The roach’s odd behavior probably stems from a bit of biochemical backseat driving. The imperious passengers are young cystacanths, thorny-headed worms who’ve made themselves at home in the unfortunate insect’s body cavity. The parasites need a lift to their adult home—a rat’s intestines—so they enlist the cockroach as an expendable chauffeur. Details still elude scientists, but the worms somehow spur their host to act against its own interests. AVERAGE SIZE: 0.03 inch (0.8 millimeter).