Source: FECYT - Spanish Foundation for Science and Technology
The tarantula species Lycosa tarantula ambushes its prey and lives in burrows around 20 cm deep topped by a structure, a kind of turret which the tarantula build from twigs, leaves and small stones, fastened with the spider's silk. From the turret, the tarantula surprises its prey and runs to pursue it, subsequently returning to the burrow from distances between 30 and 40 cm.
L. tarantula uses path integration to return to its burrow. With this mechanism, it does not follow the same path back to its burrow; instead, it moves as though it had followed the sides of a right-angle triangle, returning along the hypotenuse.
In 1999, a research team from the Autonomous University of Madrid discovered that these animals used polarised light from the sky to know their position with respect to their nest. In the new research, the scientists wanted to go beyond this, and have analysed the role of each pair of the tarantula's eyes (they have four pairs in total) in the process of distance measurement, or odometry.
"To calculate the distance it has travelled, the animal needs an odometer that registers the route, its location with respect to the finish point, which would be the burrow, and a 'compass' to track the direction of travel," according to Joaquin Ortega Escobar, lead author of a paper published in the Journal of Experimental Biology on the function of each eye in these processes.
The 'compass' would correspond to polarised light, which the median eyes use to measure the angle; direction is detected by the anterior lateral eyes. Through this research, the scientists have learned that it is principally the anterior lateral eyes (which until now had not been analysed), and to a lesser extent the posterior lateral eyes, that help tarantulas measures the distance to their nest.