Assessing the patterns and drivers of shape complexity in the amblypygid
pedipalp using elliptical Fourier analysis
Abstract
Amblypygids are an arachnid order possessing a unique pair of spined
pedipalps: appendages that perform in prey capture, courtship and
contest. Pedipalp length, hypothesised to be under sexual selection,
varies markedly across amblypygid species, and pedipalp spination,
thought to reflect selection for function in prey capture, also differs
interspecifically. Differences in pedipalp shape between species may
indicate that the relative strength of selection for prey capture and
sexual selection vary across the group. However, interspecific
differences in pedipalp shape have not been quantified, due to
difficulties in identifying homologous features. For the first time, we
quantify trends in amblypygid pedipalp shape complexity. We use
elliptical Fourier analysis to quantify 2D complexity in pedipalp
outlines across eleven species and six genera. We find that complexity
significantly decreases as pedipalp length increases. This appears to be
driven by relative spine length, suggesting that a trade-off exists
between pedipalp length and spination. Furthermore, significant
female-biased sexual dimorphism in shape complexity is present in the
tibial segment of the amblypygid pedipalp. Our results provide novel
insights into the drivers of amblypygid pedipalp evolution, and suggest
that a functional trade-off between performance in prey capture and
other functions under sexual selection exist in this enigmatic
structure.