- Characteristics of spiders
- Types of spiders
- How often are people bit by irritating or dangerous spiders?
- How dangerous are poisonous spider bites?
- Poisonous Spider Bites
- Widow Spiders
- Recluse Spiders
- Wandering Spiders
- Funnel spiders
- How to decide when to see a doctor
- Preventing spider bites
- Spider Bites FAQ
Spiders are a large group of ancient animals closely related to insects and shellfish.1-3
Spiders are found everywhere in the world and in nearly all habitats and climates.3 Scientists have found over 45,000 species of spiders, but they believe that there could be over 120,000 species in existence.2,3
Characteristics of spiders
Spiders range in size from 2 millimeters to 9 centimeters.3 They may have a wide variety of colors, markings and amounts of hair.
All spiders have two body segments. The front segment, called a cephalothorax, contains the spider’s nervous system, eyes, mouth pieces (chelicera), sensory organs (palps) and legs. The back segment, called an abdomen, is responsible for digestion, breathing, reproduction and producing silk (spinneret). The two body segments are linked by a thin shaft called a pedicel.3
Figure 1: Anatomy of a spider. Shutterstock
All spiders are predators and eat other animals. Usually they target insects or other spiders. All but a very few species of spiders use venom to paralyze and kill their prey. Technically, over 99 percent of spiders are poisonous.3
Types of spiders
Scientists classify spiders by many different characteristics. They might group spiders by their evolutionary history, their hunting style, their venom, their type of mouth pieces or dozens of other traits.
For the average person, the most important classification is whether or not a spider is dangerous for humans. In this respect, there are three classes of spiders: harmless, irritating, and dangerous.
Over 99.99 percent of spiders fall into the harmless category.4 Nearly all spiders are too small to bite through human skin.3
A few families of spiders fall between harmless and dangerous. These spiders can bite humans but their venom is not able to cause serious harm.
Symptoms of bites by irritating spiders are mild and short-term. They usually include bite-site swelling or blisters and may include a mild fever and itching.3,4 Rarely, bites from these spiders may get infected and require antibiotics.5 Common spiders that fall into this category include3,5:
- Water spiders
- Yellow sac spiders
- Hobo spiders
- Wolf spiders
Tarantulas also fall into this category, but not because of their bites. Most negative reactions to tarantulas come from small hairs the spiders release from their bodies when they feel threatened.3
Figure 2: Pictures of spiders with irritating bites.
Top left: water spider (Pavel Krasensky/Shutterstock);
top center: yellow sac spider (D. Kucharski & K. Kurchaska/Shutterstock);
top right: hobo spider (Anton Kozyrev/Shutterstock);
bottom left: wolf spider (Brett Hondow/Shutterstock);
bottom right: tarantula (pets in frame/Shutterstock)
Mild spider bites can usually be treated with anti-inflammatory and/or pain medication, as needed.6
Deciding if it was one of these less dangerous spiders that bit you is not always easy. If you have none of the symptoms of a venomous bite (described below), you may have been bitten by a non-venomous spider. If you are unsure, please visit a doctor.
Four families of spiders are able to bite humans and inject a venom that is poisonous.3,4 Bites from these spiders can cause serious illness or death.
- Widow spiders
- Recluse (or brown) spiders
- Funnel spiders
- Wandering (or armed or banana bunch) spiders
Each of these groups of spiders is described in detail below.
How often are people bit by irritating or dangerous spiders?
It is difficult to track total number of people bitten by spiders world-wide each year.
Many people with mild bites do not seek medical attention and their bites are never reported.
On the other hand, many people jump to the conclusion that they have been bitten by a spider when they have not. Up to 97 percent of people who go to the emergency room with what they believe is a spider bite are diagnosed with something else.7 In non-emergency situations misdiagnosis might be common.7
The data we do have, though, suggests that spider bites are fairly common, but poisonous spider bites are rare. An average of 142,886 spider bites per year were diagnosed in American emergency rooms between 2001 and 2010. Of these, only 3.3 percent (4,700) were diagnosed to have come from a poisonous spider.8
How dangerous are poisonous spider bites?
If you are bitten by a venomous spider, how much danger are you in?
This depends on several factors including4:
- exact species of spider that bit you
- bite location
- amount of venom in your bite
- your age (kids and older individuals tend to have more severe reactions)
- pre-existing health conditions.
