h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Incidentally, I used to collect the moulted skins of cicada larvae when I was a kid. I have no idea what I was thinking.

On a completely unrelated topic, my question was: "Why do cicadas have to be in a sparsely populated brood before they can spread out geographically?" I would have thought that spreading out geographically would be the logical response to being in a DENSELY populated brood. It would reduce the local population density.

Hi Q*bert

Where in the article does it say that "cicadas have to be in a sparsely populated brood before they can spread out geographically"?

*Furthermore, a proportion of any 17 year brood6 can, seemingly,
spontaneously accelerate their development and emerge after 13 years7. If the new 13 year brood survives, they will be separated from the
'sibling' brood by a four year gap and can, over time, gradually move away geographically. They may maintain the accelerated cycle or they
may return to a 17 year period. Either way, there will clearly be no further opportunity for the two broods to interbreed for at least a couple
of hundred years, by which time the geographical locations of the broods may no longer overlap.*
*This tends to happen in years when there is a particularly high density of nymphs below ground*

Actually I feel a little stupid now, because I can see that I misunderstood the reference. (And it's in the forums for posterity!) But I still don't understand why the thirteen year brood separates from the seventeen year brood. Does it matter whether their territories overlap? And don't similar factors affect both broods when it comes to where they migrate?

Hi Q*bert

Don't feel stupid. We all make mistakes. It's antisocial not to make the odd mistake. People who never err cause other people to feel insecure.

To answer your question: I'd speculate that there are any number of reasons that broods might drift geographically. Suppose that in one year when a brood emerges, there's a gale blowing. That could shift them quite a distance presumably. Fire, predators, farming - all sorts of things might happen some years but not other years. Given the length of their life cycles, you'd have to study them for a long time to be able to say with any certainty what the driving forces might be. The fact is that they *do* drift apart geographically. It's not a question of whether they *should* or whether it matters if their territories overlap. "You can't make an IS out of an OUGHT." It's simply what happens - and it just so happens that there's an advantage to the cicadas in this drift. They've evolved their 13 and 17 year life-cycles over who-knows-how-many millions of years and the chance geographical drift is probably just one of the factors that this life cycle strategy was built upon. The way evolution occurs involves an enormous amount of chance.

By the way, Micheal Mole is also a connoisseur of moulted cicada skins.