The latest installment in "The Hobbit" trilogy is likely to make arachnophobes squirm. That's the hope of the director Peter Jackson, whose own fear of spiders helped infuse some important scenes in "The Desolation of Smaug" with creepy, crawly discomfort.
The spiders lurking in the forest of Mirkwood are among the perils Bilbo Baggins and his traveling companions encounter on their journey to reclaim the dwarf kingdom of Erebor. Unlike some of the more fantastical creatures in the series, the spiders look and feel more familiar, albeit supersize. They were the visual-effects children of Weta Digital, led by Aliquippa native Joe Letteri, who has won four Academy Awards for special effects. He and his team tried to make the spiders immediately recognizable as such, but enhanced to amplify their menace.
The biggest change was to their faces: These spiders have jaws and fangs. "We wanted you to get the feeling that they're biting you with teeth, that they would swallow you whole if they could," Mr. Letteri said, speaking by phone from Los Angeles. "But the bodies and heads were based on realistic designs."
The design evolved from several sources, including a reference photo Mr. Jackson found of a particularly creepy spider. Those designs were fine-tuned in the textures department, which defined color patterns for the creatures and added details, including wrinkles. That department's supervisor, Gino Acevedo, worked alongside the creatures department to add distinct details, like legs that were translucent.
"We wanted what we called a 'beer bottle effect,' " Mr. Acevedo said, speaking by phone from Weta Digital's base in New Zealand. "If you look at a beer bottle, it has that amber look and color to it, especially when you hold it up to the light. When the legs were backlit, you saw an internal structure inside with veins."
The research also included discussions with a spider expert, Phil Sirvid from Te Papa, the Museum of New Zealand.
"We took a bit of a field trip, met Phil and looked at his creepy collection of wild-looking spiders," Mr. Acevedo said. "He gave us amazing reference images taken through an electron microscope of spider parts, things that you never knew spiders had." For instance, they were surprised to learn that spiders have pores, which were incorporated into the design.
Mr. Jackson wanted the spiders to look ancient, as if they'd been trudging around in the forest for many years, so Weta aged them with flaky skin, calluses and more.
"We wanted it to look like when you get a sunburn and the skin peels away and underneath is a new skin," Mr. Acevedo explained. The spiders were also given boils to make them look diseased, and soiled with dirt and mud.
"Some of this stuff is so subtle that you don't pick it up," he added. "But if you didn't have it there, the creature would look too clean, or as we call it, 'CG perfect.' "