蝇 on its own is literary. In everyday speech, the full word 苍蝇 (cāngyíng) is used for 'fly.' Common compounds: 蝇拍 (yíngpāi, flyswatter), 果蝇 (guǒyíng, fruit fly).
虫 on the left places 蝇 in the insect family alongside 蚊 (mosquito), 蚁 (ant), and 蝴 (butterfly). The radical marks anything that creeps, buzzes, or wriggles.
黾 once depicted a squat frog and gives a contracted phonetic hint that drifted toward yíng. The shape suggests something low-slung and quick to dart, fitting the fly's hovering motion above all the bug 虫 already promises.
