This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 蝅 is uncertain. It visually contains 虫 (insect) at the bottom, but the upper component's origin is unclear.
The bottom is 虫 (insect), but the overall meaning is unclear. It may be an old variant related to silkworms.
For サン, picture a silkworm spinning silk in the sun: sun -> サン, and the insect 虫 works below.