Experimental analysis of the liquid-feeding mechanism of the butterfly Pieris rapae

The butterfly Pieirs rapae drinks liquid using a long proboscis. A high pressure gradient is induced in the proboscis when cibarial pump muscles contract. However, liquid feeding through the long proboscis poses a disadvantage of high flow resistance. Hence, butterflies may possess special features to compensate for this disadvantage and succeed in foraging. The main objective of this study is to analyze the liquid-feeding mechanism of butterflies. The systaltic motion of the cibarial pump organ was visualized using the synchrotron X-ray imaging technique. In addition, an ellipsoidal pump model was established based on synchrotron X-ray micro-computed tomography. To determine the relationship between the cyclic variation of the pump volume and the liquid-feeding flow, velocity fields of the intake flow at the tip of the proboscis were measured using micro-particle image velocimetry. Reynolds and Womersley numbers of liquid-feeding flow in the proboscis were ~1.40 and 0.129, respectively. The liquid-feeding flow could be characterized as a quasi-steady state laminar flow. Considering these results, we analyzed the dimensions of the feeding apparatus on the basis of minimum energy consumption during the liquid-feeding process. The relationship between the proboscis and the cibarial pump was determined when minimum energy consumption occurs. As a result, the volume of the cibarial pump is proportional to the cube of the radius of the proboscis. It seems that the liquid-feeding system of butterflies and other long-proboscid insects follow the cube relationship. The present results provide insights into the feeding strategies of liquid-feeding butterflies.