The cicadas appear periodically but only emerge after a prime number of years. In the case of the brood appearing around Nashville this year, 13 years. The forests have been quiet for 12 years since the last invasion of these mathematical bugs in 1998 and the locals won’t be disturbed by them again until 2024.
This choice of a 13-year cycle doesn’t seem too arbitrary. There are another two broods across north America that also have this 13-year life cycle, appearing in different regions and different years. In addition there are another 12 broods that appear every 17 years.
You could just dismiss these numbers as random. But it’s very curious that there are no cicadas with 12, 14, 15, 16 or 18-year life cycles. However look at these cicadas through the mathematician’s eyes and a pattern begins to emerge.
Because 13 and 17 are both indivisible this gives the cicadas an evolutionary advantage as primes are helpful in avoiding other animals with periodic behaviour. Suppose for example that a predator appears every six years in the forest. Then a cicada with an eight or nine-year life cycle will coincide with the predator much more often than a cicada with a seven-year prime life cycle.
These insects are tapping into the code of mathematics for their survival. The cicadas unwittingly discovered the primes using evolutionary tactics but humans have understood that these numbers not just the key to survival but are the very building blocks of the code of mathematics.