A project undertaken at the Department of Biological Sciences, Macquarie University and supervised by Dr Ajay Narendra and Dr Yuri Ogawa
Ocelli are simple eyes placed in a triangular formation on the dorsal surface of the head. Ocelli are found in almost all flying insects, but in very few walking insects. Ants are ideal to investigate the function of the ocelli, since the flying forms (winged males and females) have ocelli while the regularly encountered workers typically lack ocelli. Among the renowned bull ants, that belong to the genus Myrmecia and the champion desert navigators, Melophorus bagoti and Cataglyphis fortis, workers have distinct ocelli. This project aimed to characterise the structure and function of ocelli in ants that exhibit different modes of locomotion and occupy different temporal and ecological niches.
Figure 1. Night-active ants have larger ocelli in Myrmecia ants. Top: day-active ant, Myrmecia croslandi. Bottom: night-active ant, Myrmecia pyriformis.
In closely related species of Myrmecia, we found that among pedestrian workers, the crepuscular– nocturnal species had the largest ocellar lens and the nocturnal species had the widest and greatest number of rhabdoms. Among males, the day fliers had the largest ocellar lens and the night fliers had the widest rhabdom. These adaptations improve the light gathering capacity of the ocelli. We discovered that night-flying males of Myrmecia have a reflective tapetum which allows the reflected light to be reabsorbed, thus significantly increasing their optical sensitivity which is essential for operating in dim-light conditions.
We then explored whether the ocellar is involved in detecting the pattern polarised skylight. The pattern of polarised skylight is a significant compass cue that several insects use to determine heading directions. We found that the organisation of the microvilli in the ocellar rhabdoms of crepuscular and nocturnal worker ants, suggests that it destroys polarisation sensitivity. This contrasts with the males in which the organisation of the microvilli suggests that ocelli are sensitive to changes in pattern of polarised skylight. These are the first reports of such variation found within a single species.
In the champion desert ant navigators, we found that Australian ant, Melophorus bagoti, that lives in semi-arid habitats have a peculiar ocellar retina making them unlikely to be polarisation detectors. In contrast, the North African desert ants, _Cataglyphis_spp, that live in relatively featureless landscapes have an ocellar retina that is well tuned to be polarisation detectors.
Given the ocelli are placed dorsal surface of the head, we asked whether the ocelli could be involved in spatial vision, spatial resolving power and contrast sensitivity. To investigate this, we used electrophysiological techniques to show that in _Myrmecia_ants, the time at which animals are active does not affect the ocellar spatial vision. Ocellar spatial vision improves significantly with input from the compound eyes, with a noticeably larger improvement in contrast sensitivity than in spatial resolving power.