This summer's emergence of trillions of periodical cicadas may be over, but the science arising from Brood X's dessicated carcasses has just begun.

Brood X, whose patchwork home turf runs from the Chesapeake Bay to the edge of the Great Plains, is the world's largest simultaneously emerging population of Magicicada. Its appearance after 17 years underground, while not quite a once-in-a-career event, was nevertheless an opportunity that biologists were not likely to overlook.

In many ways, periodical cicadas are as much a mystery as they are an oddity. Although they appear in huge numbers -- 100,000 to 1 million per acre -- they stay only a month and do not return for 13 or 17 years. How important a role they play in the hardwood forest communities where they live their Rip Van Winkle lives is quite unclear.

But that does not mean they do not leave ample evidence of their infestation. Washington area trees are festooned with dead branches, thanks to female cicadas that slit the bark of twigs to create nests for their eggs. Those "oviposition scars" sometimes kill branch tips, which dry out and often break off.

This flagging is the obvious damage from cicadas. But what happens out of sight is almost certainly more important.

Cicada eggs that are about to hatch produce a rain of nymphs that fall to the ground, burrow in and attach to the fine roots of trees. There they consume a thin mixture of water, minerals, and simple carbon- and nitrogen-containing compounds that the tree pulls up from the ground for its own sustenance. This fluid is the sole nutrient that takes the nymphs from their barely visible hatchling size to the clumsy, noisy, 11/2-inch-long adults that emerge 17 years later.

Three different research groups -- two in Indiana, one in Washington -- are engaged in ambitious projects to learn the effects of this below-ground parasitism.

Keith Clay, a biologist at Indiana University in Bloomington, leads the biggest of the three teams. With the help of 25 assistants, he deployed 250,000 square feet of netting over thousands of trees during Brood X's emergence.

Growing mostly on abandoned farmland, the trees are all less than 17 years old. No cicadas emerged from the ground under them to fly up into their branches and lay eggs. The netting was designed to keep them as cicada-free as possible by keeping off insects that flew in from elsewhere.

Clay and his colleagues have about 20 areas under study, each one subdivided into 10 covered experimental plots and 10 uncovered control ones. The team battled heat, chiggers and poison ivy as it draped the netting over 15-foot-tall trees.

The netting is now off. The researchers are counting oviposition scars on the netted and unnetted trees to determine how different they are in terms of cicada infestation. Then, Clay said, "it's just sit back and watch what happens."

Actually, not quite.

Over the next several years, the team will record tree growth as evidenced by height, trunk girth and stem diameter. They also hope to shed light on which species of trees cicadas prefer. In Indiana, maples "are head and shoulders above the others" in the amount of obvious cicada damage, he said, while species such as walnut and sassafras have been largely spared.

At Indiana State University in Terre Haute, biogeographer James H. Speer is trying to produce a time-lapse picture of cicada effects by measuring growth rings in sample cores taken from trees inside and outside Brood X's territory.

Two years ago, Speer conducted a pilot study on red maples in a state park after the emergence of 13-year cicadas called Brood XXIII. He found a dramatic pattern.

Growth rings became steadily narrower in the five years before the cicadas appeared. In the year of the emergence, however, the rings were twice as wide as the year before, revealing a burst of growth as the trees were temporarily released from parasitism.

In the first two years of the new cycle, however, the growth rings shrank again, although they did not get as narrow as they had been at the end of the cycle. Then they widened for several years, suggesting another growth release. Speer speculates that it may be the result of a massive, underground die-off of developing nymphs, 95 percent of which never make it through the entire 13- or 17-year cycle.

The differences could not be explained by extremes of temperature or moisture, the two best-known factors influencing tree growth.

"What we really need to do is expand our study to more species and more sites," said Speer, who with Clay shares a $400,000 research grant from the National Science Foundation. When the growing season is over this fall, a graduate student in Speer's lab will extract cores of at least five different tree species from 20 places to see if the earlier observations hold up.

Trees are not the only organisms that cicadas may affect. In a complicated experiment, George Washington University biologist John T. Lill will attempt to learn whether the subterranean action of cicada nymphs affects the above-ground activity of caterpillars and other leaf eaters.

Lill has 56 trees under study at the Patuxent National Wildlife Refuge in Laurel. In May, an average of 12 cicadas per square meter emerged under them -- a relatively small number. He is now in the process of moving twigs containing soon-to-be-hatched cicada eggs onto the ground under a sample of those trees. This will enrich their roots with cicada nymphs tenfold to 100-fold, greatly increasing the future strain on the trees.

Over the next few years, Lill will compare the amount of insect damage on the enriched and control trees throughout the growing season. He is testing a number of hypotheses.

One is that the above-ground herbivores will shun the enriched trees as the leaves decline in quality and nutritional value. The other is that enriched trees will produce unusually large amounts of defensive chemicals called phenolics that slow the growth and appetites of the leaf-munching pests.

He will even attempt to determine whether the leaf eaters are more or less at risk of being eaten by their predators, such as wasps, depending on which trees they inhabit. If so, this would suggest that out-of-sight cicadas have more profound effects on the food web than previously realized.

"It's a long-term project. I'll be potentially monitoring damage for the next 17 years," said Lill, who will be 52 when Brood X next shows its black-and-red head.

Cicadas cover grasses surrounding a tree along a road in Bedford, Ind., in May.