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How tarantulas find their way home: Scientists discover they use their side-eyes to measure distance after leaving the nest to ambush prey

  • Researchers analyzed the role of each of the four pairs of a tarantula's eyes
  • Lateral eyes help them measure the distance to their nest to find their way home
  • Researchers covered the lateral eyes of a tarantula with a water-soluble paint 
  • Then, they found that it stopped short of reaching its objective in an experiment

Tarantulas use their lateral side eyes to calculate distances, researchers have found. 

Researchers analyzed the role of each pair of a tarantula's eyes in the process of distance measurement. 

They found that their lateral eyes help them measure the distance to their nest to find their way home.

Pictured is the arrangement of the four pairs of eyes on the cephalothorax of the spider Lycosa tarantula. OLA shows the anterior lateral eyes, OMA the anterior median eyes, OLP the posterior lateral eyes and OMP the posterior lateral eyes  

The tarantula studied in the research is a species called Lycosa tarantula, which is found in southern Europe.

It lives in burrows around 20 centimeters (7.9 inches) deep, topped by a turret-like structure which it builds from twigs, leaves and small stones, fastened with its own silk.

Spying from the turret, the tarantula surprises its prey, running to catch it and then returning to the burrow from distances between 30 and 40 centimeters (11.8 and 15.7 inches). 

It uses a method called 'path integration' to return to their burrows.

With this mechanism, it doesn't follow the same path back to its burrow - instead, it moves as though it had followed the sides of a right-angle triangle and returning along the hypotenuse - the longest side of the triangle. 

The researchers, based at the Autonomous University of Madrid, studied a tarantula species called Lycosa tarantula, which is found in southern Europe. 

It lives in burrows around 20 centimeters (7.9 inches) deep, topped by a turret-like structure which it builds from twigs, leaves and small stones, fastened with its own silk. 

Spying from the turret, the tarantula surprises its prey, running to catch it and then returning to the burrow from distances between 30 and 40 centimeters (11.8 and 15.7 inches). 

It uses a method called 'path integration' to return to their burrows.

With this mechanism, it doesn't follow the same path back to its burrow - instead, it moves as though it had followed the sides of a right-angle triangle and returning along the hypotenuse - the longest side of the triangle. 

In 1999, researchers from the same university discovered that the tarantulas use polarised light from the sky to know their position with respect to their nest. 

In the new research, the researchers wanted to learn more and analysed the role of each of the tarantula's four pairs of eyes in the the process of measuring distance, also called odometry. 

'To calculate the distance it has travelled, the animal needs an odometer (an instrument for measuring distance travelled) 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,' SINC was told by Dr Joaquin Ortega Escobar, the lead author of the research published in the Journal of Experimental Biology

In this analogy, their 'compass' would correspond to polarised light - which the median eyes use to measure the angle, while direction is detected mainly by the anterior lateral eyes and to a lesser extent the posteriol lateral eyes.

Spying from the turret, the tarantula surprises its prey, running to catch it and then returning to the burrow

Dr Escobar explained that the anterior lateral eyes point downwards, looking at the ground. 

'In the experiment, we covered these eyes with a water-soluble paint and observed that instead of travelling 30 centimetres (11.8 inches) from the nest, which is the distance we initially set, they stopped 8.5 centimetres (3.3 inches) before they reached their objective,' said Dr Escobar. 

With their anterior lateral eyes covered and the others able to see, they had problems determining distance. 

'When we uncovered them, they could return to their nests perfectly,' Dr Escobar said.

In the experiment, the researchers covered the anterior lateral eyes of the tarantula with a water-soluble paint and observed that instead of travelling 30 centimetres (11.8 inches) from the nest, which is the distance that was initially set, they stopped 8.5 centimetres (3.3 inches) before they reached their objective 

'They need the lateral anterior eyes to measure the distance.'

The researchers also observed that don't move their front two legs when their eyes are covered.  

In previous studies of other animals such as desert ants (Cataglyphis fortis), the researchers observed that in a similar experiment to the tarantula one, where the ventral region of the ants' compound eyes (the part that perceives the grid) was covered, they did not present a significant difference in the return trip to the nest compared to when the eyes were uncovered.

'The situations of these two animals are analogous,' said Dr Escobar. 

'In the case of the spider, it is the anterior lateral eye that perceives the ventral field of view, while in the ant it is the ventral region of the compound eye.

'Spiders have simple eyes like our own, rather than compound eyes,' he said. 

 

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