Spring in Flagstaff means occasionally seeing the entire contents of your recycling bin flying through the neighborhood courtesy of the high winds. It’s enough to make even those who fiercely love this town wish they lived along the coast of the southeastern United States, where the wind does something so much cooler. The winds coming off the ocean there send tiger beetle larvae rolling along the beach and uphill in the sand dunes.
In response to being disturbed, these immature insects curve their bodies into the shape of a wheel and let the wind propel them to safety. In real time, it looks as though the beetles launch themselves into the air with random, out-of-control thrashing motions and that the wind fortuitously carries them away.
Analysis of slow motion video tells a different story. The beetle larvae actually execute carefully controlled and impeccably timed somersaults in the air that allow them to orient themselves to catch the wind. They then hit the ground spinning, holding their three pairs of legs out to the side for balance.
The rolling behavior is thought to allow these insects to make a quick escape from dangerous predators or parasites, and by using the wind to power their locomotion, tiger beetle larvae do reach high speeds. With favorable winds, the scientists reported that larvae could spin at a rate of 20-30 revolutions per second and travel faster than their assistant could run along the beach. (That assistant could run about 3 meters per second, or 6.7 miles per hour — the equivalent of running just under a 9-minute mile.)
There are a few other animals capable of rolling locomotion, meaning that they distort their own body into the shape of a wheel in order to move. These include web-toed salamanders, wheeling spiders, pebble toads and pangolins. In all the other species that roll in order to move, the force comes either from gravity or from the animal itself. Tiger beetles are the only ones known to use wind power to roll.
Though it is often reported that tiger beetles are an example of wheeled locomotion, that’s not technically correct. Wheels rotate around an axle, without attachment points. The tiger beetle and other rollers move more like tires that have come off a vehicle than as wheels that are functioning as part of a larger system. It has been argued that true wheels are impossible in a living system because there is no way to deliver nutrients or nervous system signals between the parts.
The population decline of several coastal tiger beetle species is associated with increased human disturbance. Human foot traffic may be an indirect cause because large numbers of people walking on the beach change the surface of the sand. The impact of disrupting the smooth, firm surface may interfere with the efficient wheeling necessary to escape from natural enemies.
Tiger beetles are only figuratively capable of rolling with whatever threats come their way when the path is smooth enough that they can literally roll away from those dangers.