The butterfly's proboscis is a long slender tube used to drink nectar from flowers. The butterfly curls the structure for storage. As it approaches a plant, the tube unrolls to reach to the bottom of the flower where nectar collects.
When a butterfly emerges from its chrysalis as an adult, the proboscis is in two parts. The insect fuses these parts together to form a long drinking tube. While often thought of as a straw, scientists believe the proboscis works more like a paper towel. Capillary action is used to wick nectar to the insect's digestive system.
The structure of the proboscis includes several very small grooves. These grooves are essential to moving fluids through capillary action. Because molecules of water are somewhat sticky, they adhere to the walls of the grooves and slowly work their way to the top. Wicking allows the butterfly to collect food too thick to drink if the proboscis worked like a straw. The viscosity of the nectar would collapse the straw and turn the nectar into a vapor.
Plants use the same capillary action to draw water and nutrients through their roots from the soil. In the human body, the tear ducts use it to provide moisture to the eyes.