How Does a Butterfly Drink Through a Coiled Straw? Two Halves Zipped Together

The structure is not a single tube grown as one piece. Two elongated appendages, one from each side of the head, each carry a groove along the inner face, and the two are brought together so that the grooves form a channel between them. They are held along their length by interlocking hooks and by a fringe of fine projections, which grip in a manner frequently described as a zip, and the join is sealed by saliva. A newly emerged adult has the two halves separate and must work them together by repeated coiling and probing, which takes an hour or so, and an individual that fails at this cannot feed and dies.

Drawing fluid along a tube that thin requires more than suction:

• •A pump in the head expands and contracts, drawing liquid up
• •The channel is narrow enough that capillary action contributes substantially
• •The tip carries slits through which liquid enters rather than an open end
• •Those slits are fringed, which filters particles that would block the channel
• •The surface inside is treated so that liquid wets it readily
• •Coiling and uncoiling is driven by muscles and by internal pressure together

Nectar is the familiar answer and the diet is broader and stranger than that. Many species take fluid from rotting fruit, from tree sap and from dung, which supply sugars and salts that flowers do not. Mud puddling, in which numbers of individuals gather at damp ground, is a behaviour driven by sodium and amino acids rather than by water, and it is performed overwhelmingly by males, which transfer those salts to females during mating as a nutritional gift. Several species feed on carrion and on sweat, and a few in South America drink the tears of reptiles and birds for the same reason. Some moths have lost the structure entirely and never feed as adults at all.

Coiling the tube away is not simply tidiness and the mechanism is worth describing. The structure is held coiled by elasticity built into its own material, so the resting state is the coil and uncoiling requires active effort rather than the reverse. Muscles within the base straighten it partially, and increasing the pressure of fluid inside extends it the rest of the way, which means the insect uncoils by pumping rather than by pulling. Coiling protects a delicate structure from damage and from drying, keeps it clear of the ground when the insect walks, and reduces drag in flight. A butterfly seen with the tube partly uncoiled is usually cleaning it, which they do frequently with the front legs.

Charles Darwin was sent an orchid from Madagascar in 1862 whose nectar sits at the bottom of a spur around thirty centimetres long, and he predicted that a moth must exist on the island with a feeding tube of that length, since no shorter one could reach the reward and the flower would not otherwise be pollinated. The prediction was mocked. A hawkmoth matching the description was described in 1903, over twenty years after his death, and was finally filmed feeding at the flower in 1992, which confirmed the whole chain. The case is a standard illustration of a testable prediction from evolutionary reasoning and of how long confirmation can take.

Two grooved appendages are held together along their length by interlocking hooks to form the channel, and a newly emerged adult must assemble them within about an hour or it cannot feed. A head pump and capillary action move liquid, entering through fringed slits near the tip. The diet extends well beyond nectar to fruit, dung, sweat and mud, and Darwin predicted a long-tongued moth from an orchid in 1862.