Periodical cicadas are insects that belong to the order Hemiptera (true bugs). They feed on plant fluids (xylem) both below and above ground.
Brood X (X is 10 in Roman numerals) is the largest brood of 17-year cicadas. This brood is found in three separate areas centering around Pennsylvania and northern Virginia, Indiana, and eastern Tennessee. The largest emergence of Brood X appears as adults only once every 17 years. Cicada nymphs go through 5 stages of development (instars) during 17 years underground. When they become adults, males come out first to begin mating. The females will then lay about 500 eggs. They rise for about one month and will die after mating. Eggs are laid in trees, and when they hatch about six weeks after they are laid, tiny nymphs will drop to the ground. These small nymphs will then burrow into the ground and disappear for another 17 years.

Periodical cicadas generally live underground for 17 years, however there are much smaller numbers that come out after 13 years and some that come out after 21 years. These "stragglers" are the result of the underground nymphs growing at different rates.

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