Final answer:
Adult butterflies use their proboscis, a long mouth-tube, to siphon nectar from flowers.
Step-by-step explanation:
The structure used by adult butterflies to siphon nectar is the proboscis. This long mouth-tube resembles a straw through which butterflies extract nectar from flowers efficiently. Other mouthpart variations among insects include antennae for sensing chemicals, wings for flight, and legs, but none of these function like the proboscis in nectar consumption.