Why Do Spiders Have Venom?

Venom is a biological weapon designed for hunting, defense, and survival. In spiders, its primary purpose is to help kill or paralyze prey.

Spiders produce venom in specialized glands located in the spider’s cephalothorax, just behind the eyes. These glands contain cells that synthesize the components of the venom, guided by the spider’s genetic code. Venom typically includes a mix of neurotoxins, cytotoxins, and enzymes that disrupt nerve transmission or destroy tissue. Venom production can be influenced by the spider’s species, age, and environment. Once formed, the venom is stored until it is needed.

How Do Spiders Deliver Venom?

Spiders deliver venom through their fangs which are located at the front of the mouth. These fangs are connected to its venom glands. During a bite, muscle tissue around the glands contract, forcing venom through ducts and out the fang tips, injecting venom directly into the prey.

Over millions of years, natural selection has fine-tuned spider venom for specific functions. The potency and mechanism vary greatly by species.

How Is Venom Toxicity Measured?

Venom toxicity is commonly measured using a scientific metric called LD₅₀, which stands for “lethal dose, 50 percent.” This refers to the amount of venom required to kill half the members of a tested population, usually lab mice, and is expressed in milligrams per kilogram of body weight (mg/kg). A lower LD₅₀ value indicates higher venom toxicity.

LD₅₀ is widely used in toxicology because it provides a standardized way to compare the potency of venoms across species. However, it has limitations. These values are derived from animal models and may not directly translate to human responses. LD₅₀ also doesn’t consider how much venom a spider can inject, how likely it is to bite, or how easily antivenom can treat a bite. Still, LD₅₀ remains one of the most objective tools scientists have for ranking venomous species by chemical strength.

The World’s 12 Most Venomous Spiders

Here is a list of the most venomous spiders, ranked by venom toxicity to humans (based on LD₅₀ in mg/kg).

1. Sydney Funnel-Web Spider — LD₅₀: 0.16 mg/kg

The Sydney Funnel-Web Spider is native to eastern Australia and is considered the most dangerous spider to humans. It is typically 1 to 2 inches (2.5 to 5 cm) long, with a glossy black or dark brown body and large, powerful fangs. This spider lives in moist, cool habitats such as under logs or in burrows lined with silk. Males become more active during mating season, increasing the risk of human encounters.

Its venom contains a powerful, fast-acting neurotoxin called atracotoxin, which affects the nervous system and can be kill a human within hours. Males are significantly more venomous than females. Immediate medical treatment (antivenom) is required for bites.

2. Brazilian Wandering Spider — LD₅₀: 0.63 mg/kg

The Brazilian Wandering Spider is found in South and Central America, particularly in Brazil. It can grow up to 5 inches (13 cm) in leg span and is known for its aggressive behavior and tendency to roam at night. Unlike web-dwelling spiders, it hunts actively on the forest floor and often wanders into homes, making accidental encounters with humans more likely.

Its venom is a high threat to humans. It contains a potent neurotoxin that can cause muscle paralysis, respiratory distress, and, in some cases, death. The venom also contains a compound that can induce priapism, an effect that has attracted pharmaceutical interest.

3. Six-Eyed Sand Spider — LD₅₀: 0.60 mg/kg

Native to southern Africa, the Six-Eyed Sand Spider is a reclusive species that lives in deserts and arid regions. It is well-camouflaged, often burying itself in sand to ambush prey. It measures about 1.5 inches (4 cm) in body length, with a flattened appearance and a tan or reddish-brown color.

Its venom contains a powerful cytotoxin that causes tissue destruction and blood vessel leakage. Laboratory tests show it can be lethal to rabbits and primates, but confirmed human bites are extremely rare, likely due to the spider’s reclusive nature.

4. Chilean Recluse Spider — LD₅₀: 1.45 mg/kg

The Chilean Recluse is the most dangerous of the recluse spiders and is found in South America, particularly in Chile and Peru. It is tan to light brown, about 0.5 inches (1.2 cm) in body length, with long legs and a violin-shaped mark on its cephalothorax.

It prefers undisturbed indoor spaces such as closets or storage areas. The venom is highly necrotic, containing enzymes that destroy tissue and red blood cells. Bites can result in large skin ulcers and, in severe cases, systemic failure. Fatalities to humans, however, are rare.

