tip-toe
蚑 centers on moving lightly and carefully on the tips of one's feet, like an insect skittering or a person tiptoeing.
蚑 combines 虫 (insect) with 支, which likely contributes the sound and may suggest the idea of branching or light movement, together evoking the skittering motion of an insect.
An insect (虫) moving with delicate, branching steps (支) is tiptoeing across a leaf.
For キ, imagine a tiny insect tiptoeing on a key: key -> キ, and the insect's light steps match the tiptoe meaning.