An old myth places a three-legged toad in the moon, so 蟾 became a poetic name for the moon itself. 蟾宫 ('toad palace') means the moon, and 蟾宫折桂 ('pluck cassia in the toad palace') means passing the imperial exams.
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The bug radical 虫 marks it as a creepy-crawly; the right side 詹 gives the rhyme. Almost always seen in 蟾蜍 (toad).
The insect radical on the left files this under small crawling creatures, the broad category old Chinese used for amphibians like toads as well as bugs.
