rainbow
蝀 centers on the rainbow, a colorful arc in the sky. The given meaning is a single concrete natural phenomenon.
蝀 combines 虫 (insect/creature) with 東, which likely provides the sound. The association of a rainbow with a creature may reflect an old belief that a rainbow was a kind of sky serpent or dragon.
The left side 虫 suggests a creature, and the right side 東 looks like a sun rising behind a tree. Imagine a mythical sky creature arching across the sky after rain, its body forming the rainbow.
For トウ, picture a rainbow stretching from one toe to the other: toe -> トウ, and the rainbow arches between them.
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rainbow