This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
蛥 combines 虫 (insect) with 多 (many), but the exact historical development is uncertain. It may have originally referred to a type of cicada, as suggested by the kun reading にいにいぜみ.
The left side is 虫 (insect), and the right side is 多 (many). Picture many insects, perhaps a swarm of cicadas, but the exact meaning is unclear.
For セツ, imagine a set of many insects: set -> セツ, and the kanji has 虫 and 多.