NOVEMBER
2008

 

By
Tom Turpin
 
Professor of
Entomology
Purdue University

 

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Check out these books by Tom Turpin:

Flies in the face of fashion

What's Buggin You Now?

 

 

 

11-26-08

Download the audio of On Six Legs: MP3, WMV.

Insects Were First Wacky Wall Walkers


To most of us, the notion of climbing the walls is not a good thing. The phrase has come to mean being extremely nervous or agitated. For example, when we get cooped up during inclement weather and can't get outside. Or when the kids run out of anything to do during vacation from school.

One of the best-selling boy bands of all time, The Backstreet Boys, incorporated this notion in song for their "Never Gone" album. The song has to do with loneliness in the absence of a girl or boyfriend. Entitled "Climbing the Walls," the lyrics include the line, "Now I'm climbing the walls 'cause I miss you."

Such figurative use of the idea of scaling vertical surfaces should not be confused with real wall or rock climbing. This is an activity that some human beings attempt under the heading of sport. Never mind that we really aren't structurally suited for such a thing. Some of us, at least, are willing to try it anyway.

So to really climb a wall, or at least to do so safely, humans need to use gizmos and gadgets. Things like appropriate shoes, carabiners, webbing, harnesses, belay devices, tapered wedges and hexes, and spring-loaded camming devices. A list, I am sure, that makes some of us happy to keep our feet firmly planted on reasonably flat earth.

Some animals, though, seem to relish living on steep inclined plains, life on the precipice so to speak. Mountain goats, for instance. But when it comes to walking up walls and across ceilings, most of the animal world says, "No, thank you." Most, but not all. A few lizards and tree frogs can do it, so can many arthropods, including all spiders and many insects.

Like the children's-toy sensation of the 1980s called a "Wacky WallWalker," these animals are equipped for the job of clinging to vertical surfaces without mechanical equipment, such as carabiners and camming devices.

Tree frogs use suction-cup-like structures on their toes to adhere to glass windows where they feed upon tiny insects attracted to the house lights inside. Arthropods also have special adaptations on their feet to adhere to smooth surfaces.

All spiders and most land-dwelling insects have legs that end with a pair of claws. These claws allow insects and spiders to cling to soft surfaces such as leaves and fabric and, when the opportunity affords itself, human skin. But claws aren't of much value for gripping hard surfaces. So many arthropods have dual-purpose feet. Such feet are equipped to hang onto both soft and hard surfaces. This is the animal equivalent to all-weather-tread tires for your car, designed to grip the road surface regardless of the conditions.

The adaptations of arthropod feet to deal with slick surfaces involve pad-like structures with names such as arolium, pulvillus or empodium. Scientists have shown that such devices function in a fashion similar to how a wet piece of cloth or paper will stick to a smooth surface. What happens is that water molecules between the two surfaces form an adhesion layer sufficient to hold the cloth or the insect to the surface. In both instances, the adhesion can be released by rolling the adhering surface, such as the cloth, away from the surface to which it clings or allowing the moisture to evaporate.

And that is how insects, including ladybugs, flies, boxelder bugs, cockroaches and even butterflies and moths, seem to magically climb along hard vertical surfaces, such as walls and window glass. It is also how these six-legged creatures magically cling to the windshield of our cars, even as we drive down the road.

Unlike human wall climbers, insects do so without benefit of safety ropes to catch them when falls occur. But insects don't need ropes for protection against falls. These creatures do have a backup plan for times when they lose their grip. They just use their wings to fly to a safe landing. Kind of takes the suspense out of being an insect wacky-wall-walker, doesn't it?

 

Writer: Tom Turpin
Editor: Olivia Maddox