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Aerodynamic arachnids in acrobatic display

19 August 2015

Title: Arachnid Aloft: Directed Aerial Descent in Neotropical Canopy Spiders

Authors: Stephen P. Yanoviak, Yonatan Munk, Robert Dudley

Journal: Interface

A paper published today in the Royal Society journal Interface investigates the aerial acrobatics of a rainforest spider

Creatures living in the rainforest canopy sometimes need to make speedy escapes from predators or lose their footing and fall from the treetops. From 30m or more above the ground, falling onto the unfamiliar rainforest floor where predators lurk could be dangerous.

Scientists already know that some ants are able to control a fall from such heights, gliding towards a safe landing site. Researchers involved in this new study set about finding out if another treetop inhabitant, spiders of the genus Selenops (nicknamed ‘flatties’ because of their flat body shape), might also be able to glide to safety.

The researchers dropped the spiders from tree tops to test how well they glide. The spiders righted themselves mid-air to swoop head first towards the safety of a tree trunk where they could avoid the unknown terrain and threat of forest floor predators awaiting below.

Selenops dropped from the rainforest canopy glides in a helix to a tree trunk.

The spiders proved themselves to be excellent gliders, possibly thanks to their flat bodies. In the drop tests 93% of the spiders landed successfully on a tree trunk. The other 7% landed on the forest floor after falling. The team tested spiders from other species and found they were not such graceful gliders. Some managed to right themselves in free-fall but didn’t show any obvious control over the direction of their descent.

The team filmed the drop tests in slow motion and analysed the spiders’ body shapes as they fell. With most of their legs spread behind them, the flatties head first with their two front legs raised. The scientists observed that the spiders steered themselves to safety using these front legs.

Sky-diving spider in a vertical wind tunnel demonstrates the gliding body shape.

‘These spiders represent a remarkable evolutionary adventure in the animal conquest of the air,’ say the team. They add that the spiders are ‘unlikely if not truly ungainly’ aerial acrobats but that their skill at body righting and manoeuvring could inspire robotic design.