Orientation in the intertidal salt-marsh collembolan Anurida maritima

We investigated the cues used by the intertidal insect Anurida maritima (Apterygota: Collembola) to orient to the appropriate zone in which to seek shelter during high tide. Our experiments clearly ruled out any significant role for magnetic, local topographic, slope or celestial cues. Instead, we suggest that the difference between the appearance of the up-shore and the down-shore horizon is the major cue used by the collembolans. When a mirror was used to duplicate either of the two horizons, the insects were not able to orient. The insects moved towards an artificially dark horizon (the reverse of the mirror) placed down-shore, the opposite of their usual direction of movement. The insects had an endogenous circatidal rhythm of phototactic behaviour: most of the population was always negatively phototactic, but between 2 and 7 h after low tide, a significant proportion of the population became positively phototactic. This is the first demonstration of an endogenous tidal rhythm of orientation in an insect.