How Cicadas Work


This cicada is no longer living, but it's still beautiful on this leaf. Note: The cicada pictured is not a member of 2013's Brood II. See more pictures of insects.
Katherine Frey/The Washington Post via Getty Images

They are crawling out from underground, where they have been hiding in the darkness for almost two decades. They are invading the East Coast in the United States, as far south as North Carolina and as far north as Connecticut and New York, as of May 2013.

They fill the skies and forests as they swarm. Thousands of them cry out together day and night. They are Brood II. Should you be frightened? Should you try to prepare yourself?

Not really. The only preparation you may need to greet the cicadas of Brood II (or any other periodical cicada) is a pair of earplugs, because the worst they can do is keep you up at night.

Cicadas are flying, plant-feeding insects that are most famous for their powerful singing voices and rare appearances. Of course, not all cicada species dramatically appear en masse at regular intervals of 13 or 17 years. Some species feature adults that pop up on a more ho-hum annual basis.

But in spring 2013, the Brood II cicadas have come to perform once again.

Know 'em When You See 'em

Why are cicadas so noisy?
Why are cicadas so noisy?
Photo courtesy Dave Davis/EPA

Cicadas are often mistaken for locusts, but they're actually more like leafhoppers or aphids. They are classified in the order Hemiptera -- a distinction given to all insects with piercing and sucking mouthparts. Entomologist have identified 1,500 different species of this mythical insect [source: Encyclopaedia Britannica].

The average wingspan of a cicada is between 2.5 centimeters and 15 centimeters (1 and 6 inches), depending on the type. The periodical variety are notoriously bad flyers (good thing they don't have to pass a flying test!), and they often run into things, if they can get off the ground at all. Why should they run, after all, when there are so many of them hanging out at once? They have four wings, and when they're not flying they fold their wings back along the sides of their body. The glassy, transparent, longer forewing covers the shorter, opaque hind wing. A network of sturdy veins strengthens the two pairs of wings.

Cicadas have three pairs of legs, all about the same length. Consequently, they aren't adept at jumping, though they do try. Large, compound eyes situated on each side of their head give them wide peripheral vision. Three tiny eyes on the top of the head (called ocelli) allow them to watch for predators from above. Small, bristle-like antennae are located just behind the ocelli.

The cicada's mouthparts are enclosed in a long, thin, beak-like sheath. The sheath, called the labium, is retracted between the legs when the insect is not feeding. The labium contains four needle-like stylets used for feeding. Cicadas feed by piercing the surface of plants with their stylets. They use them like a straw to suck up the sap from plants.

Nice Pipes, Cicadas!

Cicada with wings spread
Cicada with wings spread
Photo courtesy National Park Service

The cicada's claim to fame (in case you literally haven't heard it) is its singing. The high-pitched song is actually a mating call belted out by males. Each species has its own distinctive song that only attracts females of its own kind. This allows several different species to coexist.

Cicadas are the only insects capable of producing such a unique and loud sound. Some larger species can produce a call in excess of 120 decibels at close range. This is approaching the pain threshold of the human ear! Smaller species sing in such a high pitch that it can't be heard by humans, but it may cause dogs and other animals to howl in pain.

The apparatus used by cicadas for singing is complex. The organs that produce sound are called tymbals. This pair of ribbed membranes sits at the base of the abdomen. The cicada sings by contracting the internal tymbal muscles. This causes the membranes to buckle inward, producing a distinct sound. When these muscles relax, the tymbals pop back to their original position. Scientists still don't fully understand how this apparatus produces such extreme volume.

Cicadas usually sing during the heat of the day. In addition to attracting a mate, the loud noise actually repels birds. The cicada's song is painful to the birds' ears and interferes with their communication, making it difficult for the birds to hunt in groups. Male cicadas in the same brood will stick together when calling in order to increase the total volume of noise. This reduces the chances of bird predation for the whole brood.

Even cicadas must protect themselves from the volume of their own singing. Both male and female cicadas have a pair of large, mirror-like membranes called the tympana, which function as ears. The tympana are connected to an auditory organ by a short tendon. When a male sings, the tendon retracts, creasing the tympana so that it won't be damaged by the sound.

Love Song -- for an Insect

After succumbing to the romantic ballads, the cicadas mate. Afterward, adult female cicadas lay eggs by piercing plant stems with their ovipositor. The ovipositor is an egg-laying spike located at the tip of the female's abdomen. The spike inserts the eggs into the slit created in the stem.

The eggs eventually hatch into small, wingless cicadas known as nymphs. The nymphs fall to the ground and dig below the surface. If they're periodical cicadas, they will stay for 13 or 17 years, slowly growing into adults. The nymphs live on the sap from plant roots while they grow. They shed their skin at intervals throughout their life span.

When the nymphs reach full size, they dig their way to the surface with specially adapted front legs that act as tiny shovels. They surface around nightfall in late spring or early summer. The nymphs then climb to higher ground and shed their skin for the last time. Now fully winged adult cicadas, they leave behind their old, empty, nymphal skin.

Showtime

Bush cicada at the University of Nebraska State Museum.
Bush cicada at the University of Nebraska State Museum.
Joel Sartore/Getty Images

Before Brood II, the most recent periodical cicada to emerge, Brood I, was seen in 2012 in the relatively limited range of West Virginia and Virginia.

Brood II is occurring along the Eastern seaboard from North Carolina to New York in spring 2013. (See Radiolab's Cicada Tracker to check out a map of the sightings.) There will be many, many cicadas per infested acre. So if you are in these areas, you won't miss them.

Even though cicadas may land on you or bump into you, you don't have to worry about getting bitten or stung. Cicadas are harmless. They may cause some slowing of the growth of trees from the amount of sap the cicadas consume, but they won't cause permanent damage. The worst they will do is annoy you with their incessant singing.

The life span of an adult cicada is short, as in a few weeks short. As mysteriously as they arrive, they will disappear. Most will be eaten by birds and other predators. Even the nymphs are not safe below the ground, as they're often preyed upon by beetle larvae and other ground-dwelling parasites.

If you live to be 75 years old, you will only have about four opportunities in your life to hear the song of the Brood II cicadas. If you live in an infested area, you won't be able to miss it. But if you don't, it might be worth your time to go out of your way to hear one of nature's most powerful and elusive performers.

Sources

  • Encyclopædia Britannica. "cicada." Encyclopædia Britannica Online Academic Edition. Encyclopædia Britannica Inc., 2013. (May 16, 2013) http://www.britannica.com/EBchecked/topic/117539/cicada

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