toad
蟆 centers on a toad, a specific type of amphibian. The meaning is narrow and concrete, with no major abstract extensions.
蟆 combines 虫 (insect/creature) with 莫, which likely contributes the sound. The character is used specifically for toad.
The left side 虫 suggests a creeping creature, and the right side 莫 looks like a squat, bumpy shape. Picture a toad with a warty back sitting under a broad leaf.
For バ, imagine a toad croaking a deep 'ba' sound as it hops: ba -> バ, and the toad's call echoes from the 虫 side.