Performance Measurement
In vivo pinch forces were measured using either a Kistler force
transducer (type 9203, Kistler Inc., Switzerland) mounted on a
purpose-built holder [42], or using a similar setup using a Sauter
FH20 external force sensor (Sauter ltd., Germany). Measurements
were made in a climate-controlled room set at 23–24uC. During
pinch-force measurements, scorpions were restrained between
sponge pads in which a cutout was made to accommodate the
body, or by placing a padded clamp over the last segments of the
metasoma to allow safe handling. Five trials were performed,
separated by at least one day. Only the maximum force per
individual was retained for further analyses. In order to obtain
pinch forces corrected for body size, we attempted to use a linear
regression of pinch force on prosoma length across all species. As
these variables did not show any linear relationship (R2,0.02),
presumably due to the effects of chela design obscuring the effects
of body size, we chose to correct for size by dividing pinch force by
the square of the prosoma length. Pinch force must be scaled by
prosoma length squared, as force scales with the physiological
cross section of the muscle, which in turn scales with length
squared [43].
The LD50 of 14 species of scorpions were included as a second
defensive performance variable. Where no LD50 was available,
the value of a closely related species was used given that this
variable is thought to be conserved within genera (see table 3).