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It's a sound signaling the season and the dog days of summer - the plaintive drone of the annual cicada, seldom seen but always heard, now at the height of its summer song.

The song, which has been compared to the sound of a distant buzzsaw lasting a minute at a time, is not a lot of hot air.

A fairly large insect in comparison to others common to the area - usually two to three inches in size - the males emit their powerful tune through organs called "tympana" located at the base of their abdomens. The tympana consists of a series of three membranes inside a resonating chamber.

They can be identified by their brown bodies with black markings; their wings have green veins. And they have big eyes.

As intimidating as they may seem in sound and size, they actually cause little harm to plants and trees, even though they are populous. They have two- to five-year life cycles with overlapping broods appearing every July and August. Next season, however, could start on a different note.

Massachusetts, along with 12 other states, will see the emergence of the 17-year-brood cicadas.

These cicadas, referred to as "periodical" cicadas for their synchronized emergence, are about half the size of the dog day cicadas.

But since they mature into adults in the same year, they appear on the scene in big numbers.

In fact, as many as 1.5 million of these 17-year-brood cicadas can cover an acre, though densities of tens to hundreds of thousands per acre are more common - still far bigger concentrations than most other cicada species, according to the University of Michigan Museum of Zoology insect division's Web site.

It can make for a spectacular sight, said Robert Childs, an entomologist at the University of Massachusetts at Amherst. His specialty is the identification and management of insects that attack trees and woody ornamentals in the Northeast.

And while some people get "very freaked out" over the thought of such a massive appearance of buzzing, winged creatures - oh, and these have bright red eyes - there's nothing to fear, he said. Rather, he encourages people to pack a picnic and head to the Cape next season to view the pockets of these populations, which Childs describes as "one of those rather rare phenomenons."

While a few may make their way from underground around the state, "they're almost completely defined by areas on the Cape" in recent cycles, Childs said. The main reason for that is "the phenomenal development in Massachusetts which has destroyed their habitat." Even on the Cape, there were less of the brood in 1991 than there was in the previous cycle, he said.

If the pattern holds, the periodical cicadas are predicted to emerge in isolated pockets in places like Hyannis, Barnstable, Falmouth and along the canal, Childs said. In those pockets there are likely to be thousands. "You can look at one plant and see dozens."

"In a grove of oak trees, there could be hundreds of thousands," he added. "It tends to have a negative connotation, but they're really not harmful." Childs visited the Cape several times during the previous emergence of these cicadas, which he says are harmless to humans and only pose a threat to young trees. It also presents an impressive display of sound.

"I'm driving along, the windows down and I hear this ringing," Childs says of his trip to the Cape in the summer of 1991. At first he despaired, thinking what he was hearing was the sound of bad ball bearings. But then he realized, it was the song of male cicadas.

"They produce their mating call in harmony by the thousands," he said.

The first sign of these insects is in the spring, once the snow melts and people start raking. In those areas where the nymphs have been in the ground and are in their immature stage, holes - the size you could put a thumb into - will appear. That's the emergence hole, Childs said. The insect will then go back down in the ground for another 5 to 6 weeks. In late May or early June, "they come out in force" into bushes and trees where they molt into adults for the last time.

They don't bite and are not known to carry any diseases. And there is minimal damage caused by the onslaught, he maintains.

"The immatures in the ground mostly feed on tree roots and in rare cases cause damage to a tree," Childs said. Usually, there are other contributing factors that have played into the tree's demise.

Where the harm is done, he said, is after the adult female has mated and lays eggs down branches. It ends up killing those branches or twigs. (A female can lay up to 600 eggs after making slits in the twigs of broad-leafed trees.) For older, taller trees, that means some two feet of outer branches will be killed, but it won't kill the tree.

Younger trees, those 6 feet tall or under, are more vulnerable. Fine netting is recommended coverage for those trees during egg-laying, usually occurring early to mid-June.

So, while this year people may be sitting back and listening to the buzz of hot summer nights, next year could be a chance to hear the choir.

"It's quite spectacular to witness a peak year," Childs said.