Generally speaking, there is a very low risk of dying from any spider bite.4,9 Venomous spiders usually do not inject much venom into humans, which makes them non-lethal nearly all of the time.3,4
In the rare cases in which symptoms do become life-threatening, antivenom is available. Since the development of antivenom serums, death following spider bites has become nearly non-existent.9-10
The latest data out of the United States found the yearly death rate from venomous spiders to be 0.0267 per million people. That is one person in 37.5 million.9
This does not mean that you should not seek medical attention for a suspected venomous spider bite. On the contrary — effective treatment by doctors is why nearly no one dies from spider bites any more. If you think you may have been bitten by a venomous spider, please seek medical attention as soon as possible.
Poisonous Spider Bites
The four families of venomous spiders, their characteristics, and their bite symptoms are described below.
There are over 30 species of widow spiders found world-wide. Black widows are medium-sized and hairless with glossy black skin and red hour-glass markings.10 Brown widows look very similar, and have similar bites, but are completely brown.3
Figure 3: Black (above) and brown widow spiders (below). (Jay Ondreicka/Shutterstock)
The habitat of these spiders appears to depend on the exact species. In the Americas, most contact with widow spiders occur outdoors after disturbing wood piles or cluttered areas. In Australia, most bites occur in houses.10,11
Widow spiders are not naturally aggressive and only bite when disturbed.4
The venom of widow spiders contains a small protein called alpha-latrotoxin.10 Alpha-latrotoxin is toxic for nerve cells. It forces nerves in your body to fire uncontrollably and is responsible for the toxic symptoms of a widow bite.4,10
The bite itself may be slightly or moderately painful.5 Rarely, you may be able to see two puncture marks.11 In about a quarter of cases, the bite site turns red, swells and begins to sweat. For many bites, no bite site is visible.4
After a period of 10 to 60 minutes, toxic symptoms of the bite may begin to appear.
- Intense pain (may be shooting, around bite, or in your back or stomach)
- High blood pressure
- Muscle twitches
- Muscle cramps (especially near the bite, in your back, or in your stomach)
- Local paralysis
- Erection (in men)
- Damage to heart muscle
The exact symptoms seem to depend on the amount of venom and the specific species of spider.10
Doctors treat most widow bites with two types of medication.
First, they give pain medications. Often, they choose opioids, such as morphine. These may be given as a pill or a shot, depending on your pain level.10
Second, they administer medication to relieve the muscle cramps. Usually, this is a type of benzodiazepine.4
Children, pregnant women, the elderly and those whose pain cannot be managed with morphine may be given antivenom. Antivenom is given as a shot or IV.4
Antivenom stops the symptoms and allows full recovery within a few days. Without antivenom, symptoms may last up to five days and full recovery may take weeks.3
You may also be given other medications to help with specific symptoms, if they have become severe. If your doctor suspects there may be damage to your heart, you may be put on a heart monitor or have blood taken for testing.10
With treatment, fatality from widow spider bites is less than 1 percent.4
There are over 100 species of recluse spiders (also called brown or fiddle-back spiders) throughout the world.10 Most species live in warm climates (South America, Australia and the southern United States).10 Their habitats appear to be expanding due to global warming.12
These spiders are brown or tan with a dark violin-shaped lines on their abdomens.4
Figure 4: Picture of a recluse spider (Steve Collender/Shutterstock)
Recluse spiders hide in dark, dry places during the day. Outdoors, they may be under rocks, wood or pieces of bark. In houses, they may hide in cupboards, shoes, clothes or bedsheets.10
Recluse spiders require pressure in order to bite through human skin. Nearly all bites occur when humans accidently grab, lay on or step on a recluse spider.4,10-11
Most bites occur at night when the spiders are active. People often do not realize they have been bitten until morning.10,12
Recluse spider venom contains phospholipase D and hyaluronidase enzymes.10 These enzymes cause skin and fat cells to breakdown and die. If these enzymes make it into your bloodstream, they may also cause red blood cells to break open and die.10
Recluse bites themselves are usually painless.10
After several hours, the first symptoms begin. These are localized to the skin and may include4,10:
- Swelling and redness centered around a white or blue area
- Blood-filled blisters at the center of the bite
- Intense pain
After three days, the skin around the bite begins to die. During this stage, the area that had been swollen begins drying out and turning black, creating a stark line between healthy and dying skin.10
In the final stages of a recluse spider bite, the blackened tissue falls off. This leaves a deep hole in the skin. The hole may take months to heal and almost always scars.10
If the toxic enzymes enter the bloodstream, additional symptoms may appear. These may include: 4,10
- General itchiness
- Low fever
In very rare cases, symptoms may progress to include4,10:
- Shortness of breath
- Problems peeing
- Dark urine
- Muscle pain
- Swelling of one leg
- Numbness, tingling or paralysis on one side of the body
- Problems swallowing
- Problems speaking
These last symptoms may indicate serious complications of a recluse spider bite. They may mean you are experiencing severe loss of red blood cells, kidney failure, breakdown of muscle tissue, blood clots or a stroke. If you develop any of these symptoms, seek medical attention immediately.
