If you have ever tried to swat cockroaches, you know they have the ability to run up walls and even across ceilings to evade you. How do they defy gravity, you wonder. It's due to their specially adapted legs, which contain barbs and claws that dig into any surface and allow them to perform rapid maneuvers to avoid danger.
A cockroach has tiny, sharp claws at the ends of his legs that allow the insect to scale vertical surfaces. The surface of a wall may seem smooth as you run your hand over its surface but, in reality, the surface contains tiny irregularities. Bare wood, painted drywall, ceramic tile and stone all offer plenty of footholds for cockroaches to use, while extremely regular surfaces like glass or polished metal may be too smooth to climb. The roach inserts his claws into any hold he can find, much like a climber with six climbing axes. When a cockroach reaches the ceiling, he can continue across, upside down, using the same technique.
One trick cockroaches use to evade humans is rapid inversion. A cockroach will run across a flat surface like a tabletop, reach the edge, and swing underneath without breaking stride. According to a study published in the journal PLoS ONE, scientists observed that the cockroach uses his rear legs to grip the end of the platform as he runs off, swinging like a pendulum underneath the surface until his forelegs find purchase and he can continue to run. This ability to reverse direction suddenly while flipping from one surface to another can disorient predators and prevent pursuit.
You can take advantage of a cockroach’s ability to climb by setting a trap for it. If you spray the interior of a coffee can or other vessel with nonstick cooking spray, or coat it with petroleum jelly, it will become too slippery for a cockroach to climb. Then, you simply place some sugary bait such as non-citrus fruit juice in the bottom to attract a roach. The insect will climb up the outside of the can, flip over into the interior and fall in. The lubricant will prevent the roach from climbing back out, and you can dispose of trapped insects at your leisure.
Not all breeds of cockroach are adept climbers. The German cockroach is the one most familiar to American homeowners, and the one most adept at scaling vertical walls. The less common Oriental cockroach lacks the powerful climbing claws of its relative, limiting its ability to scale sheer surfaces. However, even cockroach species without these climbing claws may find surfaces rough enough to climb inside walls and air conditioning ducts.