Mexican Blood Leg Tarantula (Aphonopelma bicoloratum)

Physical description and identification

The Mexican blood leg is a medium-sized terrestrial tarantula with a colour scheme that makes it hard to confuse with anything outside a short list of Mexican species. The name bicoloratum refers exactly to what you see: legs split into contrasting bright and dark sections. Despite the tropical looks, it is a member of Aphonopelma, the genus best known from the deserts of the southwestern United States.

Adults

• Size: a mature spider spans roughly 110–130 mm (about 4½ to 5 inches) across the legs. The body is far shorter than that figure suggests, and females are noticeably heavier and stockier than males, which are leggier and lighter once mature.
• Colour: the carapace is golden to orange-tan, the abdomen dark and densely haired, and the legs orange to blood-red with black or blackish segments — usually the femora — producing the banded look the common name celebrates. Colour is at its most intense in the days after a moult and dulls as the setae wear.
• Eyes: eight small eyes grouped on a single raised tubercle at the front of the carapace, the standard mygalomorph arrangement. Vision is poor and the spider navigates by touch and vibration.
• Distinctive features: a patch of barbed urticating hairs covers the upper abdomen and is flicked off with the hind legs at anything that comes too close — the defining New World tarantula defence. Mature males carry tibial hooks on the front legs and swollen pedipalp tips, and their colours are often duller than those of an adult female.

Eggs and egg sac

Eggs are yellowish and laid onto a silk mat inside the burrow, which the female then folds and binds into a rounded sac. She guards it in the tunnel, turning it regularly, and stays with the clutch until the spiderlings emerge weeks later.

Spiderlings

Newly emerged young measure around 5 mm and are drab compared with adults; the orange develops gradually over successive moults. The smallest spiderlings dig into any small hole or crevice they can find, while larger juveniles become bolder and more openly terrestrial. Reaching adult size takes years, and a female continues to moult for the rest of her long life.

The web and hunting

This tarantula spins no snare. Silk is used to line the burrow and to carpet the ground immediately around the entrance, where it works as a vibration antenna rather than a trap. The spider spends the day underground and waits near the mouth of the burrow after dark; when a cricket, cockroach or beetle crosses the silk, it lunges, bites and pulls the prey back inside. Our page on what spiders eat covers how such ambush hunters differ from web builders.

Where the Mexican blood leg tarantula lives

Aphonopelma bicoloratum is endemic to western Mexico and is known from a small area in the state of Michoacan, in the dry country of the Balsas river basin and neighbouring slopes. That restricted distribution is the single most important fact about the species: everything about its conservation follows from it.

The habitat is often described loosely as desert, but it is better understood as seasonally dry tropical forest and thorn scrub — a landscape of thorny trees, cacti and grasses that is genuinely arid for much of the year and then soaked by a summer rainy season. The tarantula digs burrows in well-drained soil on slopes and banks, or occupies spaces under rocks and among roots, and the tunnel buffers it against both the dry-season heat and the downpours. It is strictly terrestrial, active at night, and completely absent from buildings and gardens outside its narrow native range.

Are Mexican blood leg tarantulas venomous?

Yes, like every tarantula — the venom is what turns a cricket into a meal. It is not medically significant for humans. New World terrestrial tarantulas of this kind have mild venom, no serious envenomations have been documented from the species, and no antivenom exists or is needed.

What deters predators is not the bite but the abdomen. When threatened, the spider rears back and combs a cloud of barbed urticating hairs into the air with its hind legs. Those bristles work their way into skin and mucous membranes and cause itching, redness and small welts, which is enough to send a curious mammal away with a very memorable nose.

Can Mexican blood leg tarantulas bite?

They can, but they are famously reluctant to. A disturbed individual retreats first, throws hairs second, and bites only when it is cornered, pinned or handled. A bite is mostly a puncture wound: the fangs are several millimetres long, so expect sharp local pain, a little bleeding, and redness or swelling that fades within a day or two, comparable to a bee sting.

Wash the site with soap and water, apply a cold pack, and keep the wound clean while it heals. Seek medical care if the swelling spreads well beyond the bite, if you develop hives, faintness or difficulty breathing — an allergic reaction rather than venom toxicity — or if the puncture later shows signs of infection.

Ecological importance and behavior of the Mexican blood leg tarantula

Within its patch of Michoacan scrub this tarantula is one of the larger ground-level invertebrate predators. It hunts by ambush at night, taking crickets, cockroaches, beetles, grasshoppers, other spiders and occasionally small vertebrates, and it can fast for long periods between meals — an essential trait where prey abundance swings with the rains. A healthy population quietly suppresses insect numbers across the ground layer.

Natural predators: spider wasps of the tarantula hawk group are the specialists, paralysing a tarantula and sealing it underground with an egg so the larva can feed on the living spider. Coyotes, foxes, skunks and coatis, snakes, lizards and large birds all take tarantulas as well, and wandering males are the most exposed of all.

Prey-predator dynamics: as both a heavy consumer of insects and a substantial meal for vertebrates, the species links the arthropod layer of the dry forest to its birds and mammals.

Seasonal behavior: females are sedentary and can hold a single burrow for many years, sealing it during the harshest weather. Males abandon their burrows after their final moult to search for females and die within months, so nearly every tarantula seen walking in the open is male. Females, by contrast, commonly live two decades or more.

Relationship with humans: encounters in the wild are rare and harmless, and the spider is docile by tarantula standards. The pressure runs the other way. A species restricted to one small region is vulnerable to land clearance and to collection of wild adults for the international pet trade, and a female removed from a burrow may represent twenty years of growth that the population cannot replace quickly. In captivity the species is kept on deep, dry substrate that allows it to burrow, and it is now bred regularly enough that wild-caught animals are unnecessary.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species name means simply "two-coloured", after the contrast between the bright and dark segments of each leg.
• Most of the famous orange-legged Mexican tarantulas belong to Brachypelma and its relatives; the blood leg is an Aphonopelma, a genus whose other well-known members are the plain brown burrowers of the Arizona and California deserts.
• Spiderlings emerge drab. The orange builds up over years of moults, so a juvenile looks nothing like the adult it will become.
• An adult female's fangs can pierce a fingertip, yet the same animal will usually choose to kick hairs and walk away rather than bite.
• Because Mexican tarantulas are so sought after by collectors, several species are protected by international trade rules — one reason serious keepers insist on captive-bred animals.

In summary, the Mexican blood leg tarantula is a beautiful, harmless, slow-growing endemic of western Mexico whose spectacular colours are also its greatest liability.