No sting, no bite, no venom, no shell worth the name, no useful speed. The Connecticut lab that has spent decades mapping these insects puts it with a straight face: a periodical cicada can hurt you only if it mistakes you for a tree branch. Birds, spiders, wasps, snakes, beetles and dogs all eat them. The strategy works anyway — not for the individual, but for the insect.
The arithmetic
In 1956 Henry Dybas and Dwight Davis censused an emergence at Raccoon Grove, Illinois: over three hundred nymphs to the square yard, roughly 1.5 million per acre. That is the ceiling. Tens to hundreds of thousands per acre is more usual, and still far outside what any other cicada manages.
Kathy Williams and colleagues watched a 13-year emergence from the first night. The earliest out are slaughtered, a large share eaten within days. Then the ground opens, and within about a week the supply so far exceeds what every bird in the county can swallow that the odds against any particular cicada collapse toward zero.
The other half is quieter and does more work: nothing can specialise in them. A predator built around cicadas would starve for the twelve or sixteen years between meals.
The individual was allowed to get worse
Periodical cicadas are not merely undefended. Set beside ordinary annual cicadas they are worse at surviving — slower, less flighty, easier to catch by hand.
That is the fingerprint of a defence that lives outside the animal. Once the crowd does the surviving, vigilance stops paying for itself, and vigilance is expensive. The insect has been permitted to get worse at being one.
Which makes being early fatal
A cicada that emerges a year off schedule does not get a smaller share of the protection. It gets none.
Off-cycle individuals are called stragglers, and researchers are blunt about the usual outcome: most are few in number and quickly annihilated. Not because a straggler is defective — its body is fine, its count was merely wrong — but because arriving at the wrong time is identical, from a bird’s point of view, to arriving alone.
So the loop closes. Synchrony builds the defence; the defence destroys whatever departs from synchrony; what is left is synchrony. Nothing inside the insect enforces the schedule. Being out of step is simply not survivable — a harder kind of rule.
The floor is the real subject
A defence made of numbers has a floor, and below the floor it does not become a weak defence. It stops existing.
That is an Allee effect in the strict sense — under some critical density, growth turns negative and the decline feeds itself. In 2009 Yumi Tanaka and Jin Yoshimura put exactly such a threshold into a model of these insects, and it changed which life cycles survived.
It also makes new broods nearly impossible to start — though fifteen are on the map, so it does happen. The only route is for enough nymphs to miscount in the same direction, in the same wood, in the same year: a quorum, not a majority, clearing the floor at the first attempt. It has come close in public: choruses rose four years early along Salt Creek near Chicago in 1969, and again in 2003.
A trait that only works above a threshold cannot be approached gradually. It has to arrive finished.
About the prime numbers
You have heard that 13 and 17 are prime in order to dodge a predator on a shorter cycle. Notice what that story needs: a predator with a periodical life cycle of its own. Nobody has ever found one.
The threshold argument needs no such animal. A prime cycle rarely coincides with a neighbouring one; coincidence means hybridising, and hybridising splits a generation across schedules — diluting it below the floor. That is what Tanaka and Yoshimura’s model turns on. The specialists add a deflation worth keeping: between 1 and 20 the primes are unusually dense, so a prime cycle may not need explaining at all.
It is a counter, not a clock
The count itself is the strangest part, and one experiment settles it. Richard Karban attached 15-year-old nymphs to potted peach trees and forced the trees through two full seasonal cycles in a single calendar year. The eleven that survived emerged after sixteen years, not seventeen. They were not measuring elapsed time or accumulated warmth. They were counting their tree’s years, and had been handed a spare one.
What they count with is still unknown; Karban’s suspicion is the surge of sugars and proteins moving through a tree’s roots when it flowers. The counting is established. The sensor is not.
What this is not
No. 8 was fireflies, where synchrony is the output of oscillators pulling each other into step. Here synchrony is the armour, and underground there is no coupling between cicadas at all: each counts alone for seventeen years, and they arrive together only because they are counting the same thing.
Nor is it No. 70’s tardigrade: nothing here is suspended. The nymph is awake throughout, feeding and moulting — not waiting out the years but spending them.
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One loop I’m watching
Next: a forest that starves its own seed-eaters for years and then buries them, in step across a whole country, with no signal passing between the trees. Next time.