Imagine encountering an extraordinarily big spider that weighs as heavy as a puppy and has fangs as long as 2 inches. Unfortunately, if you are afraid of spiders, such creature is not something out of Hollywood movies or made up to make you scared for the coming Halloween because this enormous spider does exist.

A few years ago, entomologist Piotr Naskrecki, from the Museum of Comparative Zoology at Harvard University, was out alone at night in the Guyana Rainforest looking for katydids, a nocturnal insect, when he heard a sound of what seemed to be a small animal running, possibly a rat or a possum.

When he focused the light to where the rustling sound comes from, he indeed saw a rat-sized hairy animal but it was not something he was expecting. What he saw made him regret not bringing along with him a companion that particular night. It was a South American Goliath bird-eater, named by the Guinness Book of World Records as the world's largest spider.

The enormous arachnid has legs that can reach up to 30 centimeters in length, which is about the size of the forearm of a child. The spider's weight can also reach up to 170 g which is comparable to that of a young puppy, making them remarkably large when compared to other arthropods, invertebrates characterized by their exoskeleton, jointed legs and appendages.

Naskrecki said that based on a study, the reason why the South American Goliath bird-eater gets so big is because of their metabolic rate, which is lower compared with that of their other relatives.

"This allows it to function with lower levels of oxygen reaching its tissues and organs than those required by smaller, more active spiders," Naskrecki wrote. "In other words, the bigger the body the more difficult it is to provide oxygen to all its parts if the metabolic rate is to remain constant."

As their name suggests, the Goliath bird-eating spider, which belongs to the tarantula family, is capable of killing small birds but they do not always have the chance when wandering in the forest at night, according to Naskrecki. Instead, this colossal tarantula mostly feed on what is available in their habitat.

The Natural History Museum said that the tarantula often feeds on insects including beetles and crickets as well as small mammals, reptiles and frogs. Naskrecki said these gigantic spiders also feed lots of earthworms.