Mexican Flame Knee Tarantula (Brachypelma auratum)

Physical description and identification

The Mexican flame knee is a stocky, ground-living tarantula of the family Theraphosidae, and like every tarantula it belongs to the infraorder Mygalomorphae, whose fangs strike downward in parallel rather than pinching together from the sides. It sits in Brachypelma, the small Mexican genus of banded, burrowing tarantulas, and it is regularly confused with its red-kneed relatives.

Adults

• Size: a mature female averages a leg span of about 14–15 cm (5½–6 in). Males match the span but are much lighter, with a slim abdomen and longer legs.
• Colour: the body, femora and feet are black or near-black, the carapace dark with a paler border. The patellae — the knees — are a vivid flame orange, and each one is divided lengthwise by a dark stripe. That stripe is the species' signature; the red-kneed Brachypelma have broad, unbroken orange bands edged in cream instead.
• Eyes: eight small eyes grouped on one raised tubercle at the front of the carapace. They do little; the spider works by vibration and by the touch hairs covering its legs.
• Distinctive features: dense setae cover the legs and abdomen, and the abdominal patch of barbed urticating setae is the animal's main weapon. Mature males carry swollen palpal bulbs and hooked spurs on the front tibiae.

Eggs and the egg sac

After mating the female spins a thick white silk sac in the burrow chamber and fills it with several hundred eggs — commonly somewhere between 300 and 1,000. Claims of clutches up to two thousand are well beyond what this genus normally produces. She guards the sac and turns it regularly for roughly eight to nine weeks before the young appear.

Spiderlings

Spiderlings emerge pale and unmarked and are markedly skittish, digging themselves in far more readily than adults do. The flame knees are not there at the start: orange builds up across successive moults over the first years, and the dark patella stripe becomes obvious only as the spider matures. Females need roughly six years to reach maturity, males three to four.

The web and hunting

No snare is built, but silk is central to the animal's life. It lines the burrow walls, forms a carpet of trip-threads outside the entrance that carries the footfall of passing insects, provides a flat mat for moulting and wraps the eggs. Hunting is pure ambush: the spider sits inside the doorway after dark and lunges at whatever crosses the silk. For how this differs from a true capture web, see our guide to spider webs.

Where the Mexican flame knee tarantula lives

Brachypelma auratum is endemic to western Mexico, recorded chiefly from the states of Michoacán and Guerrero on the Pacific side of the country. Like most of its genus it has a small, patchy distribution tied to particular hill country and river drainages rather than a broad range, which is part of why the whole genus is legally protected.

The habitat is dry: tropical deciduous forest, thorn scrub and rocky slopes that stand leafless through the long dry months. Within it the spider is a shelter specialist, living under fallen logs and stones and in cavities among tree roots, in banks and in old rodent tunnels, which it enlarges and lines with silk. Firm, well-drained soil matters more than vegetation, and populations concentrate on slopes, ravine sides and cuttings. Well north of its range, in the deserts and uplands of northern Mexico and the United States, the same burrowing role is filled by the duller species of Aphonopelma.

Are Mexican flame knee tarantulas venomous?

Yes, but mildly. The venom exists to kill and pre-digest insects and is not medically significant to humans. No death has ever been attributed to any Brachypelma species, no antivenom exists because none is needed, and a bite causes local pain, itching and swelling that settle within a day or two.

The defence that actually causes injury is the hair. The abdomen carries barbed, detachable urticating setae, and a disturbed spider turns its back and rakes its hind legs across the abdomen, throwing a fine cloud into the air. On skin they produce itching, burning and a raised rash lasting a week or more; inhaled, they irritate the airways. The serious risk is ocular: setae that reach the eye can embed in the cornea and cause ophthalmia nodosa, a granulomatous inflammation that is hard to treat and has required surgical removal. Wash your hands after any contact with a New World tarantula or its enclosure and keep them away from your face.

Can Mexican flame knee tarantulas bite?

They can, though it takes real provocation. The escalation is consistent: retreat into the burrow first; if cornered, turn away and kick hairs; if that is ignored, rear with the front legs raised and the fangs bared; and only then strike.

Descriptions of the bite as "a small pinch" understate it. The wound is mainly mechanical, and the fangs of a large female leave two deep punctures that bleed and ache like thorn stabs, with local pain, warmth and swelling fading inside 24 to 48 hours. Systemic symptoms are not expected from this genus; if they appear they indicate an allergic reaction rather than the venom. Wash with soap and water, apply a cold pack, and watch for spreading redness or discharge, which mean infection. Seek medical care if pain is severe, if a fang tip has broken off in the wound, or if there is breathing difficulty, facial swelling or dizziness. Never cut, suck or apply a tourniquet to a tarantula bite.

Ecological importance and behavior of the Mexican flame knee tarantula

The flame knee is solitary, territorial and strictly nocturnal, spending the day plugged into a burrow or wedged under a stone and emerging at dusk to wait at the entrance. Juveniles burrow far more readily than adults, which is a straightforward response to being small and edible.

Diet: in the wild it takes whatever ground-living arthropods pass — cockroaches, crickets, grasshoppers, beetles and moths — with other spiders and the occasional small frog or lizard taken opportunistically. Prey is crushed, flooded with digestive fluid and sucked dry, and the discarded pellet outside the doorway is the easiest sign of an occupied burrow.

Natural predators: the specialist enemy is the tarantula hawk wasp, which paralyses the spider and lays a single egg on the living body. Coatis and skunks dig burrows open, and snakes, large lizards and ground-foraging birds take wandering spiders.

Prey–predator dynamics: a long-lived ambusher removes a steady flow of large insects from one patch of hillside for two decades while feeding the animals able to breach its defences, and its tunnelling turns the surface soil in a small way. Field observations from western Mexico have reported small frogs of the genus Eleutherodactylus sharing tarantula burrows; similar commensal associations are documented elsewhere in the Americas, though the details here are not well studied.

Seasonal behaviour: mature males abandon their burrows during the rains and walk in search of females, often crossing roads and rarely surviving the season. Females live twenty to twenty-five years, moulting and breeding throughout.

Relationship with humans: the species is best known through the pet trade, and that trade is why it is regulated. All Brachypelma have been listed on CITES Appendix II since 1994, the species is protected under Mexican wildlife law, and legitimate specimens are captive-bred. Handling is discouraged: a heavy tarantula that falls even a short distance can rupture its abdomen.

Similar tarantulas and how to tell them apart

Quick facts

Did you know?

• The German arachnologist Günter Schmidt described the species in 1992, and for years before that it circulated simply as a colour variant of the Mexican red knee.
• The dark line through each orange patella is the quickest way to name it. If the knee band is clean and edged in cream, you are looking at a red-kneed species instead.
• The genus has been redrawn twice in recent years: a 2017 revision by Mendoza and Francke separated B. smithi from B. hamorii, and a 2020 revision moved the red-rump species into the new genus Tliltocatl, leaving Brachypelma as a compact group of Mexican endemics.

In summary, the Mexican flame knee is a slow, long-lived burrower of the Michoacán and Guerrero hills, black everywhere except for two striped orange knees per leg. Its venom is mild and its bite rare; the defence worth respecting is the cloud of urticating hairs, and the pressure worth worrying about is collection from a range that was never large.