The Science of Periodical Cicadas – Kristin Moon Science

For months, we’ve been hearing the hype about the groups of periodical cicadas due to emerge after years underground.

I have some friends who are as excited about periodical cicadas as I am, while others would like nothing better than to hide inside until the cicadas are gone. Because I’m a bit obsessed with these creatures, I decided to compile what I’ve learned from my research into a post so that I could share it with you.

How are periodical cicadas different from annual cicadas?

If you’re anything like me, you may be wondering what all the hype is about. Don’t cicadas often appear during the summer months?

Yes and no. There are two broad categories of cicadas: annual cicadas and periodical cicadas. Annual cicadas are those that become active each year during the warm season, typically between June and August. Periodical cicadas only emerge in cycles of 13 or 17 years. Groups of periodical cicadas that emerge at the same time are called broods, and 30 different broods have been identified.

Where are periodical cicadas between emergences?

Like most insects, cicadas undergo metamorphosis. It is the immature nymphs that live underground for 13-17 years. While underground, they survive by using their mouthparts to suck xylem from tree roots.

Once their years underground have passed, it is time for the nymphs to emerge, but they will only do so once the soil temperature has reached a sustained temperature of at least 64 degrees F. It’s not uncommon to see so-called cicada “chimneys” poking up from the ground in the days and weeks prior to their emergence. These chimneys are formed as the nymphs begin to tunnel up to the surface. If you remove the chimney from the ground, you can find a tunnel underneath.

What comes next?

The truly amazing thing about periodical cicadas is how billions of them synchronize their emergence. One day, you’ll look in vain for any sign of the cicadas. The next day, they will be literally everywhere.

Often, the first sign that they’ve started to emerge are the familiar shed exoskeletons left on trees or blades of grass. Closer inspection reveals the adult cicadas hiding in the grass. Not long after, they will disappear into the treetops.

The nymphs commonly emerge under the cover of darkness or in the early morning. They will then molt from their hard exoskeletons. Once they are strong enough, they will make their way to the treetops.

Cicada Appearance

Periodical cicadas aren’t as large as I thought they’d be: they are only approximately 2- 2.5 inches long.

The cicadas are often very pale right after molting.

Over the course of the next few hours, they will darken and their wings will expand. Adult cicadas are dark (dark brown to black) with reddish-orange eyes and transparent wings.

If you don’t mind holding a cicada, it is easy to determine its sex when you turn the insect over. Male cicadas have a domed-shaped abdomen. The abdomen of a female cicada will come to a point. Female cicadas also have an ovipositor, which may be extended.

What comes next?

After emerging, male and female cicadas will spend the next few weeks in the treetops. Males attract females by vibrating special abdominal organs called tymbals. The combined sound of their buzzing calls can reach 80-100 decibels.

After mating, the female cicada uses her ovipositor to slice into small tree branches. She will then lay 20-30 eggs in the tree. A single female may lay 400-600 eggs before her death.

After 6-10 weeks, the eggs will hatch and new cicada nymphs will fall from the trees to the ground. There they will tunnel down into the earth where they will remain for the next 17 years.