Do neotropical migrant butterflies navigate using a solar compass?

Many tropical butterfly species are well-known for their migratory behaviou r. Although these insects can maintain a constant direction throughout the day, the physiological mechanisms of orientation are unknown, It has been a rgued that tropical migrant butterflies must use a time-compensated sun com pass to accomplish their journey, but the crucial experimental manipulation s to test this hypothesis have not been conducted. This study reports the r esults of clock-shift experiments performed with two species of migrating b utterflies (Pieridae: Aphrissa statira and Phoebis argante) captured during flight across Lake Gatun, Panama. The observed constant flight bearing of natural controls suggests that these species are capable of performing time -compensated celestial navigation. Our clock-shift experiments suggest that a sun compass is involved. Individuals submitted to a 4h advance shift too k significantly different mean orientations on release compared with contro l butterflies. The direction of this difference was consistent with the use of a sun compass. The magnitude was approximately half the predicted value if the vanishing bearing of released butterflies was used as the variable to evaluate the effect of time-shifting and approximately three-quarters of that predicted if the estimated heading was the variable used. Mean vanish ing bearings of control and experimental butterflies did not correspond to predicted values, This difference can be attributed largely to the combined effects of wind and handling.