WHEN scientist Piotr Naskrecki heard rustling in a rainforest, he expected to see a possum or rat.

But his curiosity turned to shock when a puppy-sized spider bristled under his flashlight.

“When I turned on the light, I couldn’t quite understand what I was seeing,” Naskrecki, an entomologist and photographer at Harvard University’s Museum of Comparative Zoology, told Live Science.

The South American Goliath birdeater is the world’s largest spider, with a leg span of up to 30 centimetres (or the size of “a child’s forearm”) and weighing more than 170 grams.

“Its feet have hardened tips and claws that produce a very distinct, clicking sound, not unlike that of a horse’s hoofs hitting the ground,” Naskrecki wrote on his blog, The Smaller Majority.

Naskrecki, who spotted the colossal creature in a South American rainforest in 2012, saw it rub its hind legs against its abdomen, sending out hairs with tiny barbs on them. He said the barbs are “extremely painful and itchy”.

The highly venomous arachnid also has five-centimetre fangs.

But in good news, even if it bites you, “a chicken can probably do more damage,” Naskrecki said.

Naskrecki caught the female and took her back to his lab to study.

See the South American Goliath birdeater on theLive Science website.