spider millipede
螋 centers on a small many-legged creature: a spider millipede, combining the ideas of a spider and a millipede into one creepy-crawly concept.
螋 combines 虫 (insect) with 叟, which likely contributes the sound. The exact historical development is uncertain.
The left side 虫 means insect, and the right side 叟 looks like an old man with a walking stick. Imagine an old man discovering a strange spider millipede crawling on his stick.
For シュウ, picture a shoe stepping on the spider millipede: shoe -> シュウ, and the insect squishes underfoot.
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