Butterfly mouthparts are long tubes that can be inserted into the narrow channels formed by flowers to reach a nectar reward. Flying or walking while dragging an extended tube would be awkward. Packaging the mouthparts into a tightly coiled tube under the head makes movements less awkward. The mouthparts coil when at rest due to the resilin component. Resilin can be stretched under pressure, but when relaxed, will return to its original shape.
How does a butterfly extend its mouthparts? The butterfly lifts its mouthparts in an action that partially uncoils them. At the base of the mouthpart in the head, muscles repeatedly compress a tube called the stipes. The movement of the stipes pumps hemolymph into the mouthparts and the hydraulic pressure forces the mouthparts to extend. When the butterfly finishes feeding, the valve of the stipes opens and the resilin forces the hemolymph out of the mouthparts as they coil under the head.