Both characters carry the
虫 (insect/small creature) radical, the catch-all for amphibians, reptiles, and invertebrates in older biological vocabulary.
蝾 +
螈 is a binomial compound — neither character means anything on its own in modern Chinese, much like
蝴蝶 (húdié) 'butterfly' or
蜘蛛 (zhīzhū) 'spider.'