Swallowtail butterfly wing and tail measurements

The European swallowtail butterfly (Papilio machaon) is so named, because
of the long and narrow prominences extending from the trailing edge of
their hindwings and, although not a true tail, they are referred to as
such. Despite being a defining feature, an unequivocal function for the
tails is yet to be determined, with predator avoidance (diverting an
attack from the rest of the body), and enhancement of aerodynamic
performance suggested. The swallowtail, however, is sexually size
dimorphic with females larger than males, but whether the tail is also
sexually dimorphic is unknown. Here, museum specimens were used to
determine whether sexual selection has played a role in the evolution of
the swallowtail butterfly tails in a similar way to that seen in the tail
streamers of the barn swallow (Hirundo rustica), where the males have
longer streamers than those of the females. Previously identified sexual
dimorphism in swallowtail butterfly size was replicated, but no evidence
for dimorphism in tail length was found. If evolved to mimic antennae and
a head to divert a predatory attack, and if an absolute tail size was the
most effective for this, then the tail would probably be invariant with
butterfly hindwing size. The slope of the relationship between tail length
and size, however, although close to zero, was nonetheless statistically
significantly above (tail length µ hindwing area 0.107 ± 0.011). The slope
also did not equate to that expected for geometric similarity (tail length
µ hindwing area 1/2) suggesting that tail morphology is not solely driven
by aerodynamics. It seems likely then, that tail morphology is primarily
determined by, and perhaps a compromise of several, factors associated
with predator avoidance (e.g. false head mimicry and a startling
function). Of course, experimental data are required to confirm this.