The Tarantula Is Big and Hairy But Not So Scary

Some tarantulas are huge, but these ancient spiders come in all sorts of sizes. Some are about the size of a quarter with their legs spread; others, like the Goliath bird-eating tarantula (which rarely eats birds) can grow body length up to 4.75 inches (12 centimeters) with a leg span of up to 11 inches (28 centimeters).

The pinkfoot Goliath spider is another species with a diet similar to the bird-eater. Both can be found in the jungles South America. The female tarantula typically grows larger than a male tarantula of the same species, thanks to her longer lifespan.

"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.

The Largest Tarantula: The Goliath Bird Eating Spider

The goliath bird eating spider is 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. This largest species of tarantula are nocturnal hunters, lazing about in their deep burrow in the ground during daytime.

While this tarantula has been known to hunt birds, it's more likely to catch easier prey such as cockroaches, small lizards, mice or snakes. When threatened by natural enemies, the tarantula is able to rub its thick hairs together to make a hissing sound that's meant to warn the other species to back off before a fight.

While other spiders have just one set of book lungs (respiratory organs composed of thin sheets of tissue that air circulates between), tarantulas have two sets, but lack respiratory tracheae, which most other spiders possess.

About 1,000 species of tarantulas have been scientifically described so far, and they all belong to a primitive group known as mygalomorph spiders, which also includes trap-door spiders and the highly toxic Australian funnel-web spiders. All mygalomorphs have the same respiratory setup as tarantulas.

Another Difference to Sink Your Teeth Into

"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 tarantulas from either North or South America have specialized hairs — also known as urticating hairs — on their abdomen that can be flicked off or just released into the flesh of potential predators such as spider-eating birds.

Think arthropod porcupine quills or barbed bristles, 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."

Tarantulas are extremely long-lived for spiders, with male tarantulas typically living shorter lifespans. 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.

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. A terrestrial tarantula will sometimes build its own burrow, enlarging it as the spidey grows, or commandeer something that's been abandoned by another animal.

The members of the terrestrial variety 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 of arthropods 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," Hendrixson continues. "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."

Tarantulas are big and scary-looking, but, in reality, the tarantula venom has a very low toxicity to humans. Larger tarantula species tend to have more mild venom.

Tarantula bites will be painful, similar to a wasp sting, but won't cause long lasting or dangerous effects unless a specific allergic reaction occurs. Tarantulas also tend not to be aggressive to humans and typically won't bite unless provoked.

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.