What Cicadas Look Like, Eat, and Why They Stay Underground for Years Ahead of 2021 Season

Billions of cicadas are set to emerge from the ground in more than a dozen U.S. states in May and June, having spent the first 17 years of their lives underground. But what do cicadas look like, what do cicadas eat, and why do cicadas stay underground for 17 years?

There are thousands of different cicada species in the world, but those that emerge every 13 or 17 years are collectively known as periodical cicadas, and they're only found in North America.

The brood that is set to emerge this year is known as Brood X, which contains three species: Magicicada septendecim, Magicicada cassini and Magicicada septendecula.

All three species are bulky and have six legs, but Magicicada septendecim is the largest of them, typically measuring between 3 cm and 4 cm in length, with broad orange stripes running across its abdomen.

Magicicada cassini and Magicicada septendecula are between 2 cm and 3 cm long, and both have an almost uniformly black abdomen.

All three species have orange legs and orange eyes, and the veins on their otherwise clear wings are also orange.

Cicadas don't have mouthparts for biting or chewing vegetation. Instead they drink sap through the roots and branches of plants and trees.

A definitive reason for why periodical cicadas stay underground for so long hasn't yet been established, but the topic has fascinated researchers, particularly the fact that 13 and 17 are prime numbers, and there are numerous theories.

The most widely accepted of these is that both their long lifecycle and living underground have made it difficult for predators to evolve to target periodical cicadas specifically.

Another theory posits that periodical cicadas track time by monitoring seasonal cycles.

Emerging all at once, in a swarm of billions, is also believed to help these cicadas to evade predators, with the sheer number of them overwhelming potential enemies, a strategy called predatory satiation.

They are, however, vulnerable to Massospora cicadina, a fungus that only affects periodical cicadas.

It takes over their body while they're still alive, forming blocks of spores that cause the rear segments of the cicadas' abdomen to fall off, leading to infertility, disease transmission, and an extremely unpleasant death.

Once above ground, the cicadas will only live for up to four weeks, the majority of which they'll spend trying to attract a mate by making loud noises that can hit 100 decibels.

"After mating, females deposit eggs in the tips of tree branches, then billions of adult cicadas will die and fall to the ground," Michael Raupp, Professor of Entomology and Extension Specialist at the University of Maryland, previously told Newsweek.

"After several weeks, eggs hatch and tiny cicadas tumble to the earth and enter the soil to find tree roots on which they will feed for the next 13 or 17 years, thus completing the circle of life."

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A stock image shows a cicada sitting on a leaf. Trillions of Brood 10 cicadas are due to emerge in the U.S. in spring 2021. Getty