The Elements of Evolution (51-1-4) Cicadas


Reproductive strategy is only one facet of a larger life-history scheme: survival. How life is lived is paramount in propagating quality for lives of the next generation.


Cicadas are large flying insects that live in temperate and tropical climates. There are about 3,000 species, living on all continents except Antarctica.

The cicada lifestyle is predominantly subterranean. They live as nymphs for most of their lives sucking root juice. Their strong front legs are built for digging.

In their 5th and final nymphal stage, cicadas tunnel to the surface and make their way to a nearby plant where they molt and emerge as adults.

Adults mate for few weeks to few months. Males form large choruses to loudly sing to receptive females. Cicadas have species-specific songs. Some sing at up to 120 decibels: about as much racket as any insect can muster.

Once mated, females cut slits into tree barks to deposit their eggs. Newly hatched nymphs drop to the ground, burrowing to find roots.

Cicadas are individually defenseless, making them an easy meal for birds, cicada killer wasps, praying mantises, and sometimes squirrels. In Australia, bass are keen to dine on cicadas that crash-land on the water.

The en masse cicada defense mechanism is predator satiation: being so concentrated in an area – up to 350 individuals per meter – that some survive despite many being eaten alive.

Most cicadas have a life cycle of 2 to 5 years. One North American genus – Magicicada – has a number of distinct broods that go through 13- or 17-year life cycles, depending upon region. Southern Magicicada emerge every 13 years. Their northern cousins have a 17-year cycle. These are the longest insect life cycles.

The cicadas are driving the birds’ populations; they’re setting the birds on a trajectory that leads to significantly lower populations at the time of the next emergence. ~ American ethologist Walter Koenig

The deluge of dead cicadas that litter the forest floor after mating temporarily boosts tree growth. At 10% nitrogen, cicada bodies are natural fertilizer. Early stage nymphs don’t tax trees sucking roots as much as when they get older.

The forest nutrient boost does not last. Bird populations hit a cyclical low when periodical cicadas emerge.

Nymphs somehow know when they should emerge. It may be that the trees upon which they feed signal the cycle, perhaps coupled with some cicada counting.

Some cicadas occasionally emerge 4 years off-schedule. These stragglers are easy prey that do not usually survive. In the off chance that they did, it would be a way of instant speciation.