There are many species of locust that have differing years before reaching maturity, most of them 'normal' in a 2 to 8 year range with some flux back and forth.
Either from these more 'normal' species or similar ancestors, the periodical cicadas have evolved: 7 distinct species -- 4 that emerge in 13 year cycles (one recently discovered) and 3 that emerge in 17 year cycles -- and it appears that each of the 17 year species is related to one of the 13 year species, thus parallel evolution repeating a pattern of increasing cycle time.
The fact that both of these long periods are prime numbers has been one of their most facinating aspects. Of course this makes it very difficult for a predator to match the periodicity, thus allowing the large swarms to emerge and overwhelm the predator base, ensuring sufficient mating occurs in the few days the adults live for the species to survive. Last thing I remember reading was that the scientists believe that the flow of sap in the roots tells the nymphs when to emerge.
see
University of Michigan Museum of Zoology, Insect Division, Cicada Webpage (click)
for a good article on each of the species involved.
My brothers and I collected bugs as kids growing up in Ann Arbor a number of 17 year cycles ago, and the 17 year cicada was in the collection.
we are limited in our ability to understand
by our ability to understand
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