Tarantulas can climb walls because they ooze a sticky silk through their feet.
The study raises interesting questions about spider evolution
Scientists have shown how spiders made to scale vertical glass surfaces will secrete a fibrous "glue" to anchor themselves down and prevent a fall.
Arachnids are known to use claws to negotiate difficult terrain, and they also have tiny hairs that can form weak electric attractions with a surface.
But the silk represents a previously unrecognised climbing technique, a German team tells the journal Nature.
"We have discovered that the tarantula has a third attachment mechanism, which depends on fibres exuded from nozzle-like structures on its feet," Stanislav Gorb, from the Max Planck Institute for Developmental Biology in Tubingen, and colleagues write.
"These fibrous secretions function as silken tethers and, when laid down on glass plates, appear as 'footprints' that consist of dozens of fibres with diameters of 0.2-1.0 [millionths of a metre]."
The team studied how zebra tarantulas (Aphonopelma seemanni) from Costa Rica managed to hang on to vertical glass plates.
To walk up, the spiders employed their distal claws; but to come down they oozed a silky substance from all four pairs of feet.
The team believes the mechanism raises interesting questions about arachnid evolution.
Generally, spiders will extrude a silk from abdominal structures known as spinnerets. This fine thread is used in a range of activities from capturing prey to providing protective shields for developing young.
The team wonders which of the adaptations - foot silk or abdominal silk - came first; or, indeed, if they evolved completely independently.
Commenting on the research, spider expert Professor Fritz Vollrath from Oxford University, UK, said foot silk had been regarded as something of an "old wife's tale" that no scientific team had sought to describe in detail.
"It's incredible - just like Spiderman. If the stuff is so good he can pull a train around, how does he get it off?
"And that's the thing for the spiders: first they have to glue themselves down and then they have to get themselves off again. It's very clever," he told BBC News.
Some answers to the evolution questions might come from a thorough genetic analysis of foot silk, the Max-Planck-led team said.