flies, small insects produced in damp places
蠛 centers on tiny flying insects that appear in damp conditions: flies and other small moisture-bred bugs.
蠛 combines the insect radical 虫 with 蔑, which likely provides the sound. The character refers to small flies or insects that breed in dampness.
The insect radical 虫 on the left signals a bug, and the right side 蔑 suggests something tiny or insignificant. Picture a swarm of tiny flies rising from a damp corner.
For ベツ, imagine a cloud of flies so thick you have to bet someone you can walk through without swallowing one: bet -> ベツ, and the flies are the damp-loving insects.