Symptoms affecting more than just the skin are reported for up to 16 percent of recluse spider bites. Serious symptoms or death are extremely rare.10
The primary goal in treating a recluse spider bite is to stop the swelling. You may be given anti-inflammatory medications such as antihistamines, steroids or colchicine.4
If a diagnosis is made within 12 hours of the bite, you may be given antivenom. This helps keep the area of dying skin from spreading.4
If the hole in the skin is large, it may need to be surgically repaired after the inflammation has stopped.4
Wandering spiders (also called armed or banana bunch spiders) are only found in South America. They are brown, hairy spiders with very long legs.4
Figure 5: Picture of a wandering spider. (Dr. Morley Read/Shutterstock)
Wandering spiders are nocturnal and do not build webs. They are often found hiding in bunches of bananas, piles of wood, piles of trash or in shoes. Most bites occur in early spring, when the spiders are looking for mates.10
Wandering spider venom has many small proteins similar to alpha-latrotoxin in widow spider venom. These small proteins force certain nerves to fire uncontrollably and cause the toxic symptoms of a wandering spider bite.10
The initial bite of a wandering spider is usually painful.4
Symptoms are similar to that of widow spider bites, and include4,10:
- Bite swelling
- Pain radiating from the bite
- Redness around the bite
- Irregular heart beat
- High blood pressure
- Problems seeing
- Goose bumps
- Trouble breathing
Severe symptoms are very rare and occur in less than 1 percent of bites.10
Most wandering spider bites are treated by use of normal pain medications and warm compresses. 10
If pain is severe, doctors may consider giving opioids or sedatives.10 This is only done when absolutely necessary, since it increases the chance of respiratory complications.4
In severe cases, doctors may give antivenom.10
Funnel spiders are the most dangerous spiders in the world.10 They are found only in Australia. Funnel spiders are large, hairy, and dark in color. They are aggressive animals and will attack humans without being provoked.10
Figure 6: Photograph of a funnel spider. (John McQueen/Shutterstock)
Funnel spiders live in trees or in burrows, where they weave funnel shaped webs. They like to hide among leaves and other plant debris.10
Like the venom of widow and wandering spiders, funnel spider venom is toxic for nerve cells. It contains many small proteins that cause nerves to fire uncontrollably. Funnel spider venom is more powerful and acts more quickly than the venom from the other poisonous spiders, making it more dangerous.4,10
Funnel spider bites are usually very painful because of the spiders’ large fangs, which often leave puncture marks.10
If the spider injects venom, symptoms usually begin within minutes. These symptoms can progress quickly and include4,10
- Anxiety or irritability
- Abnormal pupil size
- Watery eyes
- Goose bumps
- Racing heart or slow heart beat
- High or low blood pressure
- Twitching (particularly of the tongue)
- Muscle spasms
If left untreated, symptoms may progress to low blood pressure, organ failure, coma and death.10
Pictures of credible funnel spider bites may be viewed here.
Everyone bitten by a funnel spider needs antivenom. You should wrap a tight bandage around the bite and keep the site from moving until you reach a hospital.10 Antivenom needs to be given within 15 minutes of removing the compressing bandages.4
No one has died from a funnel spider bite since the antivenom was developed in the 1980s.10
How to decide when to see a doctor
You should see doctor as soon as possible if:
- you saw the spider that bit you and could identify it as poisonous
(if you are able to catch it, you should bring it to the doctor to help with diagnosis)
- the symptoms of your bite match symptoms of a bite from a venomous spider common in the region
- you begin experiencing symptoms in parts of your body that have nothing to do with the bite itself, such as headache, nausea or vomiting
- you experience any nervous system symptoms, such as fear, confusion, problems seeing or speaking, numbness, or paralysis
- you experience muscle twitching or spasms
- your wound is growing or turning black
- your pain is severe
- your symptoms are worsening
If you have any symptoms that are causing you concern, please see a doctor.
Preventing spider bites
The only way to prevent spider bites is to prevent contact with spiders.10
Wearing protective clothing, such as long sleeves, long pants and closed-toed shoes may help prevent bites. Wearing gloves during house work or gardening in areas where spiders might be hiding may also be helpful.10
Be aware of your surroundings and check areas where spiders might be before jumping into house or yard work.10
Kids and older adults are at greater risk of severe symptoms and should take greater care.4
There are no known repellents for venomous spiders.10
Spider Bites FAQ
- How to tell what kind of spider bites you have?
