Cockroaches are infamous for their tenacity, and are often cited as the most likely survivors of a nuclear war. Some even claim that they can live without their heads. It turns out that these armchair exterminators (and their professional brethren) are right. Headless roaches are capable of living for weeks.

To understand why cockroaches—and many other insects—can survive decapitation, it helps to understand why humans cannot, explains physiologist and biochemist Joseph Kunkel at the University of Massachusetts Amherst, who studies cockroach development. First off, decapitation in humans results in blood loss and a drop in blood pressure hampering transport of oxygen and nutrition to vital tissues. "You'd bleed to death," Kunkel notes.

In addition, humans breathe through their mouth or nose and the brain controls that critical function, so breathing would stop. Moreover, the human body cannot eat without the head, ensuring a swift death from starvation should it survive the other ill effects of head loss.

But cockroaches do not have blood pressure the way people do. "They don't have a huge network of blood vessels like that of humans, or tiny capillaries that you need a lot of pressure to flow blood through," Kunkel says. "They have an open circulatory system, which there's much less pressure in."

"After you cut their heads off, very often their necks would seal off just by clotting," he adds. "There's no uncontrolled bleeding."

The hardy vermin breathe through spiracles, or little holes in each body segment. Plus, the roach brain does not control this breathing and blood does not carry oxygen throughout the body. Rather, the spiracles pipe air directly to tissues through a set of tubes called tracheae.

Cockroaches are also poikilotherms, or cold-blooded, meaning they need much less food than humans do. "An insect can survive for weeks on a meal they had one day," Kunkel says. "As long as some predator doesn't eat them, they'll just stay quiet and sit around, unless they get infected by mold or bacteria or a virus. Then they're dead."

Entomologist Christopher Tipping at Delaware Valley College in Doylestown, Pa., has actually decapitated American cockroaches (Periplaneta americana) "very carefully under microscopes," he notes. "We sealed the wound with dental wax, to prevent them from drying out. A couple lasted for several weeks in a jar."

Insects have clumps of ganglia—nerve tissue agglomerations—distributed within each body segment capable of performing the basic nervous functions responsible for reflexes, "so without the brain, the body can still function in terms of very simple reactions," Tipping says. "They could stand, react to touch and move."

And it is not just the body that can survive decapitation; the lonely head can thrive, too, waving its antennae back and forth for several hours until it runs out of steam, Kunkel says. If given nutrients and refrigerated, a roach head can last even longer.

Still, in roaches, "the body provides a huge amount of sensory information to the head and the brain cannot function normally when denied these inputs," explains neuroscientist Nick Strausfeld of the University of Arizona, who specializes in arthropod learning, memory and brain evolution. For instance, although cockroaches have a fantastic memory, "when we've tried to teach them when they had bits of them missing, it's hopeless. We have to keep their bodies completely intact."

Cockroach decapitation may seem macabre, but scientists have conducted many experiments with headless roach bodies and bodiless roach heads. Decapitating roaches deprives their bodies of hormones from glands in their heads that control maturation, helping researchers investigate metamorphosis and reproduction. And studies of bodiless roach heads shed light on how their neurons work. Plus, it provides just one more testament to the cockroach's enviable endurance.