Steve Ditko, one of the most influential comic-book artists of the 20th century, who was the co-creator of Spider-Man and developed the character of another superhero, Doctor Strange, was found dead on June 29 at his home in New York City. He was 90.

The New York Police Department announced his death, which did not become widely known until a week later. The cause was not known.

Ditko was an illustrator of remarkable flair whose colorful tales of superhuman characters made him one of the most innovative and revered artists in the world of comics. He also worked in the fantasy and horror genres and created an array of other heroic figures, but he was known above all for creating the visual image of Spider-Man.

The idea for a superhero with spider-like qualities was first floated by Jack Kirby, one of the creative forces behind the Marvel comics franchise. In 1962, Marvel’s editor in chief, Stan Lee, began to develop the idea but did not like Kirby’s illustrations.

Lee then asked Ditko, who had been working on and off with Marvel for several years, to give visual form to Spider-Man. The first appearance of Spider-Man came in August 1962 — during what aficionados call the silver age of comics — in Amazing Fantasy.

The background of the story is this: A high school student named Peter Parker has acquired remarkable powers after being bitten by a spider. In his blue-and-red tights and mask — devised by Ditko — Parker transformed himself into “The Amazing Spider-Man,” who climbs walls, crouches on ceilings and uses a weblike material to swing among skyscrapers as he overcomes villains such as the Green Goblin, Doctor Octopus and the Sandman.

Ditko supplied the illustrations and, eventually, much of the story line of Spider-Man, while Lee wrote the dialogue. The comic proved to be so popular that it soon became a separate franchise and ultimately evolved into a daily newspaper comic strip and a series of Hollywood blockbuster films.

It was considered a monumental advance in comics because Peter Parker was an ordinary teenager, like many of the readers of Spider-Man, who struggled in school to be noticed by the popular crowd. Not even his beloved Aunt May knew of his secret, arachnid-derived abilities.

Moreover, there was an undercurrent of psychological depth because Peter Parker was forever haunted by the death of his Uncle Ben, which he could have prevented if he had used his “Spidey” powers.

“Ditko took what was a very good superhero comic strip and really turned it into something revolutionary,” Blake Bell, author of Strange and Stranger: The World of Steve Ditko, told the New York Post in 2012. “It was Ditko who wanted to ground the strip in reality, to see what it was like to be a hero through the eyes of a teenager and to struggle.”

Comic-book historians often cite If This Be My Destiny, from issue No. 33 of The Amazing Spider-Man, as something of a masterpiece of the genre.

In that issue, from 1966, Ditko portrayed a besieged Spider-Man practically crushed under heavy machinery. As water rises around him, he struggles to break free.

For three pages, in panels of different sizes, Ditko depicted Spider-Man having an existential battle, practically giving up before remembering his filial duty to his aunt and uncle.

“Everything going black,” Lee wrote in a caption. “My head — aching! Hold on — I must hold on!”

Finally, in a full-page illustration with dramatic foreshortening, Spider-Man lifts the machinery off and extricates himself: “From out of the pain — from out of the agony — comes triumph!”

Generations of illustrators have considered that issue of Spider-Man a model of visual storytelling, practically the comic-book equivalent of Citizen Kane.

After 38 issues of Spider-Man, Ditko parted ways with Lee and Marvel Comics in 1966.

Fans have long speculated over the breakup, and Ditko never explained what drove them apart. In 1969, he stopped giving interviews altogether.

Years later, in an illustrated essay giving his version of Spider-Man’s origin, he wrote, “If all the web lines I’ve drawn were laid end to end, they still wouldn’t be enough to fit around Lee’s swelled head.”

Lee credited Ditko as the co-creator of the Spider-Man franchise, saying in 1999 that Ditko made the comic “more compelling and dramatic than I had dared hoped it would be. Also, it goes without saying that Steve’s costume design was an actual masterpiece of imagination.”

Survivors include a brother and a sister.