Bedbugs!

"Adults are reddish-brown, even when they haven't fed," says Dr. Sorkin. "But when the nymphs first hatch, they're pale white or clear and only about a millimeter long, so they're very hard to see." For scientists, the infants' translucent bodies offer unique opportunities for study. "Under a microscope, you can see the probosis stylet moving in and out as the head moves up and down," he says. "You actually see the gut filling up with blood." But it's not what the creatures take out of humans that causes trouble; it's what they put in—the saliva they inject often prompts an allergic reaction resulting in itchy, raised red welts.