Puppy-sized spider: How big can a bug get?

The enormous Goliath spider, spotted recently in Guyana, conjures images of horror movies. But don't worry: A spider probably can't get that big.

The spotting of a rare Goliath birdeater spider in the Amazon this week may have some wondering how far the boundaries of science can give as this arthropod skitters across into a realm more suited to comic books than science.

It makes some wonder if, perhaps not a Spiderman, but a spider the size of a man, could someday exist.

First described in 1804, the Goliath spider has a leg span of almost a foot and weighs up to a third of a pound, similar to the weight of a young puppy, according to a blog post written by Harvard entomologist and photographer Piotr Naskrecki, who spotted the spider underfoot during a nighttime walk through a rainforest in Guyana.

It is also likely the only spider in the world that makes noise as it walks, due to the “hardened tips and claws that produce a very distinct, clicking sound, not unlike that of a horse’s hooves hitting the ground,” writes Dr. Naskrecki.

The presence of a spider whose presence is announced by a clawing, thrumming skitter is the stuff of horror films.

This is the reverse of the The Incredible Shrinking Man (1957) where the hero is exposed to radioactive toxic waste and shrinks so small he ends up in a pitched battle with a “giant" spider.

However, according to the University of Chicago website called Fathom, we probably don’t have to be worried about anything too massive clacking its way out of the Amazon into, say, your shower, because science forbids it. 

Basically, if you double the size of an animal, you've got just four times the surface area trying to do the biological work – respiration, thermal regulation, not toppling over, etc. – needed to serve eight times the volume.

In other words, a spider the size of a house would probably collapse under its own weight. And because arthropods breathe through the surface of their exoskeletons, such an animal would have to figure out how to get oxygen to its cells far more efficiently than a normal spider can. Also, because animals shed heat through their surface, a house-sized spider would also likely overheat.

Of course, none of this stops filmmakers from tormenting their protagonists with monster spiders. One such filmmaker, Dustin Warburton, creator of the 2013 film Spiders, was ecstatic at the existence of a puppy-sized spider.

“I am so stoked,” Warburton said in a phone interview. “As soon as I get my next break I want to look into this because part of my film was a new species being discovered. Who knows what’s out there!”

In his zeal for gargantuan nightmarish eight-legged freak fodder Warburton speculated, “Maybe these spiders are like goldfish that grow into the environment that they’re in. Bigger bowl. Bigger fish.” 

Nobody knows what the exact upper limit for arthropods is, but the Goliath is probably near the upper range. A "bigger bowl" in this instance would probably mean a planet with lower gravity than Earth but with more oxygen in its atmosphere.

“All I can say is I’m excited,” said Warburton. “Imagine the things it could do!”