"I've got monster flies in my yard," one Arlington caller told the County Extension Office. "They're eating my children and my shrubbery," said another. "They're ugly and disgusting. What are they?" asked a third.

They are members of noisy, brood No. 6 of 17-year locusts, actually cicadas, surfacing in the Washington area after 16 1/2 boring years underground attached to the roots of trees and perennial plants.

With black, bullet-shaped bodies, gossamer wings and bright red eyes, the harmless two-inch insects have prompted thousands of calls to local officials in the last week.

"Basically what they're going to do is sing a lot, you know, humming, and then mate . . . and in six weeks they'll be dead," said Carol Bruce, Arlington's agricultural extension agent. The tiny nymphs born from eggs planted in branches by the female later fall and bore into the ground attaching themselves to tree roots, reemerging 17 years later. The insects do no lasting harm to the trees.

"People call them 17-year locusts but they're really cicadas. And locusts are really grasshoppers, the ones written about in the Bible" that swarm in clouds dozens of miles across and can sweep across a continent destroying most living plants in their path, Bruce said.

While locusts are considered pests, the many species of cicadas have been mentioned kindly for 2,000 years, as religious symbols, as pets in many lands and as ingredients in folk medicine. The Greeks ate them, apparently in their soft moulted stage.

Scientists have been numbering the cicada broods in this country since the 1800s. Brood No. 10 is the largest in the Northeast and is expected to resurface in 1987. Virginia's largest broods are No. 2, which created quite a garden stir in the Washington area in 1979, and brood No. 14, which was more noticeable in the inner suburbs here in 1974 and is not expected to be heard from again until 1991.

Brood No. 6, now in full song, is not considered a large brood.

Actually only the male cicadas sing, said Bruce. One is a congregational "song," a humming, hymn-like sound, but the male also has an irritated sound of alarm and an apparently seductive mating call.

"They're really gross, disgusting," said Michele Burbano of Arlington, whose house and shrubs were covered yesterday with buzzing cicadas and the husks of the moulted insects.