Why do spiders attack humans?

Table of Contents

Truth be told, spiders are not in the business of attacking humans.  In fact, most of the time spiders are not interested in us and will ignore us or see us as part of their surroundings if they even realise that we are there.  But, if we disturb them or get to near them they will behave towards us as though we were any regular predator.  Some spiders use threat displays like lunging or rearing up which are aimed at frightening predators away.  Spiders bite in self-defence when they feel that their lives are threatened but most of them will usually run away.

why do spiders attack humans
why do spiders attack humans

Do house spiders bite?

Spider bites are in fact quite rare since spiders actually don’t bite people that often.  If you notice a mark or bump on the skin it is easy to assume that it is a spider who is responsible but in most instances this is not the case.  Often, people are so convinced that they have been bitten by a spider when they notice any kind of bite and even doctors may misdiagnose many bites as spider bites when this is often not the case.

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Spiders are not designed to bite large mammals such as humans

Spiders are not designed for or equipped to forcefully engage in battle with large mammals such as humans but rather they are built to catch and kill other invertebrates.  While there are a number of exceptions, such as widow spiders, generally spider venom is not poisonous enough to do too much damage to humans.  Certain research has also shown that out of almost 40 000 different spider species, on a global scale there are less than a dozen types of spiders that can be dangerous to the average healthy person.  Even those spiders that have venom that is potent enough to poison humans are not able to bite us and cause any serious damage since the fangs of spiders are not designed for puncturing human skin.  This does not mean that spiders cannot bite us, what it really means is that it is difficult for them to do so.  Most arachnologists claim that they do not get bitten when handling live spiders.

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A spider will normally opt for flight over fight when it feels threatened

A spider is able to sense a threat by detecting vibrations in the environment, similar to the manner in which it senses the presence of an insect in its web.  Hence, since humans generally make a lot of noise the spider is more than aware when humans approach.  But, instead of rearing for attack, a spider will choose flight over fight when it feels threatened by humans.

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Instances where spiders do bite humans

There are certain occasions where a spider will bite someone.  This usually happens when someone unintentionally interferes with a spider’s habitat such as poking one’s hand inside the web.  This is when the spider perceives a threat and is then required to defend itself.  The Handy Bug Answer Book by Dr Gilbert Waldbauer offers this disturbing scenario:  “The majority of [black widow spider] bites are inflicted on men or boys sitting in an outdoor privy, or pit toilet. Black widows sometimes spin their web just beneath the hole in the seat, often a good place to catch flies. If the unfortunate person’s penis dangles in the web, the female spider rushes to attack; presumably in defense of her egg sacs, which are attached to the web.”

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If not a spider bite, then what is this mark on my skin?

If you suspect that you have been bitten by a spider but this is not the case, the bite can be caused by any number of other bugs such as mites, ticks, fleas, bed bugs and mosquitoes.  Skin irritations that look like bites can also be caused by other factors in the environment such as plants like poison ivy and certain chemicals.  There are also numerous other medical conditions that could be responsible for causing a skin irritation that resembles a bite ranging from diseases of the lymphatic system to vascular conditions.  Certain viral and bacterial infections are frequently misdiagnosed as bites. 

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Spiders that attack humans

While we have established that there are over 40 000 different types of spiders globally, of these, only a fraction of them are said to be dangerous and less than 30 of these different spiders have been found to be responsible for causing death in humans.  This is mainly due to the fact that there is a vast size difference between spiders and humans and that spider venom is primarily designed to be effective against smaller animals.  However, the venom of certain spider species can cause skin lesions in certain people or produce an allergic reaction that can be fatal.  Nevertheless, it is important to bear in mind that spider bite deaths are very rare since most clinics and hospitals as well as poison control centres carry spider species-specific antivenin or antitoxin, for the treatment of spider bites.

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Dangerous spiders

Brown Recluse or Violin Spider

The brown recluse spider is considered to be one of the most dangerous spiders found in the USA.  This spider’s venom can destroy the blood vessel walls near the site where the bite occurs and often causes a large ulcer on the skin.  In 2013, research indicated that a certain protein in the spider’s venom targets phospholipid molecules which are those molecules that form a large portion of cell membranes, transforms these molecules into what is known as simpler lipids causing a wound that may need many months to heal.  Often, this type of wound may become infected and can even cause death.  However, death by brown recluse spider bite is rare.

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Brazilian Wandering Spider

The Brazilian Wandering Spider is sometimes known as the banana spider because it is often found on banana leaves.  This spider’s defence posture is aggressive, whereby it raises its front legs straight up and can look quite threatening.  This type of spider is poisonous to human beings and is considered to be the most dangerous spider in the world.  This spider’s venom is toxic to the human nervous system displaying symptoms that include an irregular heartbeat, salivation, and prolonged painful erections in men.  It stands to reason that scientists are currently investigating the possible use of this venom in the treatment of men who suffer from erectile dysfunction. 

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Yellow Sac Spider

The yellow sac spider is from a family of spiders known as Clubionids.  These spiders range in body length from between 3mm to 15mm.  They build silken tubes under leaves, stones or in grass.  This spider’s venom is dangerous to humans and if often found indoors.  The Yellow Sac Spider’s venom is a cytotoxin which means that it either impairs the function of a cell or destroys it which means that it can produce necrotising lesions.  Swelling and redness around the area where the bite occurs is common.  These spiders are not docile creatures by any means and a female Yellow Sac Spider may bite to protect her eggs.

