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Fig. 1. Experimental setup. (A) In the free-coxa preparation, the dorsal surface of
Blaberus was attached using epoxy glue to 0.95 cm thick
PlexiglasTM and the tarsus removed (broken outline). A servo-motor
applied a dorsally directed impulse to the tibia, 1 mm from the distal tip.
The free response of the leg was recorded at 1000 frames s–1
and the position of a marker 1 mm from the distal tip of the tibia was
digitized. The abdomen was pulled dorsally slightly and held so that it did
not interfere with the free response of the leg. The fixed-coxa preparation
was identical except the leg was removed from the body and glued to the
PlexiglasTM at the coxa. In all instances the femur–tibia joint was
rigidly fixed by cyanoacrylate, and in half the trials the C-Tr-Fe joint was
also rigidly fixed. (B) The joint axes of rotation of the distal joints are
parallel with the applied impulse direction (into the page). The body coxa
joint's primary axis of rotation runs medio-laterally, so it is free to rotate
as a result of the dorsally directed impulse perturbation. The body–coxa
joint also has secondary joint axes parallel to the applied impulse. (C) Video
frames show the time sequence of the left metathoracic leg's response to an
impulse perturbation. The animals's dorsal surface is towards the left of the
image while anterior is towards the bottom of the image.