When I go out at night into the rainforest to search for katydids I don't like to have any company.

Not that I am particularly antisocial, but tracking skittish and cryptic animals is an activity that's better done alone. I walk slowly, trying not to disturb anything and anybody, slowly scanning the vegetation and the forest floor in the light of my headlamp.

Every now and then I turn the light off to fully immerse myself in the ambient sounds of the forest, which often helps me pinpoint a faint trill made by a katydid's wings. A few years ago I was deep in the rainforest of Guyana doing just that — listening to the sounds of the night in a complete darkness — when I heard the rustle of an animal running.

I could clearly hear its hard feet hitting the ground and dry leaves crumbling under its weight. I pressed the switch and pointed the light at the source of the sound, expecting to see a small mammal, a possum, a rat maybe.

And at first this is what I thought I saw — a big, hairy animal, the size of a rodent.

But something wasn't right, and for a split second the atavistic part of my brain sent a ping of regret that I didn't bring any companion with me on this particular night walk. But before that second was over I was lunging at the animal, ecstatic about finally seeing one of these wonderful, almost mythical creatures in person.

Goliath birdeater (Theraphosa blondi) from Suriname, displaying the full arsenal of its defenses – urticating hair, enormous fangs, and a loud hissing noise. Piotr Naskrecki The South American Goliath birdeater (Theraphosa blondi) is the largest spider in the world.

For all the arachnophobes out there this is probably a good excuse to pave over large swaths of the Amazonian rainforest, but for the rest of us this species is one of biodiversity's crown jewels. Although far from being the largest member of the subphylum Chelicerata — this honor belongs to horseshoe crabs — Goliath birdeaters are ridiculously huge for a land arthropod.

Goliath birdeater in its natural habitat in Suriname. Piotr Naskrecki

Their leg span approaches 30 cm (nearly a foot) and they weigh up to 170 g — about as much as a young puppy. They truly are Goliaths, but are they bird eaters? Alas, the truth is a bit less exciting. Although definitely capable of killing small birds, they rarely have a chance to do so while scouring the forest floor at night (however, there is some anecdotal evidence that they may feed on bird eggs if they run across a nest).

Rather, they seem to be feeding on what is available in this moist and warm habitat, and what is available is earthworms — lots of them.

But how do they get to be so big?

Apparently, according to one study (Makarieva et al., Proc. R. Soc. B [2005] 272), it has to do with their metabolic rate, which is lower than in the Goliath birdeater's relatives. This allows it to function with lower levels of oxygen reaching its tissues and organs than those required by smaller, more active spiders.

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.

Regardless of the reason, because of its gargantuan size, the Goliath birdeater is probably the only spider in the world that makes noise as it walks. Its feet have hardened tips and claws that produce a very distinct, clicking sound, not unlike that of a horse's hooves hitting the ground (albeit, admittedly, not as loud). But this is not the only sound this spider makes.

Every time I got too close to the birdeater indt would do three things. First, the spider would start rubbing its hind legs against the hairy abdomen.

"Oh, how cute!", I thought when I first saw this adorable behavior, until a cloud of urticating hair hit my eyeballs, and made me itch and cry for several days.

With the leg span of nearly 30 cm, the Goliath birdeater is an animal that should be treated with respect, even though it is pretty much harmless to humans. Piotr Naskrecki If that wasn't enough, the arachnid would rear its front legs and open its enormous fangs, capable of puncturing a mouse's skull, and tried to jab me with the pointy implements. The venom of a birdeater is not deadly to humans but, in combination with massive puncture wounds the fangs were capable of inflicting, it was definitely something to be avoided.

And then there was a loud hissing sound. For a long time the source of the sound was a mystery, but now we know that it is produced by "setal entanglement" — some of the hairs (setae) on the legs are covered with microscopic hooks that scrape against other, feather-like setae, producing the loud warning hiss.

A couple of years after my first encounter with Theraphosa blondi I was in South America again, walking alone at night in the rainforest of Suriname. Suddenly my foot brushed against something big and moving, and I nearly tripped.

I froze, expecting a snake. "Nah, it's just another Goliath birdeater. Aren't you a cutie pie?"