When cicadas lay eggs, they use their proboscis to cut little slits in thin branches and lay eggs in there. When the larvae hatch, they simply plop down onto the ground, bury themselves, and attach to the root system of the tree, where they'll remain for another 17 years, unseen. We're not really sure how they count to 17; there is a gene that differentiates the 17-year cicadas from the 13-year cicadas, but, says Gilbert, "we don't really have any way to see what the hell they're doing down there for 17 years." Occasionally, if too many cicadas make these slits in branches, the branch can break and droop. Entomologists call this "flagging," because, cut off from the rest of the tree, the leaves on the broken branch will turn brown, making them rather obvious amidst the otherwise green leaves. And the last year of the cicadas' lives underground is a bit harder on the tree, since the cicada larvae are eating more and more tree juices from the roots to get ready for their brief adulthood. But this is very minimal damage, and the cicadas repay the trees.