a female is fertilized, she carves slits into the tree
bark, laying her eggs inside. Upon hatching, cicada nymphs fall from the trees and burrow underground; they remain
buried until it’s time to mate, surviving by drinking from tree roots.
Annual cicada
nymphs emerge each summer; molt to reach
their winged adult form; then fly into the trees to mate, lay eggs, and die. But
some species – like North America’s Magicicada – are periodical instead of annual. Once
underground, Magicicada nymphs won’t
emerge for another 13 or 17 years (depending on the species). When
the time comes, populations of these bugs synchronize their emergence and mating
– sometimes in tremendous groups called broods.
Brood X (the Great Eastern Brood) first
emerged in 2004 across Illinois and Michigan, and from New York to Georgia;
they make their next appearance in 2021. Likewise, after a 13-year hiatus, Brood XIX (the Great Southern Brood) has recently reemerged across the majority
of the American South.
Cicadas' natural predators include wasps, birds, rodents –
even humans. By breeding in such large numbers, cicadas benefit from predator satiation: there are so many of them, most aren’t eaten
and get to reproduce. Yet the periodical Magicicada
gain an additional, more mathematic advantage.
You may have noticed that 13 and 17 are prime
numbers. Imagine a predator with a four-year life cycle happens to be
around when a 13-year cicada brood emerges; because 13 is prime, it will be another 52 years until that predator gets to taste another cicada.
Whatever your feelings on math – if you happen to be a cicada – numbers are a
literal lifesaver.