Why do 17-year cicadas come out more often than every 17 years?

The sudden appearance of 17-year cicadas is one of nature's most spectacular events, a phenomenon that perhaps led to the fanciful naming of the periodical cicadas' genus classification: Magicicada.

For centuries, periodical cicadas have fascinated scientists and casual observers. "Their incredible ability to merge by the millions ... is without parallel in the animal kingdom," wrote scientists Richard Alexander and Thomas Moore in 1962 [source: Penn State University].

Millions of cicadas may emerge in a single night after spending 17 or 13 years living 2 to 24 inches (5 to 6 centimeters) underground. It's a lengthy childhood that serves as an important distinction that enables them to have the longest life span of any insect in North America.

Seventeen-year cicadas spend the earliest months of their final year burrowing toward the soil's surface. By April, the cicadas have entered a nymph stage and rest slightly below ground level. When the soil temperature rises above 64 degrees Fahrenheit (17.78 degrees Celsius), the cicada nymphs venture out of the soil through half-inch (1.2-centimeter) diameter holes. Their population may number a million or more in a space smaller than 1 acre (0.4 hectares), which helps survival rates. Animals ranging from spiders and birds to dogs and snakes find the lumbering cicadas to be easy prey, but will reach their fill long before they've put a dent in the population [source: University of Michigan Museum of Zoology].

The then flightless young cicadas journey upward at sunset, crawling a foot or more up any nearby vertical surface -- tree trunks, weeds, woody shrubs, homes or outbuildings -- to begin the next phase of life.

Overnight, they will shed their nymphal skins in 60 minutes or less. Much like the fictional Incredible Hulk, the cicadas will split their shell-like clothing to unleash larger bodies. And, as they emerge through the tops of their shells, the cicadas will unfurl their wings for the first time.

Now the cicada is an adult -- soft, ashen and vulnerable -- waiting for its exoskeleton to harden and darken in color. It will leave its crunchy larval casing behind. Still clinging to the cicada's first aboveground resting place, the casing will be an empty and forgotten outline of the creature it once held [source: Penn State University].

Not all cicadas appear on the strict schedule. There are also some cicadas, known as stragglers that appear a year later or earlier than the rest of their brood. One-year early or one-year late is the most common time-frame for stragglers, but this isn't always the case. In 2000, for example, many cicadas appeared four years earlier than their brood X schedule. Some observers theorize unseasonably warm weather may cause some stragglers to emerge early; others suggest delayed development may cause some stragglers to arrive late.

Scientists are not exactly sure how cicadas know 17 years or 13 years has passed in the first place. They think it might have something to do with some property in xylem, the fluid they suck out of tree roots for nourishment when they are underground, that reflects the yearly cycle of the tree. Perhaps if the xylem signal is weaker for insects under one type of tree versus another, that will exacerbate the off-schedule appearance. Interestingly, the prime number appearances are thought to help with survival; since the cicadas are not on the same two-year or five-year cycle as their predators, the predators don't become too dependent on them for food (though they do enjoy eating them to a point when they are around) [source: Hayes]. Unfortunately, stragglers who miss the big cicada party more likely to be eaten off as their quantities are smaller.

Even if scientists don't know why stragglers exist, most agree on one thing: Stragglers make it difficult to create accurate emergence maps. Historical accounts of stragglers, because they can range from a single periodical cicada to just a few dozen -- instead of thousands -- are nearly non-existent. To date, scientists and other observers have not been able to pinpoint geographic patterns of stragglers [sources: University of Michigan Museum of Zoology, Magicicada, Cicada Mania].

Periodical cicadas, whether they are a 17-year or 13-year species, will live for just three or four weeks aboveground -- just long enough to mate and, in the case of female cicadas, deposit eggs. (Sad as this may seem, this mate-then-die cycle is quite common in the insect world.)

While cicadas don't excel at longevity as adults, they are quite good at getting noticed. Male cicadas attempt to attract females with a loud buzzing that seems to undulate up and down between two distinct notes.

Although each species of periodical cicada has its own song, the response of female cicadas varies little. Exactly one-third of one second after the male's song ceases, a female responds with a click of her wings -- but only if she wishes him to make further advances. The male counters with the same call, hoping to hear another wing click. If he does, he makes two particular sounds in quick succession, waits for a third wing click and then vibrates his forelegs as he gets busy with his new partner [source: Bug Music].

Within about 10 days, the female will deposit some 400 eggs in small twigs and branches of nearby trees and shrubs, choosing from among the 80 different species they prefer to use as a nursery. The females have protruding, needle-like devices at the end of their abdomens, which are used to create an egg cradle in the wood. Female cicadas may repeat the egg-laying process dozens of times [source: Ohio State University].

In about six weeks, white nymphs the size of ants will emerge from the eggs, drop to the ground and work their way into the soil. Like their parents, they'll spend the next 17 or 13 years feeding on fluid found in plant roots.

Although cicadas' root-feeding is considered harmless, damage caused by female cicadas burrowing into trees and shrubs to deposit eggs can be significant. Twigs may break off, which can create production issues for fruit and nut orchards. Some caretakers cover small trees with mesh cloth to prevent access to bark; others apply insecticides before egg-laying occurs or simply opt to wait out the cicadas' limited life span [source: Penn State University]. Not only are these insects generally harmless to plants, they are harmless to people, so no need to panic if one lands on you.