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How Big Can Spiders Get?

Social media has been abuzz with Piotr Naskrecki’s shocking photographs of what most news outlets refer to as a “puppy-sized” spider, the goliath birdeater (theraphosa blondi). While the goliath birdeater is undeniably huge—its apple-sized body and foot-long wingspan make it the world’s largest known spider—some readers may not realize that it’s puppy-sized in the sense that its total mass is comparable to that of a newborn puppy, which isn’t really much bigger than the tarantulas most of us have already seen. 

Are there—and have there been—any spiders that are big enough to really change our understanding of what a spider is? 

Specimen of a goliath birdeater tarantula (<i>theraposa blondi</i>). Photo: © 2011 Jedudedek (Wikimedia Commons). Licensed under Creative Commons Attribution-ShareAlike License.

Specimen of a goliath birdeater tarantula (theraphosa blondi). Photo: © 2011 Jedudedek (Wikimedia Commons). Licensed under Creative Commons Attribution-ShareAlike License 4.0.

Not yet. The 1980 discovery of the megarachne—a spider that would have been significantly larger than the goliath birdeater, with a wingspan of up to 19”—turned out, disappointingly, to be the disfigured fossil of a sea scorpion. Had the fossil checked out, the megarachne would have been roughly comparable in size to the Facehugger from the Alien franchise, but still within the general range of what most of us expect a tarantula to be.

Much larger is the legendary Congolese j’ba fofi, which is said to have a wingspan of up to six feet. While no specimens have been found or photographed, accounts from both local communities and foreign visitors have surfaced with some regularity since the 19th century. But as Sciences 360’s Terrence Aym points out, the discovery of a j’ba fofi could raise more questions than it answers:

“As some entomologists have rightly pointed out, spiders of that size would have to overcome the limitations of their exoskeletons. In addition to that hurdle, many of the more primitive arachnids have a primitive book-lung respiratory system. Modern spiders, however, often have a trachea and book-lungs. That combination allows for a smaller heart, more efficient blood flow and greater speed and stamina. If the Congolese giant spiders exist, they would most likely have both trachea and book-lungs.”

Barring travel to remote and heavily forested regions, the goliath birdeater is probably the biggest spider any of us will ever see. For most of us, this is a good thing.     

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Tom Head is an author or coauthor of 29 nonfiction books, columnist, scriptwriter, research paralegal, occasional hellraiser, and proud Jackson native. His book Possessions and Exorcisms (Fact or Fiction?) covers the recent demand for exorcists over the past 30 years and demonic possession.
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