Fig. 4. Measurement of I1/I3 muscle dimensions in transilluminated juveniles. Medio-lateral (A) and dorso-ventral (B) images from simultaneous two-axis video recordings of three consecutive swallows (Drushel et al., 1997) were scaled and aligned (using the eyespot plane, arrowheads and leftmost line) to determine the location of the lateral groove (presumably the most posterior extent of the I1/I3 muscles) in the dorso-ventral images. This was taken to be 45 % of the total buccal mass length in the medio-lateral view (Drushel et al., 1998). After dividing the resulting anterior compartment into six equal segments (to approximate six I3 rings), the medio-lateral width of each segment was measured. Note the shadow cast by the penis (B, dotted line), which is not included in the medio-lateral width measurements. The asterisk marks the flash of a 1.0 Hz timebase light-emitting diode used to synchronize medio-lateral and dorso-ventral video.