toad
蟇 centers on a toad, a specific amphibian. The meaning is concrete and does not branch into abstract extensions.
toad (esp. the Japanese toad, Bufo japonicus)
Asiatic toad (Bufo gargarizans)
common toad (Bufo bufo)
cane toad (Rhinella marina); marine toad; giant toad
Miyako toad (Bufo gargarizans miyakonis)
Japanese toad (Bufo japonicus); Japanese common toad
Japanese stream toad (Bufo torrenticola)
ranula
toadfish (any fish of family Batrachoididae, esp. the Pacuma toadfish, Batrachoides surinamensis)
large, perforated, turnip-shaped arrowhead cover made of Japanese bigleaf magnolia or paulownia; harmless arrow affixed with such a cover (that whistles when shot and is used to drive off evil spirits)
蟇 combines 虫 (insect/creature) with 莫, which likely contributes the sound. The character specifically denotes a toad.
The top 莫 looks like a squat, bumpy shape, and the bottom 虫 marks it as a creature. Picture a bumpy toad sitting under a broad leaf.
For バ, imagine a toad letting out a deep, croaking 'baaa' sound: baaa -> バ, and the toad's throat puffs out.