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Why do cicadas come out every 17 years?

Mark Kostich

This spring, trillions of periodical cicadas are emerging from the ground, where they’ve spent 13 or 17 years feeding on xylem (basically, tree juice). The two specific broods emerging this year have not come out at the same time since 1803, and kids may be hearing a lot of news about these loud insects. So today we’re tackling the cicada questions you’ve sent us: Why do cicadas come out every 17 years? What do cicadas eat? Why are there more cicadas at night than in the morning? Why do cicadas molt? How do cicadas get babies? We speak with Dan Gruner, professor of entomology at the University of Maryland, to get answers.

Download our learning guides: PDF | Google Slide | Transcript

  • Cicadas are fairly large dark-colored insects with colorful wings and eyes. 
  • There are over 10,000 species of cicadas around the world. 
  • Some species are periodical cicadas - meaning they spend  several years underground and then emerge all at once, by the millions or billions. In the US, periodical cicadas emerge in 13 or 17-year cycles. There are several different broods found in many parts of the eastern United States.
  • 17-year cicadas are general found farther north, while the warm climate in southern states may account for why 13-year cicadas complete their life cycle more quickly. 
  • Scientists aren’t sure why these insects have such an unusual life cycle, but they think it helps them evade predators. So many insects emerge at once that it’s hard for birds and other predators to eat all of them! 
  • While cicadas are underground they are growing bigger and eating xylem, fluid they find in the  roots of trees. 
  • Cicadas go through five stages while they’re underground. In the first four stages of life they don’t have wings. Shortly after coming above ground, they shed their exoskeleton for the last time, reaching maturity with a shiny set of wings.
  • While the periodical cicadas get all the attention, most cicadas around the world (and even in the US) are annual–also called dog day–cicadas, meaning they come out of the ground every year. (Well, some of them do, at least. Even dog day cicadas spend a couple of years underground, but some of them emerge every summer, rather than all together–en masse. So it’s not such an onslaught of bugs all at once.)
  • Cicadas are really noisy! Male cicadas have special drum-like organs called timbals in their abdomens that allow them to make loud calls. Female cicadas do not make any sound. 
  • Cicadas are edible! Humans can eat them and many do. They are a good source of protein. 
Jane Lindholm is the host, executive producer and creator of But Why: A Podcast For Curious Kids. In addition to her work on our international kids show, she produces special projects for Vermont Public. Until March 2021, she was host and editor of the award-winning Vermont Public program Vermont Edition.
Melody is the Contributing Editor for But Why: A Podcast For Curious Kids and the co-author of two But Why books with Jane Lindholm.

But Why is a project of Vermont Public.

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