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Fig. 9. Vertical, fore-aft and lateral excursion vs time through one
trajectory. Excursion is the perpendicular distance, standardized by SVL, of
the head (triangles) or vent (crosses) relative to the midpoint. (A) Vertical
excursion. The head is initially higher than the other landmarks. The snake
then pitches forward (indicated by bar) such that the tail is higher than the
midpoint, which is higher than the head. At the arrow, the vent is brought
down to the level of the head and midpoint and rises up and down thereafter.
The head stays at a relatively constant level. (B) Fore-aft excursion. The
head and vent are brought towards the midpoint in the S-formation phase (bar).
The head shows little movement thereafter. The vent moves in slightly cyclic
fashion fore and aft. For both fore-aft and vertical excursion, the error
increases substantially in the last 0.4 s of the trajectory. C. Lateral
excursion. Head and vent undulatory movements are clearly shown. The head
undulation (first arrow) begins before the vent undulation (second arrow). In
fully developed undulation, the head and vent are in phase, with the head
excursion smaller than the vent excursion. The gray bar represents one full
undulatory cycle. (M=36 g, SVL=69 cm.)
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