Behavioral responses to visual stimuli and underlying neural mechanisms in insect navigation
Marie Dacke (Lund University)
The unique celestial compass system of South African dung beetles has been intensely studied for nearly 20 years. Diurnal and nocturnal African dung beetles use celestial cues such as the sun and moon and the skylight polarization pattern to roll dung balls along straight paths over the savanna. Although nocturnal beetles move in the same manner through the same environment as diurnal species, they do so when it is at least a million times dimmer.
We now know that the visual ecology of the beetle is linked to its orientation strategy and the neural activity of its compass neurons. At night, polarized skylight is the dominant orientation cue for nocturnal beetles. Diurnal beetles, however, persist in using a celestial body (the sun) for their compass. Compass neurons in the central complex of diurnal beetles are tuned primarily to the position of a simulated sun, while at lunar light intensities the same neurons in the nocturnal species switch exclusively to polarized light. Thus, these neurons encode the hierarchy of celestial cues and alter their preference according to ambient light conditions. This flexible encoding of the hierarchy of celestial cues relative to the prevailing visual scenery provides a simple, yet effective mechanism for enabling visual orientation at any light intensity.
A ball rolling beetle adhere stead-fast to its chosen course by the use of a simple orientation mechanism that facilitates precise navigation with minimal signal processing - a “celestial snapshot” strategy. The enigmatic “dance”, performed on top of the ball prior to rolling, is the very moment when this celestial snapshot is taken and stored. When rolling, the beetles simply have to find the best match to this snapshot in order to hold their bearing.