Final answer:
Adult butterflies use a proboscis to siphon nectar from flowers, which functions similarly to a straw.
Step-by-step explanation:
The structure used by adult butterflies to siphon nectar is a proboscis. This special mouthpart works much like a straw, allowing the butterfly to access and suck up nectar from within the depths of flowers. It is a defining feature of butterflies and many moths, which have evolved this specialized tool for their feeding habits. Other parts of insect anatomy, such as antennae, wings, and legs, serve different purposes, like sensory input, flight, and locomotion respectively.