AUGUST
2011

 

By
Tom Turpin
 
Professor of
Entomology
Purdue University

 

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Check out these books by Tom Turpin:

Flies in the face of fashion

What's Buggin You Now?

 

 

 

08-25-11

Download the audio of On Six Legs: MP3, WMV.

The Scream of the Cicada


A number of species of insects produce sound. The most common insect approach for such an activity is to rub body parts together. This method is very much like dragging your fingernail along the teeth of a comb. That is how grasshoppers, katydids and crickets sing.

But one of the best-known songsters of the insect world - the cicada - uses a different approach to sound production. Related scientifically to the smaller leaf and plant hoppers, cicadas are percussionists. They make sound by vibrating a membrane.

The volume of the sound produced by cicadas either in a group or as an individual is some of the loudest in the insect world. An individual cicada sound has been measured at 120 decibels, a volume equivalent to a rock concert or a jet engine. It is no wonder that the Roman poet Virgil was moved to write relative to singing cicadas: "They burst the very shrubs with their noise."

Cicadas can be found in all temperate and tropical climates. Over 2,000 species exist. There are about 180 species of cicadas in North America. All species of cicadas feed underground in their immature form. When the immature cicada is ready to emerge as the adult form, it crawls from the ground and attaches itself to the bark of a tree, post or any other handy item. At this point it splits down the back and the adult emerges. The exoskeleton of the immature remains and is often referred to as a cicada shell.

The immature life of most cicadas varies from 2 to 5 years. In the United States, dog-day cicadas have such a life cycle are so named because they emerge during the dog days of summer. These are the cicadas that we hear singing during August and September. Some emerge each year so such species are sometimes called annual cicadas.

Another group of cicadas unique to North America is called periodical cicadas. These cicadas have the longest life cycle of any North American insect - either 13 or 17 years in the soil as the immature stage. So there is an emergence every 13 or 17 years in the specific regions of the country where periodical cicadas are found.

For instance, periodical cicadas emerged in several Eastern Seaboard states in 1996 and will show up again in 2013. An entomologist by the name of Marlatt numbered each emergence and called them broods in order to keep track of them. The group that will emerge in 2013 is labeled Brood II and is also called the East Coast brood.

Periodical cicadas emerge in May and June so they don't overlap with the annual cicada types. This means that the periodical cicadas don't have to deal with an insect known as a cicada killer. But the annual cicadas do. Cicada killers are the largest wasp in North America and get their name because they kill cicadas. The adult female wasp catches a cicada and stings it with a paralyzing chemical. The immobilized cicada is then placed in a burrow that the wasp has dug in the ground. The wasp then lays an egg on the cicada. The egg hatches and the young wasp uses the cicada as food. By the end of fall the young wasp has completed feeding and forms a pupa, the stage in which it will spend the winter.

As is the case in most insects, it is the male cicada that produces sound. Cicadas in the same area will often times raise their songs in unison, producing a cacophony of sound. The resulting chorus - noise according to some people - serves to attract females to the vicinity of the high-decibel concert. Once a female comes into view, the male chorister starts singing a different tune, more of a love song we are to presume.

But to a male cicada all is not love and song. Cicadas often become food items for other animals, including the cicada-killer wasp. When captured by a predator the cicada sometimes produces another type of sound, described by some as a high-pitched scream. The scream apparently works to frighten some predators away, and the cicada lives to sing again another day.

 

Writer: Tom Turpin
Editor: Olivia Maddox