Photoperiod length and the chronometric mechanism of the sun compass in Mediterranean sandhoppers

The chronometric mechanism of compensation for the apparent movement of the sun was investigated in adult individuals of Talitrus saltator (Amphipoda: Talitridae) from Mediterranean latitudes. The individuals were subjected t o alteration of the duration of illumination, while its phase remained the same as in nature. A close relation between photoperiod length and the chro nometric mechanism of the sun compass was observed, which could explain the seasonal adaptation of the sun compass. We propose an alternative to the c lassic hypothesis of daily differential compensation for the apparent movem ent of the sun.