The Goliath birdeater tarantula is a spider as large as a puppy, weighing as much as six ounces, with fangs up to one inch long. The species is the largest spider on the planet, according to the Guinness Book of World Records.  

Although the venom of the bird-eating tarantula is not fatal to humans, the massive fangs can cause a great deal of pain. Some victims of attacks by the creatures experience profuse sweating, as well as nausea.

"Another serious weapon in a tarantula's arsenal is its ability to release hairs (actually hairlike setae) from its body at any creature - including a human - it perceives as a threat. The tiny, almost invisible hairs that it voluntarily releases into the air are extremely irritating to skin, and can cause real problems if they get into delicate mucous membranes around eyes or mouth," the Smithsonian National Zoological Park reports on a Web page detailing the giant insects.

Piotr Naskrecki, an entomologist and photographer at the Museum of Comparative Zoology managed by Harvard University, was hiking through a forest in Guyana at night, when he heard a noise. Turning on a light, the researcher expected to find a rat or other small animal. Instead, he found himself face to face with one of the largest spiders in the world.

Goliath birdeaters do not build complex webs, nor do they wait for their meal to come to them. Members of the species prowl the forest floor, looking for prey. When a target is spotted, the giant spiders pounce, injecting frogs, worms and other small animals with venom. Fangs of the spider are powerful enough to pierce the skull of a mouse. For humans, the effect of being struck by one of the creatures has been compared to having a nail driven through the palm of a hand.

The Goliath birdeater is also capable of rubbing hind legs together, creating a sound much like Velcro being ripped apart. This defense mechanism is known to biologists as stridulation.

The giant spider is extremely rare, and becoming rarer, due to encroachment on its habitat by human civilization. Some biologists believe the giant huntsman spider is larger than the birdeater, but that animal is nowhere near as massive as its cousin.

"I've been working in the tropics in South America for many, many years, and in the last 10 to 15 years, I only ran across the spider three times," Naskrecki said.

The spider found by the Harvard University researcher was a female, who was captured, and was sent to a museum for public display.