larvae of the mosquito
孒 refers specifically to the larvae of mosquitoes, a very narrow biological term. It has no broader conceptual center beyond this single meaning.
The origin of 孒 is uncertain. It may be a pictographic representation of a wriggling larva, but no clear historical development is established.
The shape suggests a tiny, curled creature with a small dot for a head: a mosquito larva wriggling in water.
For ケツ, imagine a mosquito larva that looks like a tiny ketchup squirt: ketchup -> ケツ, and the larva is as small as a drop.