Newly-hatched cicadas climb or drop down to the ground from the branch their egg was in. They burrow into the ground and start feeding on the plant juices in roots. As they grow they shed their exoskeleton several times. They sometimes spend many years in the soil before they are finished growing. For some species it takes 13 years, others take 17, and some take less, but nobody knows exactly how long.
When they are mature, they climb out of the ground and complete one final molt. They emerge as an adult cicada with wings, and fly away to find a mate. Once they become an adult they stop growing and do not molt again.
Some of the species that take 13 or 17 years all come out of the ground at one time. There can be millions of adult cicadas flying around at that time. How long do they live?
Some species live for 13 or 17 years and all emerge at once. Some come out every year, but nobody knows how long they spend in the ground. Probably at least 3 years.
录自：http://en.wikipedia.org/wiki/Cicada
Most cicadas go through a life cycle that lasts from two to five years. Some species have much longer life cycles, such as the North American genus, Magicicada, which has a number of distinct "broods" that go through either a 17-year or, in some parts of the world, a 13-year life cycle.
