Riding on the back of a butterfly.

Monday dawned foggy and freezing. Not the best day to take to the air in an ultralight and do ''a little stick time,'' as Steve Brown, instructor at The Wright Solution here, phrased it.

''You'll know whether you're sold on it after the first flight,'' he said as we walked - no, lumbered - in our layers of padded ski clothes toward this recreational aircraft, which has been described as a dragonfly, a wasp, a bicycle, and the motorcycle of the air.

Extending his arm, Steve guided me over the web of wires, cables, and chrome tubing, the basic structure of the ultralight. The rainbow-colored dacron wings above me shed an eerie purple and orange light as I settled back, if one can be described as settling back on a butterfly.

''You mean this thing really flies?''

Steve handed me my helmet, fastened me into my harness, and strapped the altimeter onto my wrist. He explained that this ultralight (a Robertson B2 RD) had a capability of climbing to a maximum of 14,000 feet.

''F-f-f-fourteen thous. . . ,'' my voice crackled back to me through the live intercom that Steve was sliding inside my helmet against my right ear.

Before I could ask where to hold on he was flicking the wooden propeller to life and 50 hp. roared back from the far side of the Perspex shield.

My feet, Steve had indicated, should stay on the ground, anchoring the craft so I didn't take off - by myself.

Seating himself on my left he gave the ''lift off'' signal. I placed my feet on the grid-style pedals in front of me and we taxied down the slightly bumpy field on soft wheels about the size of bicycle tires.

''There's the throttle, here's the control stick,'' said Steve. He turned and revved the engine and suddenly, five seconds later, we were airborne and climbing toward an altitude of 500 feet. . . .

The air rushes past my right cheek, the ground spins slowly away, the horizon tilts, the rooftops shrink, and the white dots of sheep down below nuzzle the grass, totally indifferent to the waves of excitement rippling through my body.

There is nothing but empty space between my flapping ski pants and the treetops. It's like reliving a dream. Feelings of deja vu and flashbacks tumble through my confused brain stirring atavistic memories of the longing to fly unaided by a machine. Here I am doing it - well, almost.

We tilt the control stick back, we go up. When we tilt it forward we go down. With pressure on the stick to the right and by pushing the right pedal down we go right. By pressing the left pedal down and with pressure on the stick to the left we go left. It's like sailing in the air. Eat your heart out, Icarus! It defies the imagination and the birds, although it's not quite as agile. Ultralights are still in the infancy stage of their design and aerobatics are not encouraged because they tend to place too much stress on the structure.

I try to wave. The wind almost plucks my hand off my wrist. This is like sitting in a space-shuttle bug or a sky car without a roof, a floor, or sides.

It is estimated that over 20,000 ultralights are currently flying the skies of the United States. More than 60 US companies manufacture this craft, which needs no runway for takeoff and no licenses for its pilots.

Unlike regular aircraft, ultralights are not subject to low altitude or slow-speed limitations. They are classified by the federal government as those that weigh no more than 254 pounds, travel no faster than 63 miles per hour, carry no more than five gallons of fuel, and have a stall speed that cannot be faster than 27 m.p.h.

''Flick the switch,'' calls Steve through the intercom. We stall and glide toward earth, making smooth contact with the ground. Steve taxis along on one wheel then suddenly he revs the engine and we're gaining altitude again.

Through my intercom I hear myself uttering strange yelps of excitement. Steve hears them and gives me a series of Errol Flynn eyebrow twitches and nods, indicating, ''Yep, you're hooked.''

Now I know what Wilbur and Orville Wright must have felt like that first day of flight above the sands near Kitty Hawk, N.C., on Dec. 17, 1903, in their 605 -pound machine.

The real father of ultralights is Brazilian Alberto Santos-Dumont, who designed small dirigibles. In 1907 he built a miniature high-wing monoplane that weighed 243 pounds and traveled at 48 m.p.h on a two-cylinder, 20-hp. engine.

Flying of ultralights is banned from controlled airspace and is prohibited after dark. If fitted with a light that is visible for more than three miles, flying is permitted half an hour before sunrise and half an hour after sunset.

At a cost of $2,495 to $9,600, kit form or factory built, ultralights make the dream of flying one's own machine relatively inexpensive. Most kits take between 15 to 40 hours to put together and can be assembled with general workshop tools, requiring nothing more than the ability to drill holes, rig cables, and install fabric, bolts, and hardware. Essential flying gear includes a helmet, earplugs, goggles, and a parachute.

Simple as it may sound, it is essential that flying instruction be undertaken before jumping into an ultralight. Most dealers offer flight training priced from $300 to $500. It's advisable to contact an AOPA (Aircraft Owners and Pilots Association) Air Safety Foundation registered examiner and for $15 to $25 take the voluntary flight examination before taking to the air. Most dealers are examiners.

Although these carefree craft are defined as recreational, they are also catering to other markets.

Following a flight demonstration for local officials, the city of Monterey Park, Calif., decided last February to reinstitute police surveillance using a specially equipped twin-engine ultralight. Ray and David Browniee, two Union County, Mississippi, farmers, use their ultralight to spot crops and conditions from the air. They say it is more fuel and time efficient than crisscrossing fields and creeks in a pickup truck.

Flying farmers use ultralights for crop spraying, at 10 percent of normal cost.

However, there's a dark side to the ultralight. It is currently being considered for use in the insurgency wars of Central America and elsewhere. US officials are concerned that the craft could become the tool of terrorists.

There is another cloud hanging over ultralights: the question of safety. This was recently brought into focus by an ABC ''20-20'' segment that showed the death of Steve Douglas, a well-known Washington, D.C., news personality who fell out of his Pterodactyl Ascender ultralight when it collapsed in midflight. The National Transportation Safety Board says there have been 62 fatalities in ultralights since April 1983.

''The time has come for ultralight enthusiasts everywhere - pilots and manufacturers alike - to commit totally to the concept of self-regulation or face being legislated out of existence,'' said John Baker, AOPA president. ''The future of the sport as well as the lives of the participants are at stake.''

Despite these persisent and often troubling problems, ultralights are not fly-by-nights. The longing to fly for fun is too deeply embedded in everyone's dreams. ''Have fun. Enjoy it and play safe,'' said Steve as we floated back to earth and cruised to a gentle stop. Breathless, frozen, laughing, I climbed unsteadily out of the machine, which now stood in silent and stationary ignominy. The magic was over but flying and flying machines will never be the same again for me. That little ultralight has left a permanent impression on me and my dreams.