Two couples in Cancun opt for “a little culture before we leave.” A German tourist shows the sunburnt Yanks a map to Mayan ruins where his archaeologist brother was headed. He’s late getting back. Let’s go check it out. Scott B. Smith adapts his 2006 novel “The Ruins” for a generic fright that is better than you’d guess. Fearing bad reviews, Dreamworks Pictures only screened “The Ruins” for reviewers 10pm the night before it hit theaters. Smith (“A Simple Plan”) trims some of his book’s viscera—and omits his copious peeing scenes. After a jungle trek, the travelers reach the vine-covered ruins. “It was a beautiful sight: a hill shaped like a giant breast, covered in red flowers,” wrote Smith. Armed with guns, bows and arrows, local Mayans arrive on horseback, encircle the ruins and quarantine the trespassers. At the top are a vacant tent and a mineshaft. Somewhere down there a cell phone rings. The vines crawl under your skin and do you no good. The blood shed on screen is self-inflicted surgery, as the vine’s desperate victims try to carve the intruding tendrils from their own flesh. First-time director Carter Smith, although an admirer of David Cronenberg, cannot match his inner-body dread. “The Ruins” also lacks the back-story to the attacks on interlopers found in “The Descent” and “Turistas.” The clump of vines could be a mere freak of nature, and the Mayans are merely its eco-custodians. The coolest idea that Smith and Smith express is the highly evolved acoustic mimicry used by the vines’ red flowers to bait victims. Is that your cell phone ringing in the dark of the multiplex? With Jonathan Tucker, Jena Malone, Shawn Ashmore, Laura Ramsey, Joe Anderson and Sergio Calderon. 91m. (Bill Stamets)