The prologue to "The Jerk," a harmlessly jerky farce opening today at area theaters, introduces Steve Martin as a derelict who informs us that his most valued possessions is a striped thermos bottle, which he clutches to his grimy bosom.

Later, serenading his true love, an ingenue played by Bernadette Peters, from the bathtub, Martin improvises a song entitled "I'm Picking Out a Thermos for You."

Now, if a thermos-bottle fetish has you rolling on the floor and screaming for mercy, I suppose Martin's debut as a film comedy star in The Jerk" will promise hilarity. On the other hand, if it fails to tickle you instantly, you may suspect that Martin's sense of humor is a trifle inane.

Martin has a certain loyal following and I can't say I've never been amused by him. I likes his King Tut dance and his wild-and-crazy guys brother act with Dan Aykroyd on Saturday Night Live." He has his moments, few and far between as they seem to be, in "The Jerk." Like Jerry Lewis, he has a knack for dancing funny -- it may be his only spontaneous comic attribute -- and the high point of "The Jerk" is a sequence in which the hero, an orphaned white nincompoop raised by a black sharecropping family, discovers that he really does have a sense of rhythm, upon hearing a "mellow" arrangement of "Crazy Rhythm."

As a rule, Martin's style of joking and performing is more anxiety-ridden than satisfying. It's as if he doubted his own authenticity as a funnyman and and felt safer trying to ridicule the caricature of a smarmy shameless stand-up comic one associates with his exaggerated expressions, especially that crinkly-eyed, toothy smirk.

Martin might become invaluable in character roles as a comic sneak, snob or priss, roughly in the tradition of Hans Conreid and Paul Lynde. But "The Jerk" looks a little too hackneyed to launch him as an endearing boobish lead.

It is an episodic, uneven, heavily facetious account of the ups and downs of bumpkin Navin Johnson after leaving the farm to seek his fortune in urban surroundings. Navin learns about sex when he becomes the plaything of a hard-as-nails circus stuntwoman, starkly embodied by Catlin Adams. "Hey, this is like a ride!" Navin exclaims as he's deflowered.

Later, trying to ingratiate himself with the nice-girl heroine, Navin asks:

"Do you think the next time you make love to your boyfriend, you could think of me?" When she replies that she and her boyfriend have never made love, Navin dares to hope that "maybe someday I could make love to you while you're thinking of him." Cute, I suppose -- but the question remains, cute as what?

After leaving home, Navin progresses from gas station attendant to sideshow barker to successful inventor, before a flaw in his invention (a device for keeping eyeglasses from falling off noses) leads to temporary disgrace and degradation. Nothing about this story continuity feels remotely fresh or contemporary; so even if Martin squeaks by with "The Jerk" (as Marty Fledman evidently squeaked by with "The Last Remake of Beau Geste" at the same studio, Universal, two years ago), he'll be identified as a promising new comic dependent on outmoded formulas.

For my taste a little bit of Steve Martin goes a long way. Moreover, a rickety vehicle like "The Jerk" is apt to wear out as aspiring comic star's welcome in one swift stroke. The flaws in a provocative, inventive modern comedy like Albert Brooks' "Real Life" are easier to forgive. While you're conscious of Brooks occasionally groping and miscalculating in his attempt to achieve a personal comic style on film, you're also excited by the evidence of a fresh, original imagination.

Within ts limitations, "The Jerk" is a capably produced entertainment, seasoned by deft bit performances from several actors, notably Richard Ward and Mabel King as Navins foster parents, Bill Macy as the happy promoter of Navin's invention. David Landsberg as a bank officer and director Carl Reiner as an indignant victim of Navin's invention.

"Did Martin hit the long ball?" asked an ironical colleague while passing my desk. I fear not "The Jerk" is at best a Texas Leaguer, a short-ball safety if there ever was one.