Phoneutria nigriventer

Physical description and identification

Phoneutria nigriventer is the best studied of the nine Brazilian wandering spiders and the one that accounts for the great majority of recorded bites. It was described by the Baltic-German arachnologist Eugen von Keyserling in 1891, in a volume of Brazilian spiders published after his death. The epithet nigriventer means "black belly".

Adults

• Size: females measure roughly 30–48 mm in body length and males about 25–35 mm, with a leg span usually 13–15 cm and occasionally more. The commonly repeated "about 5 cm" refers to the largest female bodies, not to the whole spider.
• Colour: grey-brown to olive-brown above, densely clothed in short hairs, with paired pale spots running down a darker abdomen and faintly banded legs. The underside of the abdomen is dark — black in adult females, often with reddish or orange markings — which is where the name comes from and which is a genuinely useful diagnostic character.
• Eyes: eight eyes in the ctenid 2-4-2 arrangement: two small front eyes, four in a middle row, two set back at the sides.
• Distinctive features: dark chelicerae carrying a dense brush of red-orange hairs, and long legs with strong paired spines underneath.
• Threat posture: the surest field sign. A disturbed spider lifts the front of its body almost vertical, raises and spreads the first two pairs of legs, and sways from side to side with the red jaw hairs exposed. This is the armadeira display that gives the group its Brazilian name.

Eggs and egg sacs

The female spins a flattened white silk sac in a sheltered crevice or rolled leaf and guards it closely. A sac holds several hundred to more than a thousand eggs — far more than the figure of about a hundred that circulates online — and a female may produce several sacs over her life.

Spiderlings

Spiderlings emerge pale, disperse within days to weeks, and hunt fruit flies and other minute insects straight away. They grow through many moults and reach maturity in roughly a year. Even tiny juveniles perform the full raised-legs display.

Hunting instead of a web

This species builds no capture web. It rests by day inside banana crowns, bromeliads, rolled leaves, woodpiles and rubble, and hunts after dark on the ground and in low vegetation, locating prey by touch and vibration and seizing it in a short rush. The diet is large-bodied: crickets, katydids, cockroaches, other spiders, and occasionally tree frogs, small lizards and young mice.

Where Phoneutria nigriventer lives

The species is South American and, unlike some of its relatives, is not an Amazonian spider. Its stronghold is southeastern and southern Brazil — Minas Gerais, Espírito Santo, Rio de Janeiro, São Paulo, Paraná, Santa Catarina and Rio Grande do Sul — extending into Paraguay, northeastern Argentina and, at the margins of the range, Uruguay. The Amazonian wandering spiders are P. fera and P. reidyi, while P. boliviensis and P. depilata occupy the north and west of the genus range.

Its original habitat is Atlantic forest and its edges, but this is one of the most successful synanthropic spiders in South America. It thrives in banana plantations, gardens, sheds, rubble and building sites, and in cities it turns up in garages, stacked timber, boxes and — the classic scenario — inside shoes and clothing left on the floor. That tolerance for human habitat is precisely why it bites more people than any other South American spider.

Is Phoneutria nigriventer venomous?

Yes, and seriously so. Its venom is a mixture of neurotoxic peptides sorted into fractions called PhTx1, PhTx2 and PhTx3. The PhTx2 group keeps voltage-gated sodium channels open, so nerve endings fire repeatedly and dump neurotransmitter; this drives the immediate, radiating pain and the autonomic effects — sweating, salivation, blood-pressure and heart-rate changes. The PhTx3 family blocks voltage-gated calcium channels and the release of glutamate. One PhTx2 component is responsible for priapism and is now studied as a lead compound for erectile-dysfunction drugs.

Proportion matters. P. nigriventer has held a Guinness listing as the world's most venomous spider, which reflects how little venom kills a laboratory mouse rather than how many people it harms. Brazilian hospital series of several hundred verified bites consistently find that around nine in ten are mild — local pain and swelling only — fewer than one in ten are moderate, and roughly one in two hundred is severe, almost always in a young child. Deaths are exceptional: Brazil records thousands of wandering spider bites a year and only a handful of deaths across recent decades. A specific antivenom, produced by the Instituto Butantan, has been available since the mid-twentieth century. See our list of the most venomous spiders for context.

Bites: symptoms, timeline and what to do

This spider does not seek people out. Almost every bite is defensive and happens when it is pressed against skin — a foot pushed into a shoe, a hand into a woodpile or a banana bunch, an arm into stored boxes. Most bites are on hands and feet, and they peak in the Brazilian autumn, roughly March to May, when mature males wander in search of females.

Typical timeline. Pain starts at once, is often severe, and radiates up the limb, with swelling, redness and sweating around the puncture; in mild cases that is all, and it eases over a few hours. Moderate envenomation adds generalised sweating, agitation, vomiting, a racing heart, raised blood pressure, tremor and blurred vision, usually within the first one to two hours. Severe envenomation — rare, and overwhelmingly in small children — can bring profuse sweating, an irregular heartbeat, low blood pressure, seizures and pulmonary oedema, and requires intensive care. Priapism, a prolonged painful erection, is a characteristic sign in men and boys.

What to do: go to an emergency department, and do so urgently for any child. Keep the person calm and still, wash the site with soap and water, and if it is safe, photograph or contain the spider for identification. Hospitals control the pain by infiltrating a local anaesthetic around the bite; antivenom is reserved for moderate and severe cases, particularly in children, and works quickly. Do not apply a tourniquet, cut or suck the wound, or give alcohol.

Ecological importance and behavior

P. nigriventer is a solitary nocturnal hunter and one of the larger invertebrate predators of the Atlantic forest and its plantations. By taking cockroaches, crickets, katydids and other spiders on foot, it removes prey that no web-building spider intercepts, and in agricultural settings that role is genuinely useful.

Natural predators: spider wasps paralyse adults to stock their nest burrows, and coatis, opossums, birds, snakes and larger spiders all take them. Ants and parasitoid wasps attack the egg sacs.

Prey-predator dynamics: the threat display evolved against these predators, not against people. A spider that cannot outrun a coati gains by looking twice its size. Biting is the last resort, and a substantial share of defensive bites inject little or no venom, which is one reason so many recorded bites stay mild.

Seasonal behaviour: adults mature towards the end of the warm season; males then abandon their retreats and wander at night, which produces the autumn peak in both sightings and bites. Females live one to two years, sometimes longer, and stay close to a favoured shelter between hunting trips.

Relationship with humans: close, and manageable. The precautions are unglamorous and they work: shake out shoes and clothing, wear gloves for firewood and banana harvesting, keep floors clear of piled fabric, and never try to catch one by hand.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• Brazil's notification system for venomous animals makes Phoneutria bites among the best documented anywhere — which is exactly why the real severity figures are so much lower than the species' online reputation.
• Mid-twentieth-century practice was to give antivenom to almost every patient; modern Brazilian guidance reserves it for moderate and severe cases, because most bites settle with pain relief alone.
• The priapism-inducing toxin has been developed into experimental treatments for erectile dysfunction — a dangerous venom turned into a drug lead.
• Keyserling never saw the published description: the volume of Brazilian spiders containing it appeared after his death.

In summary, Phoneutria nigriventer earns its reputation as one of the few genuinely dangerous spiders, and also disproves the worst of it. Treat a bite as an emergency, get to a hospital, and know that the overwhelming majority of people who are bitten go home the same day.