mosquito larva
蜎 refers specifically to a mosquito larva, the wriggling aquatic immature stage of a mosquito.
蜎 combines 虫 (insect) with 肙, which likely contributes the sound. The character was created to name the mosquito larva as a type of insect.
The insect radical 虫 on the left tells you it's a bug, and the right side 肙 looks a bit like a wriggling shape with a mouth and body. Picture a tiny mosquito larva wriggling in water.
For エン, imagine a mosquito larva wriggling in a pond, and you think 'end the larvae!' — end sounds like エン.