Spiders are members of the Arachnida class, which also includes ticks, mites, and scorpions. The jaws of spiders have fangs that deliver venom via a small hole at the tips. Composition, potency, and clinical effects of venom vary among the different spider species.

Almost all species of spiders are venomous, but only a few dozen can harm humans. Of the few spiders that are of medical importance, envenomation can cause a range of clinical manifestations from skin lesions to systemic illness and, in rare cases, even death. Although tarantulas have venom, they usually cause illness from their urticating hairs. 
 
The severity of a spider bite depends on the type of spider, the amount of venom injected, the site of the bite, and the health and age of the patient.

Spiders of medical importance include the following:

Widow Spiders
Spiders of the Latrodectus genus are found worldwide and have neurotoxic venoms, with alpha-latrotoxin as the major component. The black widow spider, Lactrodectus mactans, is the most common widow spider in the United States and is found in woodpiles. Neurotoxic venoms cause systemic symptoms relating to cholinergic and catecholamine excess. The bite is often very painful, and systemic symptoms develop, which include hypertension, tachycardia, palpitations, diaphoresis, anxiety, shortness of breath, hyperthermia or hypothermia, excessive salivation, nausea, vomiting, and severe abdominal pain. The abdominal pain may be misdiagnosed as appendicitis or acute abdomen. The bite may have noticeable fang marks with development of a halo-like lesion around the bite. Female black widow spiders can easily be identified by the characteristic red hourglass figure present on their ventral abdomen.

Recluse Spiders
Spiders of the Loxosceles genus are found worldwide in temperate and tropical regions. Envenomation can cause local necrosis and, rarely, severe systemic symptoms due to a cytotoxic venom composed of the phospholipase enzyme, sphingomyelinase D. Cytotoxic venoms cause local tissue injury and necrosis. The bite is often initially painless. Pain, swelling, bullae, and ischemia develop minutes to hours later. Lesions may eventually ulcerate and become necrotic and gangrenous. Though systemic toxicity is rare, disseminated intravascular coagulation (DIC) may occur. The brown recluse spider, Loxosceles recluse, is regularly and erroneously blamed as the cause of necrotic lesions throughout the United States, although this spider is most commonly found in the Midwest and Southern states. It can be identified by the characteristic violin-shaped figure spanning its dorsal head and thorax.

Funnel-Web Spiders
In the Pacific Northwest, the hobo spider (Tegenaria agrestis), commonly known as the aggressive house spider, is often blamed as the cause of necrotic skin lesions. However, there is only one documented case of hobo spider envenomation causing tissue necrosis. Atrax / Hadronyche species in Australia include the most dangerous spider, the Sydney funnel-web spider (Atrax robustus). The venom of this spider is neurotoxic, capable of producing severe pain at the bite site and systemic symptoms that can rarely be fatal within minutes.

Tarantulas
Tarantulas, of the family Theraphosidae, have relatively harmless bites. However, they can disperse urticating hairs from their abdomens, resulting in local skin reactions, ocular problems, and allergic rhinitis.

Other spiders that less commonly cause significant skin irritation or dermal necrosis are as follows:

Yellow sac spiders of the Cheiracanthium genus are found in North America, Europe, Africa, Asia, Australia, and the Pacific Islands.

Wolf spiders of the Lycosa genus are common spiders found worldwide.

Banana spiders of the Phoneutria genus, of Central and South America, have extremely potent venom that is neurotoxic and can be lethal.

Six-eyed crab spiders of the Sicarius genus are found in Africa and South America and are considered to be extremely venomous but, fortunately, live in remote areas. Their venom is proteolytic.