chela and the manus, and the muscle insertion point furthest away
from it were either made by hand using digital calipers on the
disjointed movable finger of preserved specimens, or taken from
high-resolution CT or synchrotron scans [18]. These internal
measurements to determine the force inlever of the movable finger
were made on a subsample of the specimens available for each
species, or only once when anatomical scan data were available.
Because scorpions can vary considerably in length, girth and
weight depending on their feeding state, we used the length of the
prosoma, which does not vary between molts, as an indicator of
overall size [32]. Regression of morphological variables on
prosoma length as a proxy for size was not significant, as there
are large differences in the relative sizes of the chelae and
metasoma between species independent of overall size variation.
The calculation of regression residuals was therefore not appropriate.
Several linear measurements were combined in order to
give functionally relevant ratios. Chela aspect ratio (AR) is the
height of the chela manus divided by the total length of the chela.
This ratio has been shown to be highly correlated with pinch force
[26]. Similarly, metasoma AR was calculated by dividing the