kind of scorpion
蠆 refers to a kind of scorpion, a venomous arachnid with a segmented tail and stinger.
蠆 combines 萬 (ten thousand, scorpion) with 虫 (insect, creeping creature). The character likely originated as a pictograph of a scorpion, with 萬 originally depicting a scorpion and later borrowed for 'ten thousand'. The addition of 虫 clarifies the insect/creature meaning.
The top 萬 originally represented a scorpion, and the bottom 虫 reinforces that it's a creeping creature. Imagine ten thousand scorpions swarming together.
For タイ, picture a scorpion tying its tail in a knot: tie -> タイ, and the scorpion's stinger is ready.
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dragonfly naiad; dragonfly nymph