Why stay out of sight for so long?
How do they count to 17? Why not 16 or 18?
Scientists have theories that purport to answer these questions, but the winged creatures remain somewhat mysterious. And, yes, loud.
"There's nothing else like that amazing sound," said Jason Weintraub, manager of the entomology collection at the Academy of Natural Sciences of Drexel University.
For those who are less appreciative, fear not. Based on where the bugs' parents were sighted in 1996, experts predict that not many of the next generation will be emerging this month in the Philadelphia area.
The academy has specimens that were collected in a Franklinville, Gloucester County, cemetery 17 years ago, so some are expected there this year. Pomona, in Atlantic County, is another good bet.
And in the southern foothills of the Appalachian Mountains - in portions of Lebanon, Lehigh, and Berks Counties, near I-81 - the trees should be positively humming with the critter formally called Magicicada, Weintraub said.
"The interstate will be littered with its corpses," he said.
The bugs start to emerge when the soil temperature reaches 64 degrees. Adults live above ground for just a few weeks, but they don't all come out at once, so the noise - a mating call from the males - could persist into June.
The official name for this group sounds like something from a bad science-fiction movie: Brood II. It consists of three closely related species of cicadas.
There are 11 other broods of 17-year cicadas that emerge in various years, according to the website www.magicicada.org, maintained by University of Connecticut researcher John Cooley. To further befuddle, science also has documented four broods of 13-year cicadas, though one is extinct.
None of these is to be confused with annual cicadas, a related species that comes out in late summer.
The periodical cicadas' long underground stays are presumed to be an attempt to avoid predators. Not only would 13 or 17 years be long stretches of time for a predator to wait, but those also are prime numbers.
Imagine a parasitic wasp that lays its eggs in the bodies of other insects, with perhaps a two- or three-year life cycle. Neither two nor three divides evenly into 13 or 17. So periodical cicadas are not a reliable option, as the wasps cannot hope to line up with them every other cycle, or even every third or fourth. The wasps will pick other hosts instead.
While underground, the developing cicadas - "nymphs" - feed on the fluid from tree roots. Once they emerge, females cut slits in the branches of trees to lay their eggs. This is generally not harmful to mature trees, though people sometimes cover smaller woody plants with cheesecloth to be safe.
Exactly how the bugs count to 13 or 17 remains unknown, but Richard Karban, a former University of Pennsylvania entomologist now at the University of California, Davis, reported in 2000 that it had something to do with tree-growth cycles.
He and colleagues dug up a group of cicada nymphs that had completed 15 years of their 17-year cycle, then transplanted them beneath two sets of peach trees. One set was allowed to grow normally, while the other's life cycle was artificially accelerated by controlling heat and light in a special growth chamber, so that the plants underwent two winters and summers over the course of 12 months.
Sure enough, most of the surviving cicadas in that group came out after 16 years instead of 17.
When they first emerge, they are whitish in color, said entomologist Jon Gelhaus, a colleague of Weintraub's at the Academy of Natural Sciences. As adults, they have colors worthy of Halloween: black bodies, orange-veined wings, and red eyes.
As with the insects' sound, opinions of their appearance depend on who is doing the opining. Gelhaus, for one, is a fan.
"They're really quite nice," he said. "They're really bizarre, beautiful things."
Contact Tom Avril
at 215-854-2430 or email@example.com.