Biomechanics and Functional Morphology of Amblypygid Predation

McLean, Callum (2020) Biomechanics and Functional Morphology of Amblypygid Predation. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Amblypygids, colloquially known as whip spiders, are a charismatic order of arachnids that a characterised a unique pair of spined pedipalp appendages. Amblypygid pedipalps are hypothesised to primarily function as a prey capture device. However, the pedipalp is also used in several other functions including territorial contest and courtship, opening the possibility that the appendage could also be under the influence of sexual selection. There exists a vast degree of morphological diversity within the pedipalp, with relative length spanning nearly an order of magnitude across the group and spination varying markedly both within- and between species. The amblypygid pedipalp is therefore subject to multiple selective pressures, and both its external morphology and kinematics likely reflect this. Thus, the amblypygid pedipalp provides an ideal structure through which to study the evolution of morphological traits subject to multiple selective pressures, and the potential evolutionary trade-offs that may arise. Despite this, amblypygid pedipalp morphology and kinematics remains poorly quantified and little comparative work has been carried across the group. Here, I aim to quantify intra- and interspecific trends in amblypygid pedipalp shape and prey capture kinematics for the first time, using modern morphometric techniques and high-speed videography. In this work I present a comprehensive review of sexual dimorphism in arachnids and identify the common drivers behind this phenomenon. Building on this, I quantify sexual dimorphism in pedipalp size and shape in a single species of amblypygid using a novel geometric morphometric approach. This is followed by an broad analysis of intraspecific and interspecific trends in shape complexity of pedipalps using Elliptical Fourier Analysis. Finally, I draw a link between form and function by quantifying and comparing prey capture kinematics in a morphologically diverse set of amblypygids, using high-speed videography and motion analysis. This work provides new insights into amblypygid 8 pedipalp diversity and posits the possibility of an evolutionary trade-off between increased pedipalp length, for use in display in courtship and territorial contest, and strike performance during prey capture.