They're gross. They're loud. They're bumbling.

And, come mid-May, they're back.

With uncanny mathematical precision, and with sex on their minds, millions of red-eyed cicadas that last saw daylight in 1987 are poised just beneath the Maryland soil, raring to wriggle out, raise hell, make love and die, carpeting the ground with rotting carcasses.

"Oh, my God - the smell," said University of Maryland biologist Holly Menninger. "We're talking ankle deep in cicada carcasses" in some places.

The insects mounting this invasion are periodical cicadas, which burst forth in plaguelike proportions every 13 or 17 years, depending on the species and location.

This year, most of Maryland from Garrett County to Cecil County, excluding most of the Eastern Shore and Southern Maryland, will greet Brood X (10), the biggest of the 15 periodical clans.

"All heck is about to break loose here," said University of Maryland entomologist Mike Raupp, who is tracking the bugs from Baltimore to College Park as they tunnel toward the surface.

"They're there, happy and smiling in their holes," he said. "In every place I've been, I've found moderate to heavy populations. ... It looks like a bumper crop to me."

Brood X emerges every 17 years in nine Eastern states from Indiana to New Jersey. Often mistaken for locusts (a type of grasshopper), periodical cicadas spend all but a few weeks of their lives underground, sucking nutrients from the roots of the trees where they were born.

When the time to emerge nears, the nymphs dig pencil-size escape shafts to the surface, sometimes capped by mud piles until it's time to crawl out.

Appearing with militarylike coordination, their arrival can be dramatic: In wooded areas with undisturbed soil, 200 nymphs can squirm up from a single square yard. An acre can cough up 1 1/2 million.

Once they reach daylight, the nymphs crawl up a tree trunk, a screen door or other vertical perch and shed their skins, revealing winged adults.

Fluttering into the trees, they commence cicada karaoke. Males croon to females in a courting ritual that can drive homeowners batty and crank hand-held decibel meters to near-rock-concert peaks.

Brevisana brevis, the African cicada, is the loudest recorded insect on Earth at 106.7 decibels. But even Maryland's garden-variety Magicicada septendecim produces a racket that can overpower a rumbling lawn mower.

While scientists have only recently begun to unravel the cicada's mysteries, the insect's behavior has fascinated observers since North America was settled.

Recorded in 1633

As entomologist Gene Kritsky points out in his forthcoming book, Periodical Cicadas: The Plague and The Puzzle, the first recorded emergence was quaintly chronicled in 1633 by William Bradford, governor of Plymouth Colony:

"There was such a quantity of a great sort of flies like for bigness to wasps or bumblebees, which came out of holes in the ground and replenished all the woods, and ate the green things, and made such a constant yelling noise as made all the woods ring of them, ready to deaf the hearers. They have not, by the English been heard or seen before or since."

Commenting on the 1715 emergence of Brood X in Philadelphia, Swedish clergyman Andreas Sandel noted curiously that "people split them open and eat them." More about that later.