The common question of whether spiders are “poisonous” often stems from fear, but the reality is that nearly all spiders are venomous. While the vast majority of the over 50,000 known spider species pose no threat to humans, a small number possess venom that can cause medically significant reactions. Understanding the biological distinction between different types of toxins is the first step in separating myth from the actual risk these arachnids present. Recognizing the few species that warrant caution and knowing how to respond if an encounter occurs is important.
The Difference Between Venomous and Poisonous
The distinction between venom and poison is based entirely on the method of delivery. A substance is considered a poison if the toxin is delivered passively, meaning it must be inhaled, absorbed through the skin, or ingested to cause harm. Examples of poisonous organisms include certain frogs or mushrooms.
Conversely, an organism is venomous if it actively injects its toxin into another creature using a specialized apparatus, such as fangs, stingers, or spines. Spiders possess fangs connected to venom glands, which they use to subdue prey, classifying them as venomous. The molecules that make up venom are often too large to be absorbed through intact skin, requiring injection into a wound to be effective.
The venom produced by spiders is primarily a hunting tool, which is why almost every species produces it. However, the vast majority of these toxins are formulated to affect small insects and are too weak, or delivered in too small a quantity, to cause harm to a human. Only a tiny fraction of spider species have venom potent enough to be considered medically significant to people.
Identification of Dangerous Spiders
In North America, only two groups of spiders are generally considered to be of medical concern: the widow spiders (Latrodectus) and the recluse spiders (Loxosceles). These spiders are not aggressive and typically only bite defensively when they are accidentally trapped against a person’