Millions of periodical cicadas will emerge again from the soil this spring in 13 states across the eastern U.S., according to researchers. The 17-year Magicada cicadas come out of the soil in spring when the trees have grown their leaves.

Periodical Brood XIV cicadas are set to return to the following states sometime in May: Georgia, Kentucky, Indiana, Massachusetts, Maryland, North Carolina, New Jersey, New York, Ohio, Pennsylvania, Tennessee, Virginia and West Virginia.

The insects start to emerge when the soil beneath the ground layer reaches 64 degrees, according to researchers at the University of Connecticut.

Experts say a warm rain often is the trigger for their emergence.

These cicadas have a 17-year life cycle, so we haven't spent time with this brood since 2008.