Convergence in liquid-uptake mechanisms among fluid-feeding insects

Lepidoptera and Diptera represent approximately 70% of all fluid feeding insects. Although these groups independently evolved mouthparts modified for liquid feeding, both orders face the task of acquiring trace amounts of liquids that might be confined in small pores on substrates. Suction alone cannot overcome the capillary pressure to remove liquids from small pores, therefore, we hypothesized that fluid feeding insects rely on capillarity when feeding from porous surfaces. We tested the importance of capillarity in feeding using the limiting-pore size hypothesis, which assumes a relationship between pore sizes and mouthpart conduits; insects cannot feed on pore sizes with smaller radii than the food canal. We fed five butterflies species and two fly species a 20% sucrose solution combined with fluorescent, magnetic nanoparticles from wetted filter papers of specified pore sizes. Insects were subsequently dissected, and a magnet was waved by their crops to visualize nanoparticle movement as a positive indicator of feeding, and feeding abilities were related to mouthpart architecture. As predicted, our results indicated a trend where butterflies and flies with food canals of smaller radii fed from pores of smaller sizes. In addition, both groups evolved similarly shaped structures to support the movement of fluids from surfaces into mouthpart channels for subsequent delivery to the gut.