Some stuff about Cicadas
Clearly, the most incredible thing about Cicadas is their life cycle.
Their life cycle can last longer than most elementary students have been alive. 17 years underground as a seemingly useless (of course, that's not what BUGMAN believes!) larva, feeding on root hair juice is pretty extreme. There are some Cicadas that are on 3, 5 or 13 year life cycles, but they have overlapping generations. In other words, there are some individuals there each year, but one Cicada's babies won't be adults for 3, 5 or 13 years, depending on the species.
The 17 year Cicadas have a synchronous life cycle. In any particular area which holds a population, they all emerge within about 1 week of one another. They are everywhere. They make a lot of noise. They mate, lay eggs in tender, young tree branches and die. In a couple of weeks, the babies hatch out and climb down the tree trunk and into the ground. Nothing happens for another 17 years - nothing that we can see. Actually, they can molt up to 30 times during this period, and will "migrate" deeper in the ground in the winter and shallower in the summer. They live there nearly their whole lives. They probably think a lot happens!
Perhaps the most fascinating thing about this is that there are up to 4 different species in the synchronized group! That's pretty cool. It is only a little hard to try to tell a story about the evolution (or, development through time) of this life cycle in a single species. Consider the possible factors that could lead to it. A random mutation on the gene that produces molting hormones that confers some small advantage to that species - not competing with the others all the time, for example. But to cause the synchronization of three or four different species, seems another matter altogether.
I used to think the most likely cause was some sort of climatic condition - such as, perhaps, a glacier (of course, that would be more than 17 years), or flood (which would probably kill the underground young.) It turns out that all of these life cycles (3, 5, 13, 17 years) fall in prime numbers. That is a clue to the current belief on how such an unusual lifestyle developed. The other big clue is the observation that a Cicada is a big, juicy bag of protein that flies slowly and makes a lot of noise!
It seems to be a predator avoidance strategy! Many predators of Cicadas, such as birds, some mice-like mammals and other insects, have breeding cycles shorter than three years. So, if you emerge every year, the predators can track your emergence time and focus their resource demand to maximize your death. But if your life cycle is longer than their breeding cycle, your offspring will not get unreasonable pressure. Of course, some of the longer-lived predators could develop a pattern of population boons when they know there will be a lot of Cicadas around, and that's where the prime numbers come in.
There is no regular interval that adds up to 17 years, shorter than 17 years. So, unless the predatory species can wait that long to breed, they cannot combine life cycles to take advantage of this huge food crop. If it was 16 years, then a 2 year (every 8 years), or 4 year (every 4 years) life cycle could "track" this resource, and might even evolve to exploit it solely.
Some Stuff for Teachers
Letting your students think through hard problems before "giving" the answer is often most effective. Get 'em so they really want to know!
It is an interesting challenge to get your students, perhaps in small groups, to come up with a reasonable explanation of how this cycle develops. (Or a short essay from the Cicada's point of view might be entertaining as well - especially if you focus on the molting parts!) You can use their solutions to generate a discussion which would include evolution (or, change through time, since "evolution" has become a pc-charged issue in some areas), predator prey interaction (ecology), and math.
You could turn the question around and challenge the class with an irresistible predator which could track them through their life cycle. Give them enough info so they can make reasonable guesses at a solution. If the problem is intractable, you will lose them. It is possible - though perhaps not likely - that someone will think of prime numbers. They will surely come up with some interesting answers, and remember the value of prime numbers after this lesson!
You might start the lesson by developing a timeline of their life cycles. Offer extra credit to anyone who brings in 5 or 10 photos of themselves at different ages. Lay all the photos out in a chronological line and get the class thinking about ALL the things that have happened in their lives. Give them time to share and laugh about the photos.
You could even prepare some local, national, or world events to super-impose on the timeline. Then, perhaps even "life-size" images of baby Cicadas - very small at first and growing throughout - would make the point well. You might be able to acquire some actual specimens from a local university or science museum. Or maybe you or your students might find the empty shells (or, exuvia) on tree trunks in the late spring.
This timeline will certainly personalize the lesson for the students, and it might even help them develop a new respect for the wonders of the insect world!
1998 - Mark Berman, BUGMAN Entoprises