How Large Does A Spider Have to Be To Perforate Human Skin?

Patient Presentation
A 13-year-old male came to clinic with a pruritic rash for 1 day. He noticed it the evening before and woke up a couple of times in the night itching his legs and in the morning showed his mother. He had been playing with siblings and friends around the house and a vacant lot for the past two days including playing “hide and seek” in the garage and a storage shed. He and his mother thought this could be a spider bite. No one else has a rash and they deny any new soaps, lotions, creams or other items that could cause contact dermatitis.

The pertinent physical exam showed a well appearing male with normal vital signs and growth parameters. On his lower legs he had several linearly arranged, distinct, slightly papular, shiny, non-vesicular red lesions with no surrounding erythema. There were excoriations around the lesions. No specific bite marks were seen.

The diagnosis of of a rash due to contact with either a plant or some type of insect or arthropod was made. The pediatrician said that with the location of where the boy had been playing he could have been in contact with spiders but felt that this looked more like poison ivy. “It’s possible that you were bitten by a spider but fortunately for most people they don’t cause too many problems. It can be hard to tell sometimes when there are just a few lesions on the skin. Keep it clean and dry. For the itching you can use some diphenhydramine too,” the pediatrician advised.

Spiders (Phylum: Arthropoda, Class: Arachnida) are difficult to accurately identify as they are often confused with insects (Phylum: Arthropoda, Class: Insecta). Spiders have 8 legs and 2 body parts. They have no wings or antenna. There are no specific symptoms that are diagnostic of a spider bite. Fang marks are infrequent and the markings are difficult to even identify. Spider bites are frequently confused with other arthropod bites (fleas, ticks, mosquites, bed bugs, etc.), allergic reactions, infections and contact dermatitis (including poison ivy etc.).

Most spiders have a toxic venom. “Histamine concentration [in the venom] varies by a factor of 20 among the venoms of different spider species, and the pain in vertebrates is considered to be only a side-effect of the proposed main venom enhancing function of histamine.” The duration of the effects varies from a few minutes to a few hours but recovery is usually quick, often without any treatment.

Spiders with venom that causes a serious reactions are often called poisonous spiders. In the United States, 2 spiders are considered truly poisonous. Images can be reviewed below.

  • Black widow spiders (Latrodectus hesperus) – females have a large round abdomen and on ventral surface has a distinctive red or orange hourglass-like marking. Only large immature females and adult females are able to bite through human skin. Males have venom but are too small to puncture human skin. They like dark, dry undisturbed locations including piles of stones, wood or rubbish, sheds, barns, crawlspaces and storage areas.
    Pain can be mild to severe and muscle spasms in others areas of the body may develop along with systemic problems such as hypertension, respiratory distress and mental status changes. Symptoms usually peak by 1 day and resolve over 2-3 days. Children and the elderly may be more seriously affected, but death is rare.

  • Brown recluse spiders belong to the Loxosceles group. The American brown recluse spider is called Loxosecles reclusa. They are known to have a violin-shaped marking on the dorsum of their heads, but this can non-existent, faded or fades in preservative. They are often miscategorized by non-experts. They are found in the central mid-west (Ohio to Nebraska) and upper southern regions (Georgia to Texas). They like dark, dry undisturbed locations including piles of stones, wood or rubbish, sheds, barns, crawlspaces and storage areas. In addition to pain, the venom can cause blistering and tissue necrosis.

International standards for verified spider bites include a bite that is observed, along with collection of the spider and accurate identification by an expert, plus the bite must cause symptoms such as pain.

Symptoms of spider bites include (and these usually do not cause concern):

  • Painless, usually not noticed
  • Pinprick when bitten
  • Mild edema

Other symptoms can indicate a poisonous spider bite or an allergic reaction to the bite. These are usually rare.

  • Sharp pain when bitten
  • Significant swelling at site
  • Swelling of the lips or mouth
  • Dizziness or fainting
  • Respiratory distress
  • Tachycardia, hypertension
  • Hives
  • Mental status changes
  • Fever

Learning Point
According to Nentwig, “…a spider has to have a body length of at least 10 mm to be able to perforate the human skin.”

Questions for Further Discussion
1. What snakes are venomous in your location?
2. What are methods for getting rid of spiders?

Related Cases

    Symptom/Presentation: Rash

To Learn More
To view pediatric review articles on this topic from the past year check PubMed.

Evidence-based medicine information on this topic can be found at, the National Guideline Clearinghouse and the Cochrane Database of Systematic Reviews.

Information prescriptions for patients can be found at MedlinePlus for this topic: Spider Bites and Poison Ivy

To view current news articles on this topic check Google News.

To view images related to this topic check Google Images for Black Widow Spiders and for Brown Recluse Spiders .

To view videos related to this topic check YouTube Videos.

Vetter RS, Isbister GK, Bush SP, Boutin LJ. Verified bites by yellow sac spiders (genus Cheiracanthium) in the United States and Australia: where is the necrosis? Am J Trop Med Hyg. 2006 Jun;74(6):1043-8.

Vetter RS. Spiders. UC Statewide IPM Program, University of California, Davis, CA.Available from the Internet at: (rev. 11/07, cited 9/5/15).

Vetter RS. Pest Notes: Brown Recluse and Other Recluse Spiders. UC Statewide IPM Program, University of California, Davis, CA. Available from the Internet at: (rev. 1/08, cited 9/5/15).

Vetter RS, Isbister GK. Medical aspects of spider bites. Annu Rev Entomol. 2008;53:409-29.

Vetter RS, Hinkle NC, Ames LM. Distribution of the brown recluse spider (Araneae: Sicariidae) in Georgia with comparison to poison center reports of envenomations. J Med Entomol. 2009 Jan;46(1):15-20.

Golcuk Y, Velibey Y, Gonullu H, Sahin M, Kocabas E. Acute toxic fulminant myocarditis after a black widow spider envenomation: case report and literature review. Clin Toxicol (Phila). 2013 Mar;51(3):191-2.

Nentwig W, Gnadinger M, Fuchs J, Ceschi A. A two year study of verified spider bites in Switzerland and a review of the European spider bite literature. Toxicon. 2013 Oct;73:104-10.

Centers for Disease Control. Venomous Spiders. Available from the Internet at:
(rev. 7/31/15, cited 9/5/15).


Donna M. D’Alessandro, MD
Professor of Pediatrics, University of Iowa Children’s Hospital