mosquito wriggler
孑 refers specifically to a mosquito wriggler, the larval stage of a mosquito. It is a rare, specialized character with no broader conceptual center beyond this single biological meaning.
孑 is a pictographic character representing a mosquito larva, with the shape suggesting the small, wriggling body. The exact historical development is uncertain, but the form is closely related to 子 (child).
Imagine a tiny, wriggling creature missing a middle stroke compared to 子 (child). This incomplete shape represents a mosquito wriggler, a larval insect that is not yet fully formed.
For ケツ, picture a mosquito wriggler in a stagnant puddle, and think of the word 'kettle' (ケツ) because the wriggling motion resembles bubbles in a boiling kettle.