Tarantulas Are Big and Hairy But Not So Scary

By: Jesslyn Shields  | 

The Mexican red-kneed tarantula (Brachypelma smithi) is a terrestrial tarantula native to the deserts, scrublands and mountain ranges of Mexico. They are a large species and, like most tarantulas, have a long lifespan. ©fitopardo.com/Getty Images

In the 1990 comedy-horror film "Arachnophobia," many spiders appear on screen, but the movie's main villain is a large, hairy bird-eating tarantula named The General, with a giant nest of babies and his numerous eyes on world domination. The General (sometimes played by real tarantula wearing prosthetics to make it look beefier, and sometimes played by an animatron) is about the size of a 6-week-old kitten and shrieks like a piglet. It's also extravagantly venomous and has the dramatic flair of the lead in a community theater production of "Hamlet."

Tarantulas have a bad rap because of their size, but this family of spiders doesn't want to take over the world. Believe me, they've had plenty of time to try.


Do Tarantulas Vary in Size?

Sure, some tarantulas are huge, but this ancient group of spiders vary in size — some are about the size of a quarter with their legs spread out and others, like the Goliath bird-eating tarantula (which rarely eats birds) can grow up to 4.75 inches (12 centimeters) with a leg span of up to 11 inches (28 centimeters). That's about as big as an arachnid can possibly get, owing to their heavy exoskeletons and limitations of their respiratory systems in today's atmosphere, which is less oxygen rich than the one in which their ancestors evolved.

The Goliath bird-eating tarantula (Theraphosa blondi) can grow up to 4.75 inches (12 centimeters) with a leg span of up to 11 inches (28 centimeters).

"If you look at arthropods, especially insects and arachnids, around the time tarantulas evolved — at least 300 million years ago during the Carboniferous period — everyone was getting larger," says Dr. Rachael Alfaro, research associate at the Museum of Southwestern Biology, Arthropods division. "I believe the current large size of tarantulas stems from their ancestral condition of being a larger-bodied arachnid during a time when their prey was also larger sized."

These days tarantulas are found on every continent except Antarctica — some species are found nearly 15,000 feet (4,572 meters) above sea level in the Andes, some deep inside caves, others in harsh deserts or rainforests. They show up in showstopping colors and patterns, and are ecologically important, serving as top arthropod predators in many ecosystems.


How Are Tarantulas Different From Other Spiders?

About 1,000 species of tarantulas have been described so far, but they also belong to a primitive group known as mygalomorph spiders, which also includes trapdoor spiders and the highly toxic Australian funnel-web spiders. While other spiders have just one set of book lungs (respiratory organs composed of thin sheets of tissue that air circulates between), mygalomorphs have two sets, but lack respiratory tracheae, which most other spiders possess.

"A tarantula's fangs are situated differently than most spiders — they are mostly downward pointing and mostly parallel with respect to each other — in other spiders, the fangs are more 'pincer-like,'" says Dr. Brent Hendrixson, a biology professor at Millsaps College.

Most New World tarantulas (from either North or South America) have unique specialized hairs, also known as urticating hairs, on their abdomen that can be flicked off or just released into the flesh of a potential predator — think arthropod porcupine quills — which can cause serious irritation in whatever animal is messing with it.

"Old World tarantulas (from any continent other than North or South America) don't possess urticating hairs and are typically more aggressive, in a bite first and ask questions later kind of way," says Alfaro. "New World tarantulas tend to bite as a last resort."


How Long Do Tarantulas Live and What Do They Eat?

Tarantulas are extremely long-lived for spiders, with females outstripping the males in the longevity game: the females of some species can live up to 30 years, while the males live only about seven. They eat a wide variety of small animals, from bugs, to bigger stuff like frogs, lizards, mice and occasionally — occasionally — even birds.


Where Do Tarantulas Live?

Some tarantulas live in trees — these arboreal species are usually lighter and less cumbersome than the terrestrial species because they have to be able to quickly scurry around tree branches after prey. Arboreal tarantulas live in silken tube tents they spin for themselves.

Terrestrial tarantulas are larger and are great diggers. They sometimes build their own burrow, enlarging it as they grow, or commandeer something that's been abandoned by another animal. They have powerful mouthparts, which they use for scraping the walls of their burrow, carrying away dirt with their front legs, mouthparts and pedipalps — specialized mouth structures that arthropods have that are sort of like mouth-legs.

"It is not well understood if tarantulas remain in the same burrow for their entire lives," says Hendrixson. "For tarantulas that build their own, they probably start building their burrows shortly after dispersing from their mother's burrow. Most burrowing tarantulas — at least in North America — are very tidy. You can often find excavation mounds made up of soil, silk, leftover food and leftover bits of exoskeleton just outside the burrow entrance. Many species will place a thin veil of silk over the burrow entrance during daylight hours. This behavior is not well understood but may deter diurnal predators and reflect sunlight, to keep temperatures cooler and maintain higher humidity."


Are Tarantulas Dangerous to Humans?

Tarantulas are big and scary looking, but, in reality, the venom of these largest-of-all-spiders has a very low toxicity to humans. The most dangerous thing about these spiders is the irritating hairs of the abdomen (in New World species), which can cause inflammation of eyes and nasal passages and possible skin rashes. The best advice to avoid these problems is to wash your hands after handling a tarantula and, as if you needed to be told, try your best to keep tarantulas away from your face. Good advice for sure.