Tarantulas secrete sticky silk from their feet to help them adhere to shiny surfaces, German scientists have learned.

Spiders are already known to have two mechanisms that give them their uncanny ability to walk upside or cling to smooth vertical surfaces.

But scientists at the Max Planck Institute for Metals Research in Stuttgart have now found - in tarantulas, at least - a third gripping tool: microscopic nozzle-like structures on their feet that secrete a viscous silk-like filament.

The team carried out their research with zebra tarantulas (Aphonopelma seemanni) from Costa Rica, which were induced to walk around a slick, vertical glass surface.

To walk up, the spiders used their distal claws to keep a grip.

To walk down, they exuded fibres between 0.2 and 1.0 micrometres (0.0000078 to 0.000039 of an inch) in diameter from all four pairs of feet that adhered to the glass and stopped them sliding and falling.

The German team, led by Stanislav Gorb at the institute's Evolutionary Biomaterials Group, say the new discovery raises questions about spiders' evolutionary past.

Arachnids also have an abdominal pouch, a spinneret, which produces the silk to make webs.

Gorb theorises that spiders may have started making silk with their feet before they began to do so with their spinnerets, as this would help their survival by avoiding catastrophic falls.

The next step is to carry out a genetic analysis of the foot silk proteins to compare it to the spinneret silk proteins. Spider silk is closely studied by industrial chemists, who are eager to make a synthetic copy that mimics its strength and resilience.

The paper appears in Thursday's issue of Nature, the weekly British science journal.

"Tarantula spider. Credit: International Society of Arachnology"

Source: PhysOrg