If the offspring of 11-year and six-year cicadas are cicadas of a third cycle length - say, nine years - then the number of insects emerging on either parent's schedule in the next generation will decrease. That might affect the length of the life cycle. If the precise timing of a cicada's life cycle is produced by the interaction of several genes, then getting those genes from two different populations might change the interaction. Their offspring will be hybrids, carrying a mixture of genes. When broods with different cycle lengths emerge at the same time, some members will interbreed. It turns out that if an area contains populations of cicadas with different life-cycle lengths, broods with long cycles that are high prime numbers will share summers less often with other broods. Both of those are large prime numbers, which means they can be divided only by themselves and 1.īut why 13 or 17 years? Life cycles that long are mathematically more likely to have survived the Pleistocene era than shorter ones, but that doesn't explain the benefit of a prime number. They have also settled on specific life-cycle lengths, either 13 or 17 years. They have lengthened their life cycles and evolved into geographically distinct "broods" in which all members are on the same developmental schedule. The periodical cicadas about to emerge here once shared a common ancestor with these insects. On the other hand, many small species survive by being so well-camouflaged that "you can't see them even when they're a foot from your head," he said. For example, the large dog-day cicadas of the Deep South "are very fast, powerful flyers - that's how they get away from the birds," said David Marshall, an evolutionary biologist at the University of Connecticut. Unlike Magicicada, these other species survive by strategies more clever than simply waiting for predators to get sated. A fraction of the population reaches maturity and emerges every year, making the insects annual, not periodical. That's because in most places a person hears cicadas - often more than one species - every summer. Of course, this isn't apparent to the casual observer. If five years is the dominant length, for example, many members of a population may come out in four years - or not until the sixth. Most species of cicada have life cycles between two and eight years, with a fair amount of variability. If this occurs before they find a mate and create a new generation to carry their genes forward, they will eventually disappear completely. If too few cicadas emerge, or if they come out over an extended period of time, they are likely to be wiped out by predators. This is the survival strategy known as "predator satiation." It is a passive strategy that depends almost entirely on timing. Squirrels, dogs, cats, turtles, fish and spiders all eat cicadas, which for a few weeks are the protein equivalent of manna from heaven.Įventually, though, everyone gets full - and there are still billions of cicadas alive. Birds consume them in the greatest numbers, but many other animals get in on the act. In the parlance of animal behavior, cicadas are "predator foolhardy" - they are always available for lunch. They fly poorly, don't fight and taste great. Their few weeks of life in the open air are spent molting, calling for a mate (in the case of the buzzing males), copulating and depositing eggs in nests made in gashed twigs (in the case of the diligent females). These large, ungainly insects in the genus Magicicada spend either 13 or 17 years underground, then emerge nearly simultaneously in densities that can exceed 1 million per acre. Few strategies, however, are as strange and unlikely as the one periodical cicadas found.
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