How Cicadas Work

Introduction to How Cicadas Work

They will crawl out from underground, where they have been hiding in the darkness for almost two decades.They will invade the midwestern United States, including the Chicago metropolitan area.They will fill forests and the skies as they swarm.Thousands of them will cry out together day and night.They are Brood XIII. Should you be frightened? Should you try to prepare yourself?

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Bush cicada at the University of Nebraska State Museum.
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Not really. The only preparation you may need to greet the cicadas of Brood XIII 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. In May 2007, the cicadas are coming to perform. In this article, we'll learn more about these mysterious insects.

Know 'em When You See 'em

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 mouth-parts. There are over two-hundred species of cicada in the family called Cicadidae.

Photo courtesy EPA Dave Davis, Cicadas

The average wingspan of a cicada is between 2.5 cm and 15 cm, depending on their species. They are notoriously bad flyers, and they often run into things if they can get off the ground at all. Cicadas have four wings, and when they are not flying they fold their wings back along the sides of their body. The glassy, transparent, longer fore wing 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 mouth parts 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

The cicada's claim to fame 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 co-exist.

Photo courtesy National Park Service Cicada with wings spread

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 cannot be heard by humans, but 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. Tymbals are a pair of ribbed membranes 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

After succumbing to the romantic ballads, the cicadas mate. Afterwards, adult female cicada 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 eventually fall to the ground and dig below the surface. Here they stay for 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 the 17-year process.

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 their old, empty, nymphal skin behind.

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