DIRECT HOMING BEHAVIOR IN THE ANT TETRAMORIUM-CAESPITUM (FORMICIDAE, MYRMICINAE)

Direct homing refers to the behaviour whereby an ant with food runs in an almost straight line to the nest. We determined whether non-polari zed light acts as a directional cue in the direct homing behaviour of the grass ant, Tetramorium caespitum. We carried out tests (1) under a blue sky or in total overcast conditions in the field, and (2) with a view of the sun or with a non-polarized light within a box in the lab oratory. The ants' paths were recorded, and then analysed by means of circular and linear statistics. Ant workers with food were able to hom e directly under a blue sky, under totally overcast skies, and even un der non-polarized lighting. In a fifth test we displaced the ants at t he beginning of the homing trip; they returned to the presumed nest lo cation along a path that ran parallel to the true homing vector, as if they had not been displaced. The results suggest that the ants home b y dead reckoning, that is, they can measure directional changes and di stances travelled during the outward trip, and integrate them into a m ean home vector for direct homing depending on an external light-based reference system, that is, either the polarized skylight pattern or n on-polarized lighting. (C) 1998 The Association for the Study of Anima l Behaviour.