Ultrastructural investigations showed that amongst all the hymenopteran eyes examined to date, those of the bumblebee and honeybee exhibit the greatest similarity. Ommatidia in both species are of the apposition type. Minor, but nonetheless noteworthy differences, include the lack of interfacetal hairs in the bumblebee, the rather smooth and flat outer corneal surface, the relatively thick cornea and the larger rhabdoms and retinula cells in the bumblebee eye. Twisting of the rhabdoms or rhabdomeres was not seen. Intracellular electrophysiological responses provided the basis for study of flicker fusion frequency and absolute, polarization, spectral and angular sensitivites. The bumblebee compound eye has a flicker fusion frequency of approximately 130 Hz. Twin flashes resulted only in two distinct responses when the light levels were low. Bright light tended to lead to a suppression of the second depolarization in a twin flash response. Beyond a certain critical brightness, visual cells can no longer simply respond with a potential increase to an increase in light intensity; instead, they react with an increase in response area and a prolonged depolarization afterpotential. Since the latter disappears when the frequency of stimulation is raised, neither the Bunsen-Roscoe law nor the Weber-Fechner response/stimulus intensity relationship can be used to fully describe this phenomenon. Three basic types of retinula cells were found electrophysiologically: II cells possessed a singular sensitivity peak in the ultraviolet (353 nm), 10 cells had a clear peak in the blue (430 nm) and a secondary one in the ultraviolet, and 54 cells exhibited greatest sensitivity in the green to light of 548 nm wavelength. Green-sensitive cells had a secondary peak in the ultraviolet. Polarization sensitive cells were found only amongst ultraviolet and blue cells, but both spectral groups also contained the same number of polarization insensitive cells. It is believed that ultraviolet, blue and green cells can all occur in the same ommatidial unit but that it is unlikely that different visual pigments are housed in the same visual cell. Electroretinogramme recordings were used to measure angular and spectral response curves of the dorsal ocellus. The eye of the bumblebee appears adapted to carry out quick angular flight manoeuvres in bright sunlight. The larger rhabdom and acceptance angle, on the other hand, must give the bumblebee an advantage over the honeybee to forage in somewhat dimmer ambient light conditions. It is concluded that bumblebees, like honeybees, make use of shapes and colours of flowers, use in navigation the polarization pattern of the sky, and that their temporal and spatial resolution allows them to retain relatively fine details of their environment.
