maggots, grubs
蠐 centers on small, soft-bodied larvae that feed on decaying matter: maggots or grubs. The two given meanings are essentially the same creature in different contexts.
蠐 combines 虫 (insect) with 齊, which likely contributes the sound. The character is a phono-semantic compound for a type of larva.
The insect radical 虫 on the left hints at a bug, and the right side 齊 looks like a squirming mass of lines. Picture a writhing pile of maggots or grubs.
For セイ, imagine a maggot saying 'say' as it wriggles: 'say' -> セイ, and the grub speaks its own name.