Cicada’s 17-year life cycle

Question

Why do cicadas only come out for 17 years? Is there some biological reason that they have evolved to live this way?

Answer

In North America there are seven species of Cicada knows as the periodical cicadas. They are so called because they are only seen once in every 13 or 17 years (depending on the species). In the years when they appear above ground, the entire population of the insects will emerge from the ground within a week or two of each other, sometimes over 1.5 million of them per acre. The adults live for only a few weeks, during which they mate and lay eggs in the bark of plants and young trees. The larvae later drop and burrow into the ground, where they spend the next 13 or 17 years feeding on the vessels of plant roots, and detecting the change in seasons through variations in the sap.

Although there are several thousand species of cicada in the world, which all have similar life cycles, no others synchronise their emergence in this way – in most species, some individuals emerge every year, while the rest continue their development underground. The periodical cicada’s strategy has several advantages against predators. For one, the mass emergence ensures predator satiety – during their breeding years, there are so many that it would be impossible for all of them to be eaten by predators. Another advantage of having such a long life cycle is that none of their major predators will live long enough to become adapted or used to their presence, and the fact that their development phase is a prime number of years helps to further dysregulate them from their predators’ life cycle. If they were an annual occurrence, predatory species could adapt to take advantage of the yearly glut, as some bears take advantage of the annual salmon spawning.

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