The best way to know which type of spider bit you is to find the spider that did it.6,10 Catching the spider and identifying it is a good way to ensure a correct diagnosis.
Sometimes, you can tell which spider is responsible by testing the venom in your bite.10 A doctor might prescribe these laboratory tests to choose the right antivenom for you.
In most cases, you will have to guess which species is likely responsible for your bite based on your symptoms and where you likely came into contact with the spider.4,7,10
- What does a spider bite look like at first?
What a spider bite looks like at first depends on the type of spider that bit you. Bites range from invisible, to bites that look like bee stings or small blisters, to large red, swollen patches several inches across.4,10,13
- What does a poisonous spider bite look like?
This depends on the spider responsible and the amount of time since the bite. The bites may range from invisible, to a bite similar to a bee sting, blisters or blood blisters, large red swollen patches, blackened skin or even a hole.4,10,13
- What spider bites cause blisters?
Many spider bites may cause blisters. Nearly all of these are not dangerous. If the blisters are filled with blood, however, this may be a sign that you were bitten by a recluse spider and you should seek medical attention as soon as possible.3-4,10,13
- How do doctors treat spider bites?
The treatment of spider bites depends on the species and the severity of bite symptoms.
Many bites only require pain management.6
Severe bites from venomous spiders may require strong pain killers, medications to stop muscle spasms, medications to stop inflammation or antivenom.3-4,10,13
- Merriam-Webster. https://www.merriam-webster.com/dictionary/arthropod. Date published not available. Accessed June 26, 2017.
- Garrison NL, Rodriguez J, Agnarsson I et al. Spider phylogenomics: untangling the spider tree of life. PeerJ. 2016; 4:e179. doi: 10.7717/peerj.1719.
- Foelix R. Biology of Spiders 3rd ed. New York, NY: Oxford University Press, Inc; 2011.
- Rahmani F, Khojasteh SMB, Bakhtavar HE, Rahmani F, Nia KS, Faridaalaee. Poisonous spiders: bites, symptoms, and treatment; an educational review. Emerg. 2014;2(2):54-58. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4614586/pdf/ emerg-2-054.pdf. Accessed June 26, 2017.
- Gaver-Wainwright MM, Zack RS, Foradori MJ, Lavine LC. Misdiagnosis of spider bites: bacterial associates, mechanical pathogen transfer, and hemolytic potential of venom from the hobo spider, Tegenaria agrestis (Araneae: Agelenidae). J Med Entomol. 2011;48(2):382-388. doi: 0.1603/ME09224.
- Quan D. North American poisonous bites and stings. Crit Care Clin. 2012;28:633-659. doi: http://dx.doi.org/10.1016/j.ccc.2012.07.010.
- Suchard MD. “Spider bite” lesions are usually diagnosed as skin and soft-tissue infections. J Emerg Med. 2011;41(5):473-481. doi: 1016/j.jemermed.2009.09.014.
- Langley R, Mack K, Haileyesus T, Proescholdbell S, Annest JL. National estimates of noncanine bite and sting injuries treated in US hospital emergency departments 2001-2010. Wilderness Environ Med. 2014;25(1):14-23. doi:10.1016/j.wem.2013.08.007.
- Forrester JA, Holstege CP, Forrester JD. Fatalities from venomous and nonvenomous animals in the United States (1999-2007). Wilderness Environ Med. 2012;23(2):146-152. doi: 10.1016/j.wem.2012.02.012.
- Ibister GK, Fan H. Spider Bite. 2011;378(9808):2039-2047. doi:http://dx.doi.org/10.1016/S0140-6736(10)62230-1.
- Venomous spiders. cdc.gov. Updated July 1, 2016. Accessed June 27, 2017. https://www.cdc.gov/niosh/topics/spiders/.
- Saupe EE, Papes M, Selden PA, Vetter RS. Tracking a medically important spider: climate change, ecological niche modeling, and the brown recluse (Loxosceles reclusa). PlosOne. 2001;6(3):e17731. doi: 10.1371/journal.pone.0017731.
- Stoecker WV, Vetter RS, Dyer, JA. NOT RECLUSE – a mnemonic device to avoid false diagnosis of brown recluse spider bites. JAMA Derm. 2017;153(5):377-378. doi: 10.1001/jamadermatol.2016.5665.