Using a swimming technique to push off the air the bug could either gain momentum by using the bottle's sloped bottom as a kind of half-pipe, or start moving in

Using a swimming technique to push off the air the bug could either gain momentum by using the bottle's sloped bottom as a kind of half-pipe, or start moving in a circular path until sufficient speed were built up that the bug would begin moving up the bottle's side. Alternatively, the bug could fly out if it's that kind of bug. Or it could knock the bottle over by rocking it. Clearly I'm Google material because of my "bug in frictionless jar" degree.