All spiders have hair on their bodies, but its purpose isn’t to keep the animals warm. Most spiders have poor eyesight, so they rely on tactile hairs to provide information about their surroundings—day or night. The hairs detect air movements, like those caused by the flapping of tiny insect wings, as slow as 1 millimeter per second. “If anything moves around a spider, they’ll likely feel it,” says Eileen Hebets, an arachnologist at the University of Nebraska-Lincoln who studies spiders.
A spider’s exoskeleton, or hard outer shell, also contains thousands of microscopic holes. A thin layer of tissue covers each of these slit sensilla. The membranes act like tiny drums that vibrate when, for example, an insect crawls over a leaf that a spider is standing on. The vibration from the insect’s movements sends a signal through a nerve to the spider’s brain, alerting the spider to the presence of potential threats, prey, or mates nearby.
Such adaptations are particularly useful to nocturnal spiders, like those from the genus Cupiennius, found in Mexico, the Caribbean, and South America. These spiders don’t spin webs to catch a meal. As prey walk nearby, signals from the spiders’ tactile hairs and slit sensilla tell the spiders when to leap toward their victims. Experiments show that these spiders can capture prey even when their eyes are covered.