5. Black Widow Spider — LD₅₀: 0.90 mg/kg

Black Widows are found worldwide, with the most well-known species in North America. Females are jet black with a red hourglass marking on the underside of their abdomen. They measure about 1.5 inches (3.8 cm) including leg span. They spin irregular webs in dark, sheltered locations like garages, sheds, or basements.

Their venom contains latrotoxin, a neurotoxin that affects nerve transmission and causes muscle spasms, abdominal pain, and systemic symptoms. Despite their reputation, human deaths are almost unheard of today thanks to accessible antivenom and the typically low amount of venom delivered in bites. Antivenom is available, and symptoms usually subside with medical care.

6. Redback Spider — LD₅₀: 0.85 mg/kg

Closely related to the Black Widow, the Redback Spider is native to Australia. Females are black with a prominent red stripe on their upper abdomen and are about 0.4 inches (1 cm) in body length. They build messy, low-lying webs and prefer dry, sheltered environments like sheds or under outdoor furniture.

Redback venom is neurotoxic and causes intense pain, sweating, nausea, and muscle weakness. Fatalities are now extremely uncommon due to the availability and effectiveness of antivenom.

7. Brown Recluse Spider — LD₅₀: 1.57 mg/kg

The Brown Recluse is native to the central and southern United States. It is light to dark brown, about 0.5 inches (1.2 cm) long, and identifiable by a dark violin-shaped marking on its back. It lives in undisturbed indoor areas such as attics and basements.

The venom is necrotic, destroying skin and tissue around the bite site. In rare cases, bites can lead to systemic illness, especially in children or immunocompromised individuals. Most bites heal without treatment, but some require medical care and wound management.

8. Mouse Spider — LD₅₀: 2.5 mg/kg

The Mouse Spider is native to Australia and resembles the Sydney Funnel-Web in appearance and habitat. It has a robust body, often black or dark brown, and females are larger than males, reaching up to 1.5 inches (3.8 cm). They live in burrows and are more aggressive during mating season.

The venom is neurotoxic and can cause symptoms similar to those of the Funnel-Web Spider, though serious envenomations are rare. Some species’ venom is effectively neutralized by Funnel-Web antivenom.

9. Chinese Giant Gold — LD₅₀: 2.6 mg/kg

Also known as the Chinese Bird Spider and Chinese Earth Tiger, this aggressive tarantula species is found in southern China and Vietnam. It has a stocky body, about 2 inches (5 cm) long, with thick legs and dark coloration. It lives in deep burrows and strikes rapidly when threatened.

The venom affects the nervous system, causing severe pain, muscle spasms, and in rare cases, respiratory issues. Its large fangs make the bite mechanically damaging as well. Though not lethal, the effects can be intense and longer-lasting than those of most common spiders, rivaling bites from larger recluse species.

10. Fringed Ornamental Tarantula — LD₅₀: 2.7 mg/kg

Native to Sri Lanka, this tarantula is one of the largest and most venomous of its kind. It has a leg span up to 10 inches (25 cm), with striking black, white, and yellow markings.It lives in tree hollows and is highly defensive if disturbed.

The venom is not fatal to humans but causes intense pain, muscle cramps, and systemic symptoms such as sweating and nausea. Though bites are rare, they require medical attention due to the volume of venom delivered.

11. Yellow Sac Spider — LD₅₀: 3.0 mg/kg

Found across North America, Europe, and parts of Asia, the Yellow Sac Spider is pale yellow or greenish, about 0.4 inches (1 cm) in length. It lives indoors and is active at night, often found on ceilings or walls.

Its venom contains cytotoxins that can cause localized pain, redness, and blistering. Though not deadly, bites are painful and can resemble those of the recluse spiders. It is one of the more common indoor biting spiders.

12. Hobo Spider — LD₅₀: 4.0 mg/kg

Originally from Europe, the Hobo Spider has established populations in the northwestern United States. It is brown, about 0.5 inches (1.2 cm) long, with a mottled abdomen and no distinct markings. It lives in crevices, woodpiles, and basements, building funnel-shaped webs.

The venom is mildly necrotic and can cause localized skin damage. However, recent studies suggest its danger may be overstated. Bites are often misdiagnosed, and confirmed severe cases are rare.