Proboscis adaptations for flower feeding

Butterflies use a proboscis, a microfluidic probe engineered by natural
selection, to feed on nutritive fluids. The structural configuration of
proboscises relates to feeding habits; however, the adaptations that
enable proboscis entry into narrow floral corollas lack experimental
evidence. Here, we investigated proboscis adaptations that enable entry
into corollas using funnel-shaped glass capillary tubes and performed
feeding trials with six butterfly species of different feeding habits.
Proboscises were either guided (natural treatment) or forced (forced
treatment) into the capillary tubes that were filled with a 20% sucrose
solution. The treatments were video recorded to determine the depth the
proboscises reached into the tube and how long they remained there. The
results were interpreted in terms of proboscis morphology, friction
forces, and the material properties of the cuticle. In the natural
treatment, butterflies classified as flower visitors were more efficient
at feeding from the tubes, reaching an average 1.83x deeper into the tubes
than the other species and never getting their proboscises stuck. The
non-flower visiting species, in contrast, had their proboscises remain in
the tube 17x longer than the flower-visiting species, with 90% of them
getting their proboscises at least partially stuck. The butterfly species
with generalist feeding habits fed more efficiently than the non-flower
visitors, but less than the flower visitors. A similar pattern was
observed in the forced treatment. Flower-visiting butterflies had smoother
and more tapered proboscises, lower friction forces, and a semicircular
cross-section that would reduce bendability and was augmented by a more
sclerotized cuticle. Proboscises of flower-visiting butterflies,
therefore, have a suite of adaptations that operate synergistically to
optimize their feeding habits.