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Bull ants have right eye for the job
Worker bull ants have military-style night vision, while their higher status winged nest mates see best during the day, Australian researchers have discovered.

The research led by Dr Ajay Narendra from the Australian National University and colleagues is published this week in the Proceedings of the Royal Society B.

It is the first research to show that individual ants of the same species living in the same colony have huge variation in the structure of their eyes, depending on what job they do and when they do it.

A colony of bull ants contains three types or castes: sterile female workers who forage on the ground, fertile females who briefly fly then live in the dark nest as queens for up to 15 years, and winged fertile males who have a short life on the wing searching for a queen to mate with, before dying.

Biologists already puzzle over how a single colony of genetically identical ants can have such different body shapes. But now it seems even the fine structure of the eye can show dramatic variation.

The team studied four different species of bull ant (Myrmecia) living in eucalypt forests on the outskirts of Canberra.

They recorded at what time of day or night each caste member of each species was active. They then preserved the eyes of the insects and examined the fine structure under a microscope.
Varying routines

They found that the rhabdom (a light collecting structure in the eye) was much larger in ants active at night than those active during the day.

What surprised them even more was that in one of the four species, Myrmecia pyriformes, the workers ventured out at night, whereas the winged forms were active during the day.

"In other species, workers and winged forms are both active at the same time," says Narendra.

"It was exciting because it was happening within a single species."

The researchers were stunned that genetically similar individuals could have such different eye structures.
Pixelated world

Unlike us, ants see a pixelated world in a 360-degree panorama. Their compound eyes are made up of facets, like separate windows, which divide up what they see.

Worker ants spend most of their time foraging on the ground, and have relatively few, large facets located at the front of their eye to help them see up close. Flying males, on the other hand, have smaller facets with lenses that 'overlap'. This gives them a greater chance of seeing a distant moving object like a flying queen bee.

"There has been a lot of evidence that says animals which are closely related have to be active at the same time of the day," says Narendra.

"It was thought that if you are closely related you have very little flexibility to evolve different structures physiologically and anatomically. But in this case, it turns out that even within closely related or even a single species you can have different eye structures tuned to different times of the day."

While Narendra says he has 'no idea' how genetically identical individuals can be so different, he says more work is needed to understand how the eye structures grow and develop in the different caste types.
Important insights

Invertebrate biology expert, Emeritus Professor Dr Andrew Beattie of Macquarie University in Sydney says the discovery could be an ideal vehicle for explaining the mechanism and importance of epigenetics - the study of inherited changes in the body that don't appear to have a genetic basis.

He says is also provides important insights into how caste differentiation can affect individuals in a colony, such as "being able to fly versus being confined to running on the ground, and providing special equipment for day or night activity."

"The study also shows, incidentally, how a very common, and often maligned species can be of profound scientific importance should we not be conserving every species on the grounds that you never know which ones are going to be important to us?," says Beattie.

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