A British tourist and his wife were left stunned when they came across this strange sight on a hike in northern Spain: a "snake" that was actually hundreds of caterpillars marching in line.

Video shows what appears from a distance to be a long snake on a dusty track, filmed along the Camino trail between San Sebastian and the village of Pasaia.

But the reality might be even scarier: these fluffy caterpillars line up head-to-rump in single file can fire out their barbed hairs, causing stinging in the eyes of anyone that aggravates them.

Writes filmer Will Oliphant: "I'm fairly scared of snakes, so I approached with caution, but when I got there I was surprised to see that it was actually hundreds of little caterpillars.

"We had a look around and there were a couple of other smaller chains of caterpillars walking in a similar fashion.

"Luckily we didn't go too near them as it turns out that they are a fairly dangerous breed called the pine processionary caterpillar (thaumetopoea pityocampa), which can actually fire out its barbed hairs, causing stinging and irritation to the eyes.

"We had noticed lots of cotton-wool like structures in the surrounding pine trees during the walk, which it turns out are actually a form of a nest that these animals build."

According to reports, the species are native to southern Europe.