What are cicadas, and why do periodical cicadas emerge so seldom?
Cicadas are insects in the order Homoptera, and they are related to planthoppers and aphids. They have piercing-sucking mouthparts, which they use to pierce plants and suck out the plant juices. Immature cicadas live underground for years, where they feed on plant roots. Adults live aboveground, typically for about a month, where they mate and lay eggs.
There are two main types of cicadas, dog-day cicadas and periodical cicadas. The dog-day cicadas live underground for varying numbers of years, but some adults emerge each year. Mass emergences of periodical cicadas in odd-numbered years are apparently a means to avoid being eaten by predators such as birds, thus assuring survival of the cicada species.