Mexican Red Rump Tarantula (Tliltocatl vagans)

Physical description and identification

The Mexican red rump is a plain spider with one loud feature. The carapace, legs and underside are a uniform velvety black or blackish brown with no bands, rings or pale borders anywhere. The abdomen carries a dense coat of metallic red to coppery setae, so the animal looks as though its back end has been dipped in rust. Nothing else in its range combines a completely unmarked black body with a red rump.

Adults

• Size: a diagonal leg span of roughly 12–16 cm, about five to six inches. The body measures some 5–6.5 cm from the front of the carapace to the spinnerets. Females are heavier and thicker-legged; males are longer-limbed and much lighter.
• Colour: velvety black throughout, with long red-brown setae covering the top and sides of the abdomen. The red fades and the abdomen can become visibly bald where the spider has kicked its defensive hairs away — a worn patch on the rump is normal, not a sign of illness.
• Other features: the body and legs are covered in bristles, including the barbed urticating setae on the abdomen. As in all tarantulas there are two pairs of book lungs, downward-striking fangs and eight small eyes grouped on a low mound. Mature males carry tibial hooks on the front legs and swollen pedipalps.

Eggs and egg sac

The female spins a thick silk sac inside her burrow and fills it with a large clutch — commonly 100 to 300 eggs, with counts approaching 500 reported from big females. She guards and rotates the sac for roughly six to eight weeks, blocking the burrow mouth and feeding little until the young emerge.

Spiderlings

Hatchlings are a pale yellowish tan with no red at all. The rump colour appears gradually with each moult. Spiderlings remain near the mother's burrow briefly before dispersing to dig their own shallow tunnels. Males mature at around three to four years, females at about seven — the slow schedule typical of long-lived mygalomorphs.

The web and hunting

This species builds no capture web. It is an ambush predator that excavates a deep burrow, often under a rock or a root, lines the walls with silk and lays a thin mat of threads across the entrance. Those threads are a vibration sensor: an insect walking over them tells the spider exactly where to strike. During moults and the dry season the burrow is sealed with a silk plug. For the contrast with true snare-builders, see our guide to spider webs.

Where the Mexican red rump tarantula lives

Tliltocatl vagans is a lowland spider of southeastern Mexico and northern Central America. Its core range runs across the Yucatán Peninsula — Campeche, Yucatán and Quintana Roo — and west into Tabasco, Veracruz and Chiapas, then south through Belize, Guatemala, Honduras and El Salvador. Older accounts calling it endemic to Mexico are wrong: a good deal of its range lies outside the country.

It favours open ground with sparse vegetation rather than closed forest — scrubby grassland, dry tropical forest edges, pasture margins, roadside banks and cleared land where the soil drains well and a burrow will hold its shape. Tolerance for disturbed habitat is one reason it is the tarantula people in the region most often meet.

There is also a well-documented introduced population in Florida, in St. Lucie County, established since the 1990s and generally traced to escapes or releases connected with the exotic pet trade. It has persisted for decades but has not spread widely.

Is the Mexican red rump tarantula venomous?

Yes, like every tarantula — and like every other New World theraphosid its venom is not medically significant to humans. There are no recorded deaths, no necrosis and no systemic envenoming syndrome for this species. Clinical reviews of theraphosid bites describe local pain, redness and swelling settling within hours to a couple of days.

The genuine hazard is the abdomen. Tliltocatl vagans is one of the most enthusiastic users of urticating setae in the family: barbed hairs kicked off the abdomen with the hind legs at the slightest disturbance, often leaving a bald patch. On skin they cause an itchy, burning rash that can last for days and sometimes weeks. In the eye they can embed in the cornea and conjunctiva and cause a persistent inflammation that requires an ophthalmologist. Never bring your face close to a disturbed tarantula, and wash your hands before touching your eyes.

Can a Mexican red rump tarantula bite?

It can, and it will if it is cornered — but it is not an aggressive animal, despite older descriptions that say so. What it is, is nervous. A disturbed red rump bolts for the burrow, then rears and flicks hairs, and only strikes when pinned, grabbed or squeezed. The result is mostly a mechanical injury: fangs several millimetres long punching two deep punctures, with local pain, bleeding and swelling that fade in a day or two.

Wash the bite with soap and water, apply a cold pack and take an ordinary painkiller if you need one, then keep the punctures clean while they close. Seek medical care if the pain intensifies rather than settling, if the site becomes hot and spreading, or if you develop a widespread rash, faintness or difficulty breathing — signs of an allergic reaction rather than of the venom. Do not cut, suck or apply chemicals to the wound.

Ecological importance and behaviour of the Mexican red rump

The red rump is a solitary, nocturnal ambush hunter that spends the day underground and the night at its burrow entrance. Its diet is overwhelmingly insects — crickets, beetles, cockroaches, grasshoppers — with small lizards and frogs taken when the chance arises. Claims that it routinely eats rodents overstate the case: a vertebrate meal is an occasional event, not a habit. As a long-lived resident predator it exerts steady pressure on the arthropod community of open lowland soil, and its abandoned burrows shelter frogs, geckos and beetles.

Natural predators: coatis, skunks, armadillos, snakes and large birds take adults, and tarantula hawk wasps of the genus Pepsis paralyse a spider and lay a single egg on it. Spiderlings are eaten by ants, scorpions and other spiders.

Seasonal behaviour: activity peaks with the rains. Mature males leave their burrows and walk long distances looking for females, which is when most people see one crossing a road at dusk. A male seldom lives a year beyond his final moult; a female can keep breeding for two decades.

Relationship with humans: it is harmless to people and a familiar sight around villages and pasture in the Yucatán. Its main contact with us is the exotic pet trade, where its hardiness made it one of the most commonly kept tarantulas — the source of the Florida population. Like the rest of the red-rump group it is covered by CITES Appendix II, so international movement requires permits and documented legal origin.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The Austrian naturalist Anton Ausserer described the species in 1875, originally as Mygale vagans. The epithet vagans means “wandering”, after the roaming males.
• In 2020 Mendoza and Francke split the plain dark-bodied red-rump group out of Brachypelma into a new genus, Tliltocatl, built from the Nahuatl words tliltic (black) and tocatl (spider). That is why almost every older book and label calls this animal Brachypelma vagans.
• The split was based on anatomy and DNA, and it left the brightly coloured red-kneed and fire-legged species in Brachypelma while moving the black ones — including the curly hair tarantula — to Tliltocatl.
• A bald patch on the abdomen is a record of how often the spider has been frightened: each defensive burst strips hairs that only grow back at the next moult.
• The Florida population is one of very few established non-native tarantulas anywhere in the United States, and it has stayed within a small area for nearly thirty years.

In summary, the Mexican red rump is a common, hardy, harmless burrower of open Central American lowlands — unmistakable for its black body and rust-red rump, more likely to shower you with itchy hairs than to bite, and the species whose 2020 move to Tliltocatl renamed a whole group of familiar tarantulas.