tadpole
蚪 centers on a tadpole, the aquatic larval stage of a frog or toad.
蚪 combines 虫 (insect/creature) with 斗, which likely provides the sound ト. The character specifically refers to a tadpole, a small water creature.
A tadpole is a small creature (虫) that looks like a tiny measuring scoop (斗) with a tail. Picture a tadpole shaped like a little ladle swimming in a pond.
For ト, imagine a tadpole that looks like a tiny toe: toe -> ト. The tadpole's round body and tail resemble a toe wiggling in the water.