Cicadas, Brood X Cicadas are emerging again. Oh my!

Cicadas, Brood X
Magicicada septendecim
Cicadas are emerging again. Oh my!
In North America we have a kind of cicada that only emerges from the ground every 17 years. Isn’t that
weird? Can cicadas count?
1. If cicadas only emerge in North America every 17 years and they are expected this year.
a. What was the year of their last emergence?
b. What year will they emerge next after this year?
c. How old will you be when the Magicada emerges again?
2. Find a definition for the second part of our Cicada’s name, septendecim. Write the definition here.
3. Before you read on, write down what you think is special about the number 17?
We did some research and found that emerging only every 17 years is another kind of survival technique.
Some animals and insects can blend into their surroundings. Some animals or insects have sharp spines
or hard shells so that they are dangerous or unpleasant to eat. Some insects look like something that
tastes bad. Cicadas do something different. They use the number 17 to avoid predators.
These seem to be the important facts that contribute to the survival of cicadas:
• Predators can’t depend on cicadas for food in their life cycle.
• When the cicadas are finally present, there are too many to eat to impact cicada survival.
First, you need to understand what a life cycle of an insect is? An insect’s life-cycle length is the time that
it takes for the insect egg to become an adult and then for the adult to lay eggs again. So, the life cycle of
anything is the time it takes to go from being an egg to laying an egg.
Magiciada Septendecim (the North American cicada) has a life cycle of 17 years. Predators of the
magiciada cicada are snakes, birds, rodents, small mammals, lizards, and fish. But since their life cycles
don’t coincide with the cicada’s very long 17-year cycle, they need to be able to feed consistently on other
insects and animals.
5. If an insect had a life cycle of 1 year, how many generations of that insect would have to go by
before that insect species could depend on eating cicada meat?
6. If a predator’s life cycle was 2 years, how many generations would it take before it could eat
cicada?
7. If a predator’s cycle was 3 years, how many generations would it take?
8. What if the predator had a life cycle of 5 years?
9. 7 years?
10. 11 years?
You should have noticed that 17 isn’t divisible by any of these life cycles. That is none of these life
cycles coincide with the 17-year cycle. Since these life cycles don’t coincide with 17 evenly, no
predator is going to start its life cycle or live its life cycle consistently with the Cicadas. Thus it
won't be able to rely on the Cicadas as a food source. When a number is only divisible by itself
and one it is a prime number. This prime number life cycle of the Cicadas is its defense
mechanism.
11. Why didn’t I ask about life cycles of 4, 6, 8, 9, 10 12, 14, 15 or 16 years?
12. What are some other life cycles that Cicadas could have that would be a prime number life cycle?
However there are plenty of predators who will eat whatever is available, even if that isn’t its usual meal.
Those predators would naturally eat cicadas when they appeared ... like humans sometimes do ... fried,
chocolate-covered, in sushi, or whatever.
But will those opportunistic predators be able to devour so many cicadas to endanger the cicada
population’s survival? There is a species survival strategy called "predator satiation". Which means that
as millions of cicadas emerge from the ground at the same time, there numbers ensure that they won’t be
all eaten and enough will survive to mature and lay eggs in order to continue the species.
13. Write a good definition for a prime number. Give examples and non-examples. Do you think one is
a prime number?
14. Can you find any other biological or physical events that depend on prime numbers?
Brood X
Map of where cicadas are expected this year in the U.S.
Sources: http://sciencenetlinks.com/science-news/science-updates/cicada-cycles/
http://en.wikipedia.org/wiki/Cicada
http://www.baltimoresun.com/bal-te.ms.cicada10may10,0,5638148.story
http://www.cicadamania.com/where.html
http://hydrodictyon.eeb.uconn.edu/projects/cicada/databases/magicicada/magi_search.php
http://project.wnyc.org/cicadas/
http://news.nationalgeographic.com/news/2004/05/0524_040524_cicadaterminator.html
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