Bull ants fine-tune ocelli to their lifestyle

Abstract

Aeronautical engineers would love to know more about how flying insects control their manoeuvres. While human pilots are equipped with an artificial horizon for orientation, flying insects depend on three simple eyes (ocelli) set in the tops of their heads, which probably track the position of the horizon for stability. Intriguingly, some walking insects, such as members of the Myrmecia bull ant family, are also equipped with these unusual mini-eyes, even though they are probably of most use when members of a nest take to the wing to establish a new colony. Knowing that different members of the Myrmecia family, which often share the same territory, tend to be active at different times of day, Ajay Narendra wondered whether the insects have fine-tuned the specialised minute eyes to their different lifestyles.Armed with boots for protection from the ants’ fearsome stings, Narendra and Willi Ribi ventured out into the mountains and national parks in the vicinity of Canberra to collect workers of the walking day-active M. croslandi and dawn/day-active M. tarsata bull ants, while the dusk-active M. nigriceps and the nocturnal M. pyriformis ants had to be collected by infrared light at night, so that the pair could ‘spot them before they spotted us’. However, collecting flying members of each family was more challenging: ‘They fly out of the nest only once a year and were produced by only a few of the nests’, recalls Narendra, who eventually gathered night-flying M. nigriceps males and day-flying M. pyriformis males.Back in the lab, the pair took detailed images of the ants’ mini eye structures using light and electron microscopes. ‘The ocelli of the [walking] worker ants were surprisingly small compared to those of the flying ants’, says Narendra, who had to take great care handling the delicate structures. The day-active M. croslandi had the smallest ocelli lenses (76 μm), the species that were active in dim light and during the night had larger (129–201 μm) lenses, and the lenses of the flying ants were at least 10 times larger than those of the walking ants. The duo then compared the size of the light-sensitive portion of the eye, the rhabdom, across the four species of walking ants and found that the rhabdoms were widest in the ants that are active in dim-light conditions, probably to increase their light sensitivity, while the eyes of the flying ants were packed with 3 times as many rhabdoms as the eyes of the walking workers.Knowing that many insects are capable of sensing polarised skylight with their ocelli for navigation, Narendra and Ribi looked for structures in the ant ocelli that might detect light polarisation and were impressed to see that the two walking day-active species (M. croslandi and M. tarsata) had the structures, while the species that were active toward the dim end of the day lacked them. They also found that the ocelli of both species of flying ants were equipped with structures that may be sensitive to light polarisation, suggesting that the mini-eyes may be polarisation detectors, even though M. nigriceps flies at night.In addition, the two scientists was surprised when they discovered a reflective crystal structure in the ocelli of the flying night-active M. nigriceps males, toward the top of the rhabdom. Explaining that other animals use similar reflective eye coatings to enhance their sensitivity to light in dim conditions, Narendra suspects that the nocturnal fliers take advantage of the increased light sensitivity to make the most of what little light there is when they take to the wing.