Response to self-motion in waterstriders: visual discrimination between rotation and translation

Waterstriders (Gerris paludum) often try to maintain a nearly stationary position on a moving water surface. Passive motion is restricted to 3 degrees of freedom: yaw, longitudinal, and transverse displacement. They correct for passive rotation and translation by distinct behavioral sequences. The compensatory behavior is predominantly visually controlled.

1.	When waterstriders are rotated and translated simultaneously, they are able to discriminate their own rotation and translation visually.
2.	They discriminate their rotation from their translation even if the visible pattern is restricted to a monocular visual field.
3.	They detect rotation only if they see an extended pattern.

The restriction of degreess of freedom reduces the complexity of the motion-induced visual flow field. Each motion component induces its own flow field component. We propose that those areas of the visual field are preferred for analysis where the directions of the 3 flow field components differ most. These areas (Figs. 2 and 3) have their largest extent at 45° above the horizon.