Biomechanical drivers of the evolution of butterflies and moths with a coilable proboscis

The ability of butterflies and moths to drink nectar through a proboscis up to 3 times longer than their body has remained a puzzle. Although current biomechanical models suggest that the proboscis is used as a straw, the forces required to pull liquids through these long straws would require adaptations that have not yet been described, such as disproportionately large muscles that create strong pressure differentials. Our analyses revealed a common feature that allows butterflies and moths to drink through an elongated proboscis: the presence of bubble trains in the conduits of the feeding system. We combined X-ray phase-contrast imaging, optical video microscopy, micro-computed tomography, phylogenetic models of evolution, and fluid mechanics models of bubble train formation to understand the biomechanics of butterfly and moth feeding. Our models of evolution suggest that the bubble train mechanism appeared in the early evolution of butterflies and moths with a proboscis long enough to coil. We propose that the bubble train mechanism is a key feature in the diversification of butterflies and moths with a coilable proboscis.

Methods
The data collected to characterize the sucking pump and associated muscles were taken from micro-tomography scans in the software Slicer 3D.