Lurking at the top of a burrow concealed in a cunningly constructed turret of twigs, leaves and stones, hungry wolf spiders (Lycosa tarantula) keep all eight eyes peeled, ready to ambush the next passing meal. However, after pursuing and subduing the hapless victim, the ravenous arachnid has to drag its quarry home before feasting. Joaquín Ortega-Escobar, from the University Autónoma of Madrid, Spain, explains that the spiders keep track of the direction and distance travelled on the outbound leg of the hunt, so that they can return home along the most direct route instead of retracing their steps directly. The animals rely on a polarized light compass associated with the minute pair of anterior median eyes at the front of the head to determine their orientation, and an odometer that measures the movement of images across the retina to determine the distance covered. However, it was not clear which of the four pairs of eyes the wily arachnids use to keep track of how far they have travelled.
After training the spiders to run 30 cm along a channel lined with stripy wallpaper to their burrow, Ortega-Escobar fitted water-soluble blinds to the spider's posterior lateral eyes and posterior median eyes, and then encouraged the spiders to scamper home. Impressively, the spiders were able to accurately gauge the return distance when the large posterior median eyes were covered; however, when the posterior lateral eyes were obscured, the spiders pulled up 3 cm short of their home site. And the spiders’ ability to estimate the distance travelled was most compromised when the stripy wallpaper was replaced with a stripy carpet. Coating the tiny anterior lateral eyes and the posterior median eyes in black paint, the spiders stopped 7 cm short of the full 30 cm home run when the anterior lateral eyes were covered, but only stopped 3 cm before the burrow when the large posterior median eyes were covered.
So the two outermost pairs of eyes and the posterior median eyes in the front of the spider's head hold the key to the arachnid's ability to keep track of how far it has travelled, and Ortega-Escobar says, ‘L. tarantula probably integrates the information gathered through the anterior lateral eyes and the posterior median eyes to get an image of the changes observed in the substratum that can be used for orientation when returning to their burrow after looking for prey’.
