Henschel, Joh, Natural History
A strobe of sky-blue alternating with dune-red flashes past the spider's eyes twenty times a second as it hurtles down a dune. With its eight legs neatly tucked in, so that one set of joints points outward to form a circle, the spider has become an upright wheel, rolling down the steep sand slope with tremendous speed. Near the top of the dune, its archenemy, a pompilid (or spider) wasp searches in vain for its quarry, which has suddenly disappeared.
In 1977, when David Hughes and Anthony Bannister first filmed the wheel spider's behavior in the Namib Desert of southern Africa, they postulated that it might allow the spider to escape attacking wasps. Wondering if predators could really be the impetus for the evolution of this unusual mode of travel, I decided to learn more about the spider.
Wheellike locomotion is rare in nature. Pangolins, hedgehogs, armadillos, and pill bugs may tumble downhill when they curl up into protective balls, but their spherical bodies roll without orientation and are thus not proper wheels. Snakes that take their tails in their mouths, becoming hoops, remain in the domain of myth, but looping stomatopods (small, shrimplike crustaceans found in intertidal zones) exist. They somersault backward when stranded, turning like bulldozer treads. Besides that, Namib dune spiders are the only animals we know of--other than ourselves--that purposefully and readily use a planar wheel for locomotion.
Of course, wheels with axles are ruled out in multicellular animals because such a rotating organ would be severed from the body's nutrient supply. This limitation, however, does not rule out wheels without axles, like tires removed from a car. But wheels work best on roads. On natural terrains, soft ground yields under the weight of a rolling object, slowing it down. And obstacles greater than a wheel's radius would be absolute barriers.
One of the few natural places where wheels might roll freely is across open desert sand, especially if it is steeply sloping. With hundreds of square miles of regularly spaced, high dunes, the Namib Desert is an ideal place for an animal to have evolved this mode of travel. Weighing little, the wheel spider makes almost no impression on the sand, but its barely discernible tracks allowed me to measure the circumference of the wheel and calculate the number of revolutions and speed of the spider. With the help of gravity, the one-half inch spider can accelerate to speeds of 1.5 to 5 feet per second. The average of twenty revolutions per second that I measured for the spiders is the same as that for the wheel of a sedan traveling at 137 miles an hour. On slopes greater than fifteen degrees, the speed increases as the spider rolls. In the fastest rolls I measured, the spiders made forty-four revolutions per second. Large spiders turn more slowly, and heavy females carrying eggs in their abdomens are unbalanced and topple easily.
As its specific name Carparachne aureoflava suggests, the wheel spider is a beautiful golden yellow, although individuals may vary from white to bronze. To escape the scorching heat of the Namib Desert, the spider shelters in a burrow in the sand during the day and hunts for insects on the surface at night. To prevent the walls of its burrow from collapsing, the spider pushes its very long spigots--fingerlike projections at the tip of the spinnerets--deep into the sand and binds the loose grains with silk. In this way, the spider can dig angled burrows more than a yard deep into a dune face and seal them shut with a trapdoor also fashioned from silk.
Depth provides safety from most enemies. On the surface, wheel spiders may occasionally encounter lizards, dune larks, large, voracious dancing white lady spiders, or cannibalistic neighbors of their own species. Wheel spiders can escape by dashing into their burrows, where only one predator seems to be able to extract them with ease: a palpimanid, or hook-footed, spider, will patiently sit on the trapdoor of a wheel spider burrow, dangling its enlarged front legs into the burrow. …