Nectar foaming by butterflies and moths with long proboscises*

The ability of many insects to drink nectar from long floral tubes questions the conventional “drinking straw” model of proboscis: the model requires an enormous suction pressure to move nectar through the proboscis to the suction pump in the insect head. Imaging of the behaviour of liquid films and droplets on proboscises of hawk moths, allowed us to show that liquid gets inside the food canal by penetrating through the fence of the linking dorsal and ventral hooks. And a thin liquid film is formed on the internal surface of the canal. X-ray phase-contrast imaging reveals that as the drop penetrates into the canal, the internal liquid films undergo an instability and form liquid bridges. Thus, the insect gets nectar from the bubble trains not from a continuous nectar column. These observations show the deficiency of the drinking straw model. We also report on the mechanism of bubble train formation theoretically and numerically showing that the bubble length and the time of its formation depend on permeability of the proboscis linking structures. Estimates for the energy dissipation of moving bubble trains show that fluid feeding insects with long proboscises may benefit from this mechanism as it reduces the required suction pressure.