red spotted lizard (used as medicine)
蚧 refers to a red spotted lizard, specifically one used in traditional medicine. Its meaning is narrow and concrete, centered on this particular creature.
蚧 combines 虫 (insect/reptile) with 介, which likely contributes the sound. The character is a phono-semantic compound for a type of lizard.
The insect radical 虫 suggests a small crawling creature, and 介 looks like a person with legs apart. Imagine a red spotted lizard scurrying between someone's legs.
For カイ, picture a red spotted lizard crawling onto a piece of pie: pie -> カイ, and the lizard is used as medicine.
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