What’s in a band? The function of the colour and banding pattern of the Banded Swallowtail

Butterflies have evolved a diversity of colour patterns, but the
ecological functions for most of these patterns are still poorly
understood. The Banded Swallowtail butterfly, Papilio demolion demolion,
is a mostly black butterfly with a greenish-blue band that traverses the
wings. The function of this wing pattern remains unknown. Here, we
examined the morphology of black and green-blue coloured scales, and how
the colour and banding pattern affects predation risk in the wild. The
protective benefits of the transversal band and of its green-blue colour
were tested via the use of paper model replicas of the Banded Swallowtail
with variations in band shape and band colour in a full factorial design.
A variant model where the continuous transversal green-blue band was
shifted and made discontinuous tested the protective benefit of the
transversal band, while greyscale variants of the wildtype and distorted
band models assessed the protective benefit of the green-blue colour.
Paper models of the variants and the wildtype were placed simultaneously
in the field with live baits. Wildtype models were the least preyed upon
compared to all other variants, while grey models with distorted bands
suffered the greatest predation. The colour and the continuous band of the
Banded Swallowtail hence confer antipredator qualities. We propose that
the shape of the band hinders detection of the butterfly’s true shape
through coincident disruptive coloration; while the green colour of the
band prevents detection of the butterfly from its background via
differential blending.  Differential blending is aided by the
green-blue colour being due to pigments rather than via structural
colouration. Both green and black scales have identical structures, and
the scales follow the Bauplan of pigmented scales documented in
other Papilio butterflies.