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North America's most venomous spiders

Of the 35,000 species of spiders described worldwide (out of an estimated 170,000 species), only a handful are considered to be dangerous and only 27 are known to have caused human fatalities.

Though most people have a big fear of spiders, it seems that venomous spiders are not such a big threat to humans as many people believe.

First of all, studies have shown that most serious bites attributed to spiders were, in fact, due to other causes, notably other arthropods.
Secondly, antivenin against spider bite has become very effective and thus made the occurrence of death resulting from a spider's bite a very rare thing.
Lastly, spider venom, in general, is usually almost harmless to humans, since it evolved for capturing or killing small invertebrates and not big mammals like ourselves.

There are, however, a few exceptions, and bites from certain species can cause severe dermatological lesions and other symptoms which require urgent medical treatment and care.

3,000 species of spiders have been described in North America.
Of the spiders found in North America, only the Loxosceles and Latrodectus species are considered really dangerous and an average of 4 people die from venomous spider bites a year in the USA.

1. Recluse Spiders (Loxosceles)

11 species of Recluse Spiders are found in North America.

- Brown Recluse Spider (Loxosceles reclusa)

This highly venomous spider is thought to be the most dangerous Recluse Spider. It is found in the USA, from the Atlantic to the Pacific coast, and mainly in the south, in an area with radius of 2000 km measured from the center of Arkansas: south-eastern Nebraska, Kansas, Oklahoma, Texas, Louisiana, Arkansas, Missouri, Kentucky, Tennessee, Mississippi, Alabama, northern Georgia, southern portions of Ohio, Indiana, Illinois, and Iowa.

This species measures 6 to 18 mm (1/4 to 3/4 inch) in body length. A dark violin shape is located on the top of the leg attachment region with the neck of the violin pointing backward toward the abdomen. When most spiders have 8 eyes, Recluse Spiders has only 6 arranged in pairs - one pair in front and a pair on either side.

The Brown Recluse Spider's venom can cause significant cutaneous injury with tissue loss and necrosis, and can be deadly to humans. However, though it is very dangerous to people, it is not an aggressive species and it only bites when threatened.

- Brown Violin Spider

This close relative to the Brown Recluse Spider is found in Hawaii.

2. Widows (Latrodectus)

This genus comprises thirty-two identified species. It is found worldwide and in places like Africa, the Middle East, Iberian Peninsula, Australia, New Zealand, and North and South America and Hawaii.

The recognition of taxa within Latrodectus has been considered problematic for a long time due to the difficulty associated with morphological features exhibiting geographic variation.

- Black Widow Spider, Southern Black Widow (Latrodectus mactans)

This species occurs throughout North America, from Canada to Mexico, and in the West Indies.

Black Widows measure about 1.2 cm (1/2 inch) in body length. The female is normally shiny black, with a yellowish orange to red hourglass (sometimes just a dot) marking on the underside of the abdomen.

Black Widow Spiders need some kind of a shelter and can often be seen in small crevices or in woodpiles, rubble piles, under stones, in hollow stumps, in sheds and garages. Indoors they are sometimes found in undisturbed, cluttered areas in basements and crawl spaces.
They build strong retreats close to the ground but might also spread their webs over plants.

The Black Widow Spider is notorious at being one of the most venomous spiders in the world. It inflicts a painful bite which can be fatal, especially to the young and elderly. Only a small amount of venom can cause serious illness, as the neurotoxic poison (which is 15 times as toxic as the venom of a prairie rattlesnake) attacks the nervous system. Systemic envenomisation usually results in headache, nausea, vomiting, abdominal pain, pyrexia and hypertension. Though the pain around the bite area can be extremely painful, it sometimes goes unnoticed. First aid and medical attention should be sought as soon as possible. People with heart condition or other heart problem, may require hospitalization.

However, less than 1% of all people bitten by this spider run the risk of dying, and most of them are saved with the use of an effective antivenin, which was developed in 1956.

Other North American widow spiders are:

- Red Widow (Latrodectus bishopi): found in Florida
- Brown Widow, Grey Widow (Latrodectus geometricus): it is unclear where this species originated. It is found in warm places in Africa, Australia, South America, and North America, from the northeastern US to Texas with predominance in the southeast, and as far west as California.
- Western Black Widow (Latrodectus hesperus): found in western Canada, the United States, and Mexico.
- Northern Black Widow (Latrodectus variolus): found from the extreme southeastern part of Canada and south to northern Florida. It is mostly common in the northern part of this range.

3. Hobo Spider (Tegenaria agrestis)

These spiders belong in the family Agelenidae.

They are found in the western USA, from Washington, to Oregon, Idaho, Montana, Wyoming, northern Utah and Colorado, as well as in southwestern Canada . They commonly live in dry or arid environments.

These brown spiders measure roughly 7 to 16 mm (1/3 to 2/3 inch) in body length and 16 to 50 mm (2/3 to 2 inches) in leg span. Their abdomens often have several chevron shaped (or a herringbone) markings. Males have two large palpi (mouth parts) that look like boxing gloves. Females tend to have a larger and rounder abdomen then males.

Hobo Spiders are common around and inside human structures, however, they rarely climb vertical surfaces and are uncommon above basements or ground level. They build a horizontal web which retreats into a hole or crack in brick walls or wood piles where they await prey.

Hobo Spiders are reputed to be quite aggressive and sometimes bite with little provocation. In recent years, they have been implicated in dermonecrotic lesions in the US Pacific Northwest. Strangely, although this species was accidentally introduced to North America from Europe as recently as the early 1900's via commercial shipping, it is not considered poisonous there, perhaps because it is not often found around human structures.

The symptoms of a Hobo Spider bite are similar to that of Loxosceles, only with lesser severity. Though the bite of a Hobo Spider is initially painless, it can, nevertheless, be serious. A redness will first appear around the bite site, which will develop into a blister after 24 hours, before breaking open, leaving an open, oozing ulceration.

The bite might also cause severe headaches lasting 2 to 7 days and even nausea, weakness, fatigue, malaise, temporary memory loss and vision impairment.

Severe lesions may take several months to heal and the Hobo Spider has been identified as a contributory factor to at least one death. Medical attention should be sought, when bitten, and when any adverse health effects are observed.

4. Yellow Sac Spiders, Black-footed Spider (Cheiracanthium inclusum or mildei)

100 species of Sac spiders (Family Clubionidae) of the genus Cheiracanthium are found worldwide. Several species have caused envenomations. In the USA, bites have occurred from Massachusetts to California and in Hawaii.

This medium-sized spider (6-10 mm in body length) is pale yellow to greenish or even pinkish or tan colored, with black chelicerae (the anterior structures which house the fangs). The body color depends on the prey recently eaten.

Yellow Sac Spiders are often found in homes, throughout the American continent. They build silk retreats in curled leaves or in crevices. Bites usually occur at night when these nocturnal hunters encounter a sleeping human. In fact, Yellow Sac Spiders are believed to be responsible for the majority of nuisance bites to humans in North America.

This species has a mildly toxic venom. Bites can be painful and cause skin irritations: erythema, edema and pruritis. Yet, though wounds may become slightly necrotic, they usually heal in rather rapidly without severe scarring.


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Bites and Stings of medically important venomous arthropods
Richard S. Vetter and P. Kirk Visscher
Department of Entomology, University of California
Published in the International Journal of Dermatology, volume 37, page 481-496, July 1998,


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