Scienceman, why is it that cicadas hatch only every 17 years and WHAT is their purpose?
Photo courtesy of Arthur D. Guilani
Well actually, there have been over 3,000 cicada species identified and some of these do hatch more frequently - for instance the “dog day” species emerges annually in mid-summer. Others generally take 13 years or, as you mentioned, 17 years. Unless a math lover, you may not realize that these are both prime numbers? It is said that the cicadas have adapted to emerge in this number of years in order to escape their predators. The logic is that, emerging once every 13 or 17 years, the cicadas are less likely to fall in sync with their predators. After all, the cicadas only get one chance to mate and once they’ve done that, they die (well the male at least, the female dies subsequent to laying her 400-600 eggs!). This notion of the significance of the prime numbers is still to be confirmed, but as the cicadas lack other more ordinary types of defense, i.e., bad taste, bright colors, spikes, etc., it seems like a logical assumption.
As far as their purpose, I cannot be certain, but I can ascertain that as plentiful as they are, when they emerge, they’re certainly a good source of nutrition. Be it while they’re alive or dead, the cicadas offer abundant nutrition to the surrounding ecosystem, as due to their great abundance, as many as 85% are left uneaten by predators and have been identified as a great fertilizer for plants. Ecosystems thrive subsequent to cicada emergences. In some societies, cicadas are even a delicacy, favored for their high protein and low fat contents. Lastly, this is obviously their greatest means of success – their existence en masse. While the solo cicada may be a bit clumsy and prone to predators, with so many emerging at once, it is inevitable that some will be successful at mating! For further information on the lives of cicadas (which I think you’ll find quite fascinating!) see Cicada Facts: Understand the Invasion.