Many butterflies visit flowers to obtain nectar, a fluid produced by flowers that contains sugars and other nutrients. Flowers often restrict access to nectar by their shape. A long tube such as the proboscis of a butterfly is necessary to reach the nectar. To ingest the nectar, a butterfly must pump the liquid such that it travels through the mouthparts and into the head.
Inside the head of the butterfly, the mouthparts are connected to a pump called a cibarium. The cibarium, is like a tube with a cavity in the center. When muscles pull upward on the top of the cibarium, the cavity opens allowing liquid to flow into the cavity and pulling liquid up the proboscis. Contraction of circular muscles surrounding the cibarium, close the cavity in a manner that pushes a bolus of liquid into the esophagus. The actions of the muscles pump liquid into the digestive system.
*Seung Chul Lee, Bo Heum Kim and Sang Joon Lee. Experimental analysis of the liquid-feeding mechanism of the butterfly Pieris rapae. The Journal of Experimental Biology (2014) 217, 2013-2019