Since their smashing introduction in the 1930s, comic-book heroes such as Superman and Batman have been fighting evil right and left, keeping cities safe and delighting fans.

But these days they and other stalwarts of the industry are stuck in the grip of a sticky Web that could ensnare even Spider-Man. They face foes that couldn't be imagined 70 years ago: digital pirates.

Digital scanning and sharing of comic books have begun to make a dent in the business, driven by easy-to-use file-sharing tools and a culture in which enthusiasts eagerly pass along their copies to one another.

"No comic sells enough to lose some of the market," said Chris Gage, who has written for DC Comics Inc. and independent publisher Arcana Studio. "If your comic book doesn't sell a certain amount of copies, it can get canceled."

Gage, a writer for "Law and Order: Special Victims Unit" and other television shows, is well aware of piracy in all forms of media. The resident of West Hollywood, Calif., worries most about his first creator-owned comic, "Paradox," which debuted last year and whose survival depends on healthy sales.

Estimates for the number of comic books shared online are fuzzy because it is difficult to track specific downloads on so-called peer-to-peer networks.

But an informal Web poll of 4,621 readers from December 2004 to December 2005 by Comic Book Resources, an online magazine, found that more than 30 percent had downloaded a comic book at least once. Twelve percent said they downloaded comic books regularly.

"Are there downloaders in the tens of thousands? Possibly," said Todd Allen, an independent online media consultant and adjunct professor of e-business at Columbia College Chicago. "Are there millions? Not likely."

It's certainly not as widespread a practice as music file sharing, but the comic-book business is much smaller. Although the top 10 comic books may have runs of 100,000 to 150,000 copies for each monthly issue, even giants such as DC Comics and Marvel Publishing have books that sell about 20,000 to 30,000 copies. And many comics don't even break 5,000 anymore, Allen said.

It's the little guys that stand to lose the most. Guys such as Robert Burnett, whose Los Angeles-based production company, Ludovico Technique, launched a comic series last year. "Living in Infamy," about a witness-protection program for super-villains, runs about 2,000 to 5,000 copies an issue.

"Comic-book piracy for us would be a problem," Burnett said. "Our retail price is $3. Every book that we sell matters."

Burnett said he was unaware of piracy of his comic books but was concerned it might happen in the future.

"Comic books are a labor-intensive proposition," he said. "Our people have to get paid."

Driven by collectors and hard-core fans, the comic-book industry will always have its share of loyal paper-copy readers. Aficionados will continue to head to the comic-book store Wednesday mornings, when new titles go on sale, to leaf through the colorful pages and breathe in the freshly printed ink.

"The collector mindset says, 'I need the paper issue,' " said Gene Kannenberg Jr., director of ComicsResearch.org, a Web site devoted to scholarship on comic books and strips.

Still, readership is declining. Comic-book publishers are having a hard time, in particular, catching the attention of younger readers, who either are tuned in to a plethora of other media — video games, movies, music, social networks — or would rather get their fix of the action characters free online.

There are multiple-image readers and formats for passing around comics. Users typically scan the files, turn them into readable formats and upload them onto sites such as Pirate Bay, www.thepiratebay.org, for users of the BitTorrent protocol.

These sites work by breaking a large file into smaller pieces stored on different users' computers. When a user wants to download a specific file, BitTorrent searches around the Web for the fragments and pieces them back together, allowing for the faster transmission of large files.

"It's fairly easy to download them," said Yero Dermenjian, a 22-year-old microchip designer from Saugus, Mass. But "it's not as easy as it's become to download, say, music."

A self-described serious comic-book reader, Dermenjian said he still regularly buys paper copies of series he likes.

"I have typically downloaded books that I generally wouldn't purchase," he said.

Some comics are never printed after their initial releases. In such cases, the digital copies are all readers may have, said Michael Pemberton, an associate professor at Georgia Southern University who teaches a course on comic books in American culture.

"It gives people the chance to read one or two of these issues, and then they might go to the comic-book store," said Pemberton, himself a devoted collector. "I see this as a kind of free publicity for the comic-book companies."