Poisonous spiders are sometimes divided into four broad categories, but there are no exact criteria for each group and it is not considered an exact scientific way of categorizing poisonous spiders.

• Group one: Extremely dangerous poisonous spiders

In this group you will find poisonous spiders capable of killing even adult persons with no underlying health problems. Fortunately enough, antivenom is today available for many spider species from this category. If the bitten person receives medical attention, it is also possible to lower the risk by treating the symptoms even when no antivenom exists. Examples of extremely dangerous poisonous spiders are the Brazilian wandering spider species and the Australian venomous funnel-web spider species.

• Group two: Very dangerous poisonous spiders

In this group you will find poisonous spiders that can cause death or severe injuries in persons who are extra sensitive. The venom of very dangerous poisonous spiders is dangerous for children, the elderly, and people with underlying health problems, especially if the bitten person does not receive medical attention.

• Group three: Dangerous poisonous spiders

The poisonous spiders in this group will rarely cause death, but the venom is still potent enough to cause significant reactions. Generally speaking, few or no deaths have been reported. The reactions can be local as well as systemic. Medical attention is still recommended, especially for risk groups. Medical treatment can also limit the unpleasant symptoms.

• Group four: Painful poisonous spiders

A bite from a poisonous spider in this group will typically cause pain and sometimes swelling and itching around the bitten area. Medical attention is usually not required, unless you are already seriously ill, have an allergic reaction to the venom or suffer from prolonged symptoms.

One example of an extremely dangerous poisonous spider is the Sydney funnel-web spider, Atrax robustus. This is a comparatively aggressive species that will not hesitate to attack if it feels threatened by a human. It is important to remember that this poisonous spider can deliver several bites in a row when agitated. Its native range is limited to a comparatively small region around Sydney in Australia. The Sydney funnel-web spider is often found nesting under houses and other man made buildings, which naturally increase the risk of human encounters. 

The Sydney funnel-web spider has a venom that contains robustotoxin, a toxin highly dangerous to primates - humans included. The venom will open up sodium channels in the body. Interestingly enough, the venom seems to be less dangerous to cats, dogs and other non-primates. Cats and dogs are renowned for being able to neutralise venom from this poisonous spider within 30 minutes. This can be compared humans, where death has been reported within 15 minutes in a small child.

The term poisonous spider is not really correct, since there is a difference between venomous and poisonous. A poisonous animal or plant is harmful when ingested, while a venomous animal or plant will cause harm by injecting venom into another organism. Since most spiders will not cause unpleasant symptoms if ingested, they are not really poisonous. There is however several examples of spiders that are in fact venomous as well as poisonous spiders, since they can cause discomfort in an organism when ingested.