[4] Phoneutria are easily confused with several other non-medically significant ctenids, especially Cupiennius, in which the recently described C. chiapanensis also has bright red hairs on the chelicerae.

[7][8] Additionally, some Phoneutria species lack red hairs on the chelicerae, making it an unreliable identification feature.

[8] The characteristic defensive posture with frontal legs held high is an especially good indicator to confirm a specimen is Phoneutria,[4] especially alongside correct colour patterns.

P. nigriventer mates during the dry season from April to June, which leads to frequent observations of the species during this time.

Phoneutria are found in forests from Costa Rica southwards throughout South America east of the Andes including Colombia, Venezuela, the Guianas, Ecuador, Peru, Bolivia, Brazil, Paraguay, and into northern Argentina.

Spider mouthparts are adapted to envenomate very small prey; they are not well-adapted to attacking large mammals such as humans.

[20][21] Symptoms may appear within 10 to 20 minutes after the bite, and death within two to six hours, where severe pain radiates to the rest of the limb, systemic effects include tachycardia, increased blood pressure, vertigo, fever, sweating, visual disturbances, nausea, vomiting, difficulty breathing and paralysis.

A 45-year-old man, with no nervous background, employed in the agricultural section of the Butantan Institute, working barefoot, was bitten at 10:40 AM on the small toe of his left foot by a medium-sized Phoneutria.

He was sweating profusely, there was hyper nasal secretion and salivation, which made him blow his nose and spit constantly.

He was agitated, with generalized tremors and continuous cramps in his left foot and leg, and an irregular pulse with 112 beats per minute.

[24] A 23-year-old market worker moving a bunch of bananas was bitten on his hand by P. nigriventer in São Paulo, Brazil.

It was reported that the wound was extremely painful, with the victim noticing that the bite area was sweating and the hair on his skin stood on end.

Immediately after the bite he experienced severe local pain, blurred vision, profuse sweating and vomiting.

He received anesthetic infiltration, dipyrone and two vials of serum, the right hand remained red and swollen, with improvement in anxiety symptoms and controlled blood pressure.

[29] Another case occurred in Minas Gerais, Brazil, where a man bitten by a Phoneutria developed numbness in his legs, redness, headache and loss of sense of time and space.

[30] In a case that occurred in Itanhaém, São Paulo, a 40-year-old man bitten in the foot, presented significant pain and generalized contractures, dying six hours after the accident.

[31] A 7-year-old child, bitten in the ear, presented convulsions, opisthotonos and progressive paralysis, dying 17 hours after the accident.

[31] In a case that occurred in Franca, a 10-year-old child bitten on the middle finger of the right hand, presented severe pain, trismus, tremors in the right arm and face, evolving to permanent contracture, respiratory paralysis, cyanosis and convulsions, dying in 30–40 minutes after the accident.

The father removed the sheets and found the spider, which was referred to the Butantan Institute, identified as a large female Phoneutria nigriventer.

Afterwards, there was disseminated pulmonary stertoration, and worsening of agitation, dyspnea and bradycardia, while being performed orotracheal intubation, manual ventilation, adrenaline and external cardiac massage, she later died.