What might be obnoxious, however, is the smell of deepening piles of cicada carcasses, which began with nymphs that failed to emerge successfully from their shells and will grow as more of the billions of adults mate, lay their eggs in the branches, die and fall fluttering to earth.

To Williams, it is a nostalgic odor. "The smell brought back 1970 [the year of the cicadas' last emergence] to me," she says.

But to those less appreciative, she says that "when the flesh flies and the blow flies really start to work on them … that's going to knock it down quite a bit."

Her best advice is to rake up the carnage and bury it in the garden. It's good compost. The nymphs' empty shells, by the way, don't smell, and they can be returned to the soil, too.

Area cats and dogs, meanwhile, are doing their best to rid the world of cicadas.

"They're fascinated by the movement and the crunchy taste," says veterinarian Bill Benson, of the Falls Road Animal Hospital. He says his office receives 20 to 25 calls a day from people whose dogs are "eating them like crazy. They're like potato chips; it seems like one is not enough."

One caller reported his black Labrador had eaten at least 50 cicadas, then vomited and didn't eat again for several days.

But that seems to be the pattern. No fatalities and no toxic effects have been reported, only vomiting and gastric irritation. Mostly, vets agree, the cicadas are a harmless protein supplement.

If you haven't heard any cicadas in your neighborhood, it does not necessarily mean you live on the site of an abandoned toxic waste dump.

First, if you live in the northern reaches of Maryland, the cicadas may only now be tuning up. The phenomenon moves north with the hot weather.

Second, unlike ants, cicadas simply aren't everywhere, Williams says. You also may not have any cicadas if you have no trees, or if the soil in your yard was disturbed while the nymphs were living underground.

And third, areas barren of cicadas are colonized very slowly. "They don't seem to do much migrating. None of the old reports talk about them flying more than a mile and they only have every 17 years to make the jump," Williams says.

Which raises the ultimate cicada questions: Why 17 years? And why cicadas?

Nobody knows for sure why these cicadas have 17-year cycles. But among the theories is one which reasons that because 17 is a prime number – evenly divisible only by 1 and itself – Magicicada septendecim's cycle is less likely to intersect with that of a threatening predator or competitor.

For example, an 18-year cicada's cycle might intersect with predators or competitors with cycles of two, three, six or nine years, increasing the threat to the species.

Interestingly, Williams says, there are three species of cicadas in the Southern states that are virtually identical to the three 17-year species, except that their cycle is 13 years. Thirteen also is a prime number evenly divisible only by 1 and 13.

The ranges of the two sets of cicadas overlap in some areas, but the danger of competition is minimal because – thanks to the prime numbers – the two cycles coincide only every 221 years. The last time the two emerged together was in 1970. They won't meet again until 2191.

As for the cosmic question of why cicadas exist at all, Williams tells of the woman who called to ask how she could get rid of the miserable creatures, which appeared to her to serve little purpose other than to annoy people.

If their life cycle is too long for them to be a reliable food source for other creatures, what purpose do they serve in nature?

"It depends on your religious upbringing," Williams says, "whether everything in nature serves a purpose or whether they just belong to the cosmos.

"These guys accomplish what they are here to do – to preserve the species," Williams says she told the woman. "She didn't want to hear that we [humans] are a major annoyance to a lot of other species, too."