The local optical factor as an orientation cue for the sandhopper Talitrus saltator Montagu (Amphipoda, Talitridae).

The aim of the present study was to assess the influence of the local optical factor (i.e., inhomogeneity in spectral composition between the hemidomes of sky over the sea and over land at the beach) on the ability of zonalorientation of the amphipod Talitrus saltator. The experiments were conducted in the laboratory usingcoloured filters and an apparatus that provides for orientation in artifcial conditions comparable to thatobserved under natural sun and sky.The results show that T. saltator can effectively use the inhomogeneity between the two hemidomes, butonly if the colour distance between them is much greater than that found in the wild. This makes thepossibility of perception - and thus use - of the local optical factor rather unlikely in natural conditions.