"Deputy Doc" Guidry sat among the ducks, seeking respite from heat that seemed as oppressive in Washington as it did in his Terre Bonne Parrish, La. You had to look a little closer to see that the ducks were really exquisitely carved decoys.

But everything else at the 42nd National Folk Festival at Wolf Trap was the real thing -- including Guidry. The deputy sheriff sitting quitely in the shade of the craft tent is also a mean Cajun fiddler with Vin Bruce and the Acadians.

He is representative of the grassroots approach that the 42nd National Folk Festival (celebrating its 10th year at Wolf Trap) and its predecessors have taken. There are no revivalists here, just the genuine article.

The performers have had a wide variety of musical, craft and cultural traditions handed down through their families. They work as peach farmers or gravediggers, and live in towns with names like Sulphur, La., and Zebulon, N.C. More than 150 of these down-home traditionalists will be featured during the daytime programs today and tomorrow, and at an evening concert tonight.

The festival opened yesterday at noon, and even the heat and humidity couldn't keep away several thousand fans. They tended to find security in the many shady spots -- some of the five stages spread throughout Wolf Trap's grounds were under gospel-style tents -- and the woods provided their own shifting shade patterns. There were a great many families, and for once it seemed that no one had trouble keeping the kids on a short leash -- although some got up to snake-dance to the music. A Family Affair

At times, it seemed that the festival theme was The Musical Nuclear Family. Georgia septuagenarians Lawrence and Vaughn Eller revived the string-band era. The Joaquin Brothers (Daniel, Angelo and Fernando) played in a Pagago Indian polka band with their nephews Leonard and Jerome. At the age of 80, Thomas Burt sustained the Piedmont blues and gospel traditions with his wife, Pauline.

And Floyd and Lloyd Armstrong arrived from Little Rock, Ark. The indentical twins are easy to spot; siver-gray hair, the stout bodies of well-lived lives, identical checkered jackets worn over solid-color pants. They look like country stars, and that's what they were in the late '40s -- over the 500,000-watt radio stations just across the Mexican border that beamed their programs clear across the country and to the farthest reaches of Canada.

Border radio was the late'30s brainchild of J.R. Brinkley, the infamous "goat gland doctor," after he was booted off stateside radio for selling specious mail-order products. Much of the charm of border radio lay in the sales pitch, and a little of that old habit has stayed with the Armstrongs. Every few minutes their conversation is punctuated with variations on "We sure would like all the good folks in this area to come out and see us . . . we have a new record out next month."

In fact, an album of 35-Year-old transcripts was released two years ago by Chris Stratchwitz on Arhoolie, a small San Fransisco-based label. At the time, he didn't even know whether the Armstrongs were still alive. They were, in Little Rock, where Floyd was a carpenter and Lloyd ran a gas station and repair shop. It had been 20 years since they'd stepped out of the limelight, 30 since they'd been heard twice a day on their own program, with guests like Rose Maddox and the Maddox Brothers and T. Texas Tyler.

As performers, they went back even further. At the age of 5, in 1935, they were already doing concert harmonies. As 13-year olds, they did live radio shows early in the morning. Border radio was fun ("You could order anything from a bowl of soup to a hairpin," said Floyd).So was their brief stint as hosts of their own television show in Odessa, Tex. "Times changed when Elvis [Presley] come along," said Floyd. Said Lloyd, who, true to twin tradition, often finishes Floyd's thoughts: "We were very proud of Elvis. He was, believe it or not, on our show out of Odessa. Elvis give the girls what they wanted, I believe. He just took off."

So did the twins, in a different manner. Now they are looking for a manager, a band and a comeback. They were surprised recently to find themselves enshrined in the Country Music Association's museum. "They had all our records and transcripts and magazines!" said Floyd, obviously impressed. "We're ready to try again. No, we're gonna do it," he laughs. Tough Old Birds

This year the festival is accenting the culture and music of South Louisiana -- often called America's "Caribbean Coast" -- and the northwest region of North Carolina. Irvan Perez is from the Islena community of St. Bernard Paris in Louisiana. The Islena people are descended from Canary Islanders imported to Louisiana by Spain in the 1770s, and still speak the same Spanish dialect that flows through their ballads.

Perez's community, totally isolated until a road was built through it in the '30s, has never abandoned its traditions of hunting and shrimping. And its arts, such as decoy carving and the singing of decimas (10-line stanzas), have remained functional rather than historic. Perez, who works at a Kaiser Aluminum plant as well as being a shrimper and trapper, has carved decoy's since he was 10. Hurricane Betsy blew away his house and 80 family decoys, half carved by his father from whom he learned the art.

"We searched for two days," says Perez "and only found one duck." The surviving decoy, like the tradition, remains on display. A Creole Quintet

The five Sam Brothers from Scott, La., have been enthralling crowds since Thursday with exuberant Zydeco dance rhythms. Zydeco is a Creole French derivative of Cajun music, paced by the accordion but with heavier flavorings from soul and rhythm and blues; it's often called "push and Pull" music, or La La.

"When you hear talk of a Zydeco, then all the people gather," Said Herbert "Good Rockin" Sam, the 56-year-old patriarch of the clan. Speaking in a Creole-flavored patois and hiding from the sum under a Southwest Louisiana version of a Panama hat, the elder Sam is obviously pleased that his five sons (who range in age from 12 to 19) are continuing the tradition.

The family group came about by attrition. Six and a half years ago, Herbert Sam led his own bands throughout the Louisina-Texas Gulf Coast region. All the instruments were his and were stored in his garage, but he had less control over the musicians. Every time "one wouldn't show, I'd replace him with one of mine, "Sam said. As a result, the sons are the only young musicians playing this type of dance music today, just as the Sams are one of the few black families still speaking Creole French.

The band's leader is Leon, 16, a singer/accordionist who has been playing since he was 10. He plays blues, rock 'n' roll, jazz and a bit of country and western, but insisted "I prefer the Zyedco." Leon has little trouble keeping his soft-spoken siblings in line: They're fairly well trained," chuckled the father. "If one of 'em gets sick, the dance will have to be postponed," Herbert Sam said, "because they will not get someone else. They do not want to perform with no one else but them." n Hand-Me-Downs

Heywood "Woodie" Blevins is in his 70s. When he walks up to perform, your hopes go with him. The piano on Stage 4 seems to welcome him as an old friend. He's been playing for 50 years in the Galax, Va., region, as well as rebuilding and tuning aging pianos through Maryland and North Carolina for the same number of years.

Gary Patton is 25 and lives in Woodlawn, near Galax. Years ago, Blevins started coming to the Patton house to tune the family piano. Nine-year-old Gary started to learn from and imitate Blevins. Now they often share the same stage, playing in the old style. Gary finishes a spunky "Cripple Creek" and says "I'm going to let Woodie play a little more for you," and he walks to the edge of the stage and becomes a member of the audience. It is the younger generation stretching out its hand in appreciation, maintaining yet another tradition. Blevins then introduces "Old Molly Hare," saying, My dad played this on five-string banjo and I caught it from him." And his father learned the songs from his father.

At Wolf Trap this weekend, there are a hundred other living traditions to be seen and heard: buck, dancers and blues, duet singing and jubilee-style gospel. The performers speak in tongues familiar and unfamiliar, but in rhythms that all will recognize.