mole
蚡 centers on the mole, a small burrowing animal. The meaning is narrow and specific to this creature.
蚡 combines 虫 (insect/creature) with 分, which likely contributes the sound. The character is used for 'mole', though the exact historical development is uncertain.
The insect radical 虫 suggests a small creature, and 分 (part/divide) hints at an animal that divides the earth by burrowing. Picture a mole parting the soil like a tiny insect.
For フン, imagine the fun of watching a mole pop up from its hole: fun -> フン, and the mole is the star of the show.