Tarantulas are large, hairy spiders of the family Theraphosidae. More than a thousand species live in warm parts of the Americas, Africa, Asia, Australia and southern Europe, from Arizona deserts to the canopy of the Amazon. Despite their reputation, most are shy, slow animals that would rather back away than bite, and for a healthy adult the venom of most species is roughly as painful as a bee sting.
The name first belonged to a different animal. Medieval Italians called the wolf spiders around the port of Taranto tarantola, and blamed their bites for a frenzy that could only be cured by dancing the tarantella. European travellers later gave the name to the much bigger hairy spiders they met in the Americas, and it stuck.
Anatomy
A tarantula's body has two parts joined by a narrow waist called the pedicel. The front part, the cephalothorax, is covered by a hard shield, the carapace. Its eight small eyes sit together on a little mound near the front, but eyesight is poor, and the spider learns about the world mostly by touch.
Each of the eight legs has seven segments: coxa, trochanter, femur, patella, tibia, metatarsus and tarsus. The knee is the joint at the patella, which is why the Mexican red-knee's orange bands sit where they do. Dense pads of hair called scopulae on the feet, along with tiny claws, let many species walk up glass.
The hairs are the real sense organs. Some detect vibrations through the ground, some feel the faintest air currents, and some on the feet and pedipalps are chemical sensors. A tarantula really does taste with its feet, which is exactly what your crawler does when it walks across a sentence.
In front of the legs sit the pedipalps, which look like short legs and are used to taste and handle food. In adult males their tips are swollen into bulbs used to transfer sperm. Below them are the chelicerae, which end in fangs that strike straight downward.
Defence
Many tarantulas from the Americas carry patches of barbed urticating hairs on the abdomen. When threatened, the spider turns and kicks these off with its back legs, sending a small cloud of itchy bristles toward the attacker's eyes and nose. A bald spot on a pet tarantula's rump usually means it has been flicking.
Species from Africa and Asia lack these hairs. They tend to rely on a threat posture instead: the spider rears up, raises its front legs and pedipalps, and shows its fangs. Some species can hiss by rubbing stiff bristles together, a sound called stridulation. Tap your crawler and it will show you both.
Molting and growth
A tarantula grows by shedding its whole exoskeleton. Before a molt it stops eating and often spins a silk mat, then rolls onto its back, which regularly alarms first-time keepers who assume it has died. Over several hours it pulls itself free, leaving behind a hollow copy that includes the linings of the fangs and the book lungs.
Molting lets tarantulas repair themselves. A lost leg grows back over the next few molts, small at first and a little larger each time.
Life span
Females are some of the longest-lived spiders. Many species live 15 to 30 years, and keep molting as adults. Males grow faster and reach maturity in a few years, but once they mature they stop molting, leave the burrow to search for females, and usually die within a year or two.
Hunting and silk
Tarantulas do not spin catching webs. Most are night-time ambush hunters that wait at the mouth of a burrow or a silk retreat and feel for footsteps through threads laid on the ground. Prey is mostly insects and other arthropods, though larger species also take frogs, lizards and the occasional small mouse.
Silk still matters. They use it to line burrows, build tube retreats in trees, wrap eggs in a sac, and make the mats they lie on during a molt. Adult males weave a small sperm web before courting. The greenbottle blue is famous for filling its whole enclosure with silk.
The name birdeater comes from an engraving published in 1705 by the naturalist Maria Sibylla Merian, which showed a large tarantula eating a hummingbird in Suriname. Birds are a rare meal, but the name stuck to the whole group of giant South American species.
Species your crawler can be
| Common name | Scientific name | Native range | Known for |
|---|---|---|---|
| Mexican red-knee | Brachypelma hamorii | Pacific coast of Mexico | Orange knee bands, calm temper, long life |
| Greenbottle blue | Chromatopelma cyaneopubescens | Paraguaná Peninsula, Venezuela | Metallic blue legs, orange abdomen, heavy webbing |
| Cobalt blue | Cyriopagopus lividus | Myanmar and Thailand | Deep burrower, no urticating hairs, quick to rear up |
| Goliath birdeater | Theraphosa blondi | Northern South America | Heaviest spider alive, leg span near 28 cm, loud hiss |
| Pink-toe | Avicularia avicularia | Northern South America and Trinidad | Lives in trees, pink tips on every foot |
The sixth option, the neon ghost, is not a real animal. It draws your tarantula as glowing lines, which reads well on bright websites.
How your crawler works
Your crawler is drawn fresh sixty times a second. Each leg plants its foot and only lifts it when the body has moved too far ahead, and neighbouring legs wait for each other, so it walks with the rippling gait of a real spider. When a foot lands on a word, the crawler reads that word straight from the page. Some words it keeps.
While it rests, it feeds the words it has kept into a tiny language model built from the text of the page it is on: a table of which word tends to follow which. Starting from a word it tasted, it strings together a short phrase and lets the words drift off its back. On a cooking blog it will mumble about onions; on the news it will mumble about the news.
Further reading
The World Spider Catalog, kept by the Natural History Museum Bern, lists every described spider species, including all tarantulas.