If you’ve ever experienced rush hour traffic, you know firsthand that most humans base our schedules roughly around the rise and setting of the Sun, during daylight hours. However, the Australian intertidal ant, Polyrhachis sokolova, must instead schedule its busy day of foraging in the mangrove forest according to the rise and fall of the tide. Low tide can occur day or night, and to function effectively in both the brightest and darkest conditions, these ants possess several useful eye structures—not unlike the pupils in our eyes, or night vision goggles—that help them adjust to different light levels so that they can find food.
There are thousands of ant species that can have a variety of habitats, morphologies (shapes), and navigation methods. Australian intertidal ants use vision to identify landmarks like trees, and celestial cues like the angle of starlight to find their way. Low tide, whenever that may be, is the best time for foraging, so these ants need to see in all light levels without the assistance of flashlights or sunhats. Exactly how they manage to adapt to such a wide range of light conditions was investigated and described in a recent PLOS ONE study.
To learn more, researchers made tiny casts of intertidal ants’ eyes using fingernail polish. They flattened the casts and examined them under a microscope. Ants have compound eyes, meaning that their eyes are made of many tiny facets, or eye units, compared to simple eyes like ours that only have one eye unit each. Researchers counted the number of facets in each compound eye and measured each one’s diameter. The eyes were cast at different times—10am and 10pm—to inspect how the eye structures changed in dark versus light conditions. The light sensitivity of the eyes was calculated based on this morphological data.
Intertidal ants’ compound eyes each have around 596 facets and are similar to the eyes of other ant species specifically adapted to darker conditions. Eyes that “see” in the dark tend to have larger lenses and be extremely sensitive to light to get the most out of the little available light. This night vision adaptation would typically limit an ant’s ability to function in daylight because bright light would overload the photoreceptors in these highly sensitive structures, but the researchers found other mechanisms that protect these ants’ eyes, restricting the amount of light that can enter—like a pupil—by making the openings that allow light to pass smaller. This mechanism helps the ants adapt their night-vision eyes to bright daylight. This type of pupil is seen in other nocturnal ants but had not been found previously in ants that forage during the day.
Finally, to assist in navigation, the researchers found yet another structure in the ants’ eyes: special light detectors that act like skylights and help determine direction by sensing the angles of light sources in the sky. Therefore, Australian intertidal ants do not have the very best day or night vision, but they instead sacrifice some of their ability to see well in each condition in order to see “adequately” in both.
Citation: Narendra A, Alkaladi A, Raderschall CA, Robson SKA, Ribi WA (2013) Compound Eye Adaptations for Diurnal and Nocturnal Lifestyle in the Intertidal Ant, Polyrhachis sokolova. PLoS ONE 8(10): e76015. doi:10.1371/journal.pone.0076015
Image Credits: Images are from Figures 1, 2, and 3 from the manuscript.