Left insect radical - originally a pictograph of a coiled snake-like creature, broadened over time to cover all small crawling animals: insects, arachnids, worms, even some reptiles. The indexing element. Puts
螨 in the tiny-creature family with
蚊 (mosquito),
蛛 (spider),
蚂 (ant),
蝇 (fly),
蚤 (flea). The radical states: this is a small living thing that crawls.