Planet Earth

Inverted Insects


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If you’re fleeing a large indoor predator, it helps to be able to dash up walls and upside down across ceilings, pausing now and then to wave your antennae provocatively at your oafish floor-bound pursuer. Thanks to animal physiologists Alexa Tullis of the University of Puget Sound and Robert Full of the University of California at Berkeley, we are at last beginning to understand how cockroaches have perfected this art. They found that a roach burns three times as much energy running up a vertical wall as it does running upright on a flat surface--and, surprisingly, 50 percent more than it does running upside down.

Tullis and Full measured the oxygen consumption of roaches running on a tiny treadmill that was inclined at 45- and 90-degree angles, and also completely inverted for some tests. The treadmill was enclosed in a small chamber so the researchers could precisely monitor oxygen levels. Tullis and Full were surprised by their findings. The cost of moving a given distance is highest when the roach runs up a vertical wall, says Tullis.

Why do roaches find it easier to run upside down than straight up a wall? Tullis and Full aren’t entirely sure. A roach, says Full, apparently does more work against gravity when climbing a 90-degree incline than it does when moving on a level surface, even if that surface happens to be above the insect. The roaches have little hooks on their feet that probably help them cling to small bumps and fissures on a surface. Those hooks may help the roaches fight gravity, but the insect also takes fewer and longer steps when it’s upside down.

It’s kind of like he knows he’s unstable, and if he lets go, it’s the end of him, Tullis says. The change in gait, she speculates, may call different muscle groups into play that use less energy. What does it take to tire a roach out? Tullis had the roaches run for at least 15 minutes to get an accurate reading, and they showed no signs of tiring.

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