Making itself conspicuous in broad daylight, the roach becomes an easy target for predators—including rats. The rodent dines on the roach and becomes the new—and final—home for the thorny-headed worms. No other host will do for this species. Worms settle in the rat’s gut, using their formidable heads to secure themselves to the intestinal wall. The parasites reach adulthood in five or six weeks. They may disturb the rat’s digestion but do no serious harm. AVERAGE SIZE: 4 to 12 inches (10 to 30 centimeters).