Elegantly sipping from drops of nectar, most butterflies have no idea of the mystery surrounding their drinking technique. Konstantin Kornev, from Clemson University, USA, explains that although the delicately tapered proboscis looks like an elaborate drinking straw, calculations show that the insect would paradoxically have to produce sucking pressures of more than 1 atmosphere to draw sugary fluids through the structure. Kornev and his colleagues wondered whether the insects were overcoming these challenges by flexing and moving the proboscis to alleviate the constriction and reduce the pressures required. Teaming up with Chen-Chih Tsai, Daria Monaenkova, Charles Beard and Peter Adler, Kornev began characterising how monarch butterflies use their proboscises for sipping (p. 2130).

After filming how the proboscis moved while the butterflies sucked, the team saw that the insects use a combination of four strategies: they splay the tip of the proboscis, slide both sides of the tube back and forth, pulse the proboscis and press the tip against the surface that the droplet is sitting on. The team suspects that these factors aid the passage of fluid through the proboscis by widening the tapered tip, altering the way in which the meniscus travels along the structure and augmenting the suction power of the cibarial pump to reduce the suction required to pull fluid through the proboscis.

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Paradox of the drinking-straw model of the butterfly proboscis
J. Exp. Biol.