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Wolf Spider

The wolf spider is from a family of spiders known as Lycosidae which are found worldwide.  The reason they were given their name is likened to the wolf in the sense that like wolves, these spiders have a tendency to chase and then pounce on their prey.  Ranging from small to medium-sized, the largest of the wolf spider grows to approximately 2.5cms with long legs of similar length. 

The majority of wolf spiders are dark brown coloured with hairy bodies that are as wide as they are long and their legs are equally stout.  Wolf spiders are mostly found to inhabit places that they can crawl under or into such as under stones, leaves, fallen tree branches or in grass.  However, they can also be found in houses that are infested with other types of insects.  The are also renowned for their running speeds.  Many different varieties of wolf spider can be found north of the Artic Circle and there are only 50 different types of wolf spiders in Europe whereas they are far more diverse in North America with approximately 125 different kinds. 

As a rule, wolf spiders build silk-lined nests in the ground that are tubular in structure.  Certain ones hide their nests with pieces of rubbish or debris while others might build a turret-shaped structure above the nest.  Only a small number of wolf spiders build webs. 

What is also interesting about these spiders is that once they have laid their eggs female widows look like they are pulling a huge ball along as they contain their eggs in a greyish silk sac which is attached to the silk-producing organs, otherwise known as spinnerets of the female widow.  And once the eggs have hatched the baby widows piggyback on the mother’s back for many days afterwards. 

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Black Widow Spider

In the United States, the black widow is the reason for more than 2500 visits to poison control centres by people who have been bitten.  The black widow is common in the United States, Canada, The West Indies as well as Latin America.  This spider lives among piles of wood, burrows and plants that are ideal for supporting its web.  The female black widow is a shiny black colour with an hourglass design on the abdomen that is reddish to yellow.  Instead of the hourglass shape it sometimes has two small triangles with a body of approximately 2.5cms long.  Since the male black widow is most often eaten by the female once mating has taken place, it is rarely spotted.  The male black widow is only a quarter of the size of the female and usually has red and white stripes on the abdomen.  When the black widow bites it feels like a pin prick which can result in nausea, muscle pain, cramping and mild paralysis felt in the diaphragm which can lead to breathing difficulties.  While most people can recover without serious problems, bites in small children and the elderly can be fatal although there have been no deaths reported in the United States.

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Brown widow spider

While it is thought that the brown widow spider may have evolved in Africa, the first noted member of this species came from South America and has been classified as an invasive species in other parts of the world.  These spiders have been found in South Africa, Japan, Australia, Cyprus and Madagascar.  It builds homes on the inside of old tyres, in buildings, under cars and in shrubs and plants.  This spider is usually brown but can range from light brown to black.  They have brown, black, yellow, white or orange markings on the abdomen and the notorious hourglass markings of these spiders are orange.  The venom of the brown widow spider is two times more powerful than that of the black widow but brown widow spiders are not really aggressive and only produce a small amount of venom when they bite.  However, two people died in Madagascar during the 1990s from brown widow spider bites but they were in poor health to begin with and did not receive antivenin treatment. 

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Red widow spider

The red widow spider or red-legged widow as it is otherwise known, is different from other widow spiders due to its red cephalothorax and legs and the reddish colour of its abdomen which sometimes can also look black.  Red widows are also characterised by a red mark on the abdomen which can either be triangular or hourglass in shape.  There are spotted red or orange markings on the top of the abdomen with a white or yellow outline surrounding each spot.  This widow spider mainly eats insects and is not known to be aggressive towards people in general.  Yet, it may bite if it is protecting its eggs in the case of a female widow or if it becomes trapped against the skin of a person or restricted by footwear or clothing.  When bitten by a red widow spider, the same symptoms appear as per the black widow and again, since the amount of venom produced when it bites is relatively small, human deaths are rare.  The elderly and small children are once again those at greatest risk from red widow spider bites. 

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House spiders that bite

Spider bites are less common than we think

While there are house spiders that do bite, the main thing to bear in mind about spiders is that they really don’t want to attack humans physically and spider bites are indeed much less common than we have been led to believe.  Instead, they much prefer to attack small insects that they can easily manage.  This is the reason that they run away from us.  Hence, spiders usually only attack humans if they feel like they themselves are under attack and this usually happens if they are accidentally touched or interfered with by humans.   Many spiders are too small to inflict that much damage to human skin, often being unable to sufficiently pierce the skin to cause any notable damage.  All things considered, spider bites in humans are not really that common and these creatures would rather feast on insects than go after us since not only are we way to big to handle but unlike mosquitoes, humans provide them with no nutritional value whatsoever.

 

 

Hazel Buckley
Hazel Buckley

About Hazel Buckley
Hazel is an animal enthusiast and educator who grew up on a farm which her parents owned in Ingogo, KwaZulu-Natal, South Africa.  The farm was situated right under the Majuba Mountains - the site where the Anglo-Boer War was fought. 

Disclaimer: Whilst every effort has been made to ensure that the information published on this website is accurate, the author and owners of this website take no responsibility  for any loss or damage suffered as a result of relience upon the information contained therein.  Furthermore the bulk of the information is derived from information in 2018 and use therefore is at your on risk. In addition you should consult professional advice if required.