Reminder to tiger beetles: If you chase prey at high speeds, you'll go blind.
Entomologists have long noticed that tiger beetles stop-and-go in their pursuit of prey. But until now, scientists have had no idea why this type of beetle attacks its food in fits and starts.
The answer is that the insect's ability to see shuts down after it accelerates toward prey.
"If the tiger beetles move too quickly, they don't gather enough photons (illumination into the beetle's eyes) to form an image of their prey," explained Cole Gilbert, Cornell professor of entomology. "Now, it doesn't mean they are not receptive. It just means that at their speed during the chase, they're not getting enough photons reflected from the prey to make an image and locate the prey. That is why they have to stop, look around and go. Although it is temporary, they go blind."
In nature, such stop-and-go chase patterns are unusual, but the tiger beetle is unique. In the midst of hot pursuit, it stops three or four times to reorient itself toward the prey. Even after a few stops, the tiger beetle has enough time to overtake its prey during its high-speed pursuit.
Results from Gilbert's laboratory observations have been published in a peer-reviewed article, "Visual control of cursorial prey pursuit by tiger beetles (Cicindelidae)," in the Journal of Comparative Physiology Fall 1997.
But, just how fast is fast for a tiger beetle? Gilbert compared an ordinary tiger beetle to Olympic superstar Michael Johnson. Johnson, the world-record holder, can run 200 meters in 19.32 seconds, which averages to a speed of 10.35 meters per second (or 23.1 mph.)
"The top speed for my tiger beetles is 0.5387 meters per second (1.2 mph)," said Gilbert. "This is not very impressive, but the beetles are a lot smaller than Michael Johnson. If we scale the speed for body length, we get a much different picture."
Considering Johnson is about 6 feet tall (1.83 meters), his 10.35 meters per second becomes 5.6 body lengths per second -- "Obviously impressive," Gilbert said. He then explained the tiger beetle has a body length of only 10 millimeters, and its running speed of 0.53 meters per second becomes 53.87 body lengths per second, or relatively 10 times faster than our best human sprinter.
"And the species of tiger beetle I work with (Cicindela repanda) is not even the fastest," explained Gilbert. "There is an Australian species, Cicindela hudsoni, which is 20 millimeters long and can run 2.5 meters per second. This translates into a relative speed of 125 body lengths per second. Michael Johnson would have to run a 200 meter race in 0.87 seconds to equal the relative quickness of the Australian species, or in 2.03 seconds to equal the relative speed of the beetle I work with."
Found throughout the world, tiger beetles come in a variety of species. More than 100 species of the tiger beetle are found in the United States. A common species, Cicindela sexguttata, is a large, metallic-green beetle seen commonly along woodland paths in the spring, said Gilbert. The largest variety in size in New York state is Cicindela formosa. One variety of tiger beetle, Cicindela puritana, which might soon find itself on the endangered species list, lives along the banks of the Connecticut River throughout New Hampshire, Vermont, Massachusetts and Connecticut.
As natural predators, tiger beetles eat just about anything they can catch. Gilbert said they dine on other beetles, hoppers, ants and caterpillars. Tiger beetles are also well-known to scavenge for their food and have been known to scavenge on vertebrate animals.
Gilbert said that after seeking out the reason for the insect's staccato-style chase, the next research step is to learn about the sensitivity of the beetle's photoreceptors, which receive photons and then process those photons into the neurological information that is sent to the beetle's brain. Through this research, he said, science will find out more about this biological tracking system, and he believes this knowledge may one day be used for optimizing artificial tracking systems.
"For example, the Mars Rover needs optical sensors to look around Mars, but it also needs to move around the planet. With speed of movement, there's a trade-off. You want to move quickly to explore a large area, but if you move too fast for the optical sensors to gather enough information to form an image, the exploration is fruitless," said Gilbert. "Through knowledge of biological tracking systems, we can learn how nature has coped with this trade-off, and we may then design better systems to see what is going on around us."