There are about 2,000 cicada species distributed throughout the tropical and temperate regions of the world; they are most numerous in Asia and Australia. There are about 180 species in North America; adults of these species range from approximately 1 to 2 in. (2.5–5 cm) in length. The periodical cicadas (Magicicada species), found in the eastern half of the continent, have the longest known life cycles of any insect. Because of their periodic appearance they are often called locusts, although they are not related to true locusts.
Their life cycle takes 17 years in northern species (the so-called 17-year locusts) and 13 years in southern species; the two types overlap in parts of the United States. The female deposits her eggs in slits that she cuts in young twigs. In about six weeks the wingless, scaly larvae, or nymphs, drop from the tree and burrow into the ground, where they remain for 13 or 17 years, feeding on juices sucked from roots. The nymphs molt periodically as they grow; finally the full-grown nymphs emerge at night, climb tree trunks and fences, and shed their last larval skin. The winged adults, which generally emerge together in large numbers, live for about one week. Different broods mature at regular intervals, so that at least one colony is conspicuous in some part of the United States each year, and even in a given locality a brood may appear every few years.
Other North American cicadas (Tibicen species and others) are known as dog-day cicadas, or harvest flies, because the adults appear in late summer. Their life cycle is thought to be similar to that of the periodical cicadas, but in most species it is completed in two years.
Cicada larvae do little damage, but when adults appear in large numbers their egg-laying may damage young trees. Cicadas are sometimes kept for their song in Asia, as they were in ancient Greece. They are classified in the phylum Arthropoda, class Insecta, order Homoptera, family Cicadidae.
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