gnat, mosquito
蜹 centers on small, bothersome flying insects that bite or swarm: gnats and mosquitoes. The two meanings are closely related as tiny, annoying pests.
蜹 is a phono-semantic compound: 虫 (insect) gives the meaning category, and 芮 likely contributes the sound. The exact historical development is uncertain.
The insect radical 虫 on the left tells you it's a bug. The right side 芮 looks like a tiny creature with a cap and legs, like a gnat or mosquito ready to bite.
For ゼイ, imagine a swarm of gnats buzzing with a sharp 'zay' sound: zay -> ゼイ, and the insects are all around you.