insect moulting
蛻 centers on the process of an insect shedding its old skin or shell to grow, leaving behind an empty husk.
蛻 combines 虫 (insect) with 兌, which likely contributes the sound and carries a sense of shedding or exchanging. The character specifically denotes the moulting of insects.
The insect radical 虫 on the left and 兌 on the right can be imagined as an insect wriggling out of its old skin, leaving the empty shell behind.
For ゼイ, picture a cicada shedding its shell with a sharp, buzzing 'zay' sound: zay -> ゼイ, as the insect moults.
shed skin (of a snake, insect, etc.); ecdysis