Skeptic: How do you know?
Believer: If I pick up a cockroach and tell it to walk, it will walk when I put it down.
Skeptic: How does that demonstrate that it hears through its legs?
Believer: If I pull off its legs, it won't obey my command."
Seems silly? We get the joke because it is obvious, so it does seem silly. Unfortunately, it's not that far removed from the illogic of some believers.
Just in case you have to know about cockroach hearing:
"There must be a God.
How do you know?
Sunsets are beautiful."
"Hearing is not very well developed in the cockroach. They respond to air movements and ground vibrations which are detected by hairs on the body and cerci. Cerci are feeler-like extensions located on the posterior. No tympanum or eardrum such as is seen in grasshoppers and crickets is present." . found here .