oots from which they gently suck the juices. Nourished by this root sap, they begin to grow. They shed their skin four times before they reach adult size.Once matured, a cicada does not necessarily leave its underground nursery. All cicadas of the same generation in a region wait for a seventeenth spring before they come creeping forth from the ground as a group. The eeriness of this group effort has puzzled humans for centuries. People have responded to the mystery with a host of superstitions, educated guesses, and scientific theories.One of the earliest explanations for the mass appearance of cicada populations after their long absence in an area was that the insects had come to foretell war. This idea stems from an observation of the adult cicada shortly after it appears above ground. It immediately sheds its skin for the last time and begins to darken in color. Near the outer edge of its front wings, a black mark appears that looks distinctly like the letter W. Some thought this W stood for "war." In the past, people who saw a group of cicadas emerge from the ground like an invading army were filled with panic. The sight was especially frightening because literally millions of insects can appear within an area of a few square miles.Later explanations for the mass appearance of cicadas stem from more scientific observations. Dr. L. L. Pechuman, a professor at the New York State College of Agriculture and Life Sciences at Cornell University, has suggested that coming above ground only once every seventeen years is an excellent way for a species to discourage its natural enemies. Perhaps the cicadas have evolved a special kind of biological time clock to protect them from predators. James Heath, an insect physiologist at the University of Illinois, theorizes that the cicadas all emerge at around the same time in a certain year because the soil has reached a temperature of 64 degrees. Theories like this have still not been proved absol...
