Researchers at the Australian National University found that not only did bull ants of the genus Myrmecia have these specialised night vision structures, but that only those individuals in the colony that worked at night were equipped with night vision. Other members of the colony that walked or flew by day had differently structured eyes.
Dr Ajay Narendra of the ANU’s Division of Evolution, Ecology and Genetics, led the research into bull ants, which were already known to have highly complex optical structures. With 3000 facets in each eye, Myrmecia ants have the second largest eyes in the ant world, and are highly responsive to visual moving targets, such as possible prey.
Ant Caste System Determines Eye Type
All ant colonies are divided into castes of different sorts of workers, breeding females that do not work, and short lived males.
The worker ants are sterile wingless females, while the breeding female and the males have wings. The winged female flies for a brief period of a few days in the mating season, flying with the males in the ‘nuptial flights’ and mating, before settling in a dark part of the nest to produce eggs.
Dr Narendra said the caste system made it possible to study the differences in eye structures within the same species, as different duties required different types of vision.
Night Working Ants Have More Efficient Light Sensors
The team investigated several nests of four different Myrmecia species: M croslandi, M tarsata, M nigriceps and M pyriformis in Canberra, Australia. Although different species, their habitat ranges often overlaps, and all four species are often found foraging on Eucalyptus trees.
The team observed the times of days that flying ants and walking workers left their different nests and returned. They then examined the eye structures of the different castes of each of the species.
Only the worker ants that foraged at dusk or later in the evening were equipped with the night vision structures – more and larger facets and wider rhabdoms (photosensitive structures) to their already large eyes.
In complete contrast, the flying males, which live only for a few days during the nuptial flights, had small eyes with comparatively few facets.
“Our research shows that the time of day the insects are active and the task they carry out influences eye design,” Dr Narendra said.
Ant Species Active at Different Times of Day
Different species of ants are active at different times of day, from sunrise to twilight or even at night. Many ants forage in the evening or during the night to avoid predators and to look for suitable food resources.
The four species of Myrmecia ants were able to share the same habitat, as they were active at different times of day, so were not directly competing with one another.
These different periods of activity were reflected in their different eye sizes and complexity, from the workers of diurnal M. croslandi, out only in daylight; diurnal-crepuscular M. tarsata, active late afternoon and evening; the crepuscular-nocturnal M. nigriceps, which foraged during the evening and night; and the nocturnal M. pyriformis, who stayed in their nests until night.
Dim Light Led to Ants’ Better Photosensitive Structures
Dr Narendra said the striking change that occurs as any animal moves from day to night is the dramatic decrease in light intensities. Light levels at night are up to a million times dimmer than day light intensities.
Ants that predominantly foraged at night needed to develop much better eyes to make the most of the low light levels. The size of the eye increased in all castes of each species as their activity moved from day to night. Thus, the eyes of the diurnal M. croslandi were the smallest and those of the nocturnal M. pyriformis were the largest.
In addition, those ants that foraged at dusk and night had developed not only bigger eyes, but eyes that were more efficient in capturing as much of the dim light as possible. As well as more facets than the eyes of daylight-active ants, they had bigger facets – nearly twice as wide, and the photosensitive structures, the rhabdoms, were twice as long and nearly three times wider.
Night Vision Might Not Be All Ants Need at Night
However, despite this being the first evidence that different castes within the same species of ant can develop specialised vision depending on the level of light they are working in, Dr Arendra said night vision might not be all that nocturnal ants use.
“While our work shows stunning differences in eye design tuned to ambient light levels, the improvement in optical sensitivity is modest considering the dim light intensities,” he said.The increase in size of the facets and rhabdoms needed to get really clear vision at night would result in very large and heavy heads that would be unwieldy and require too much energy and strength to carry.
Dr Narendra speculated that night-foraging ants might have other strategies to help them navigate in the dark. “Insects might further require temporal or spatial integration of signals to improve signal-noise ratio at dim light. We aim to address this in our future work.”
The research, which was partly funded by the Vision Centre and the Centre for Visual Sciences at ANU, was published as ‘Caste-specific visual adaptation to distinct daily activity schedules in Australian Myrmecia ants’ in the October 5 online edition of the Proceedings of the Royal Society B.
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Sue Cartledge - I'm a science, health, nutrition and lifestyle journalist, fascinated by the way the physical world operates in all its forms, and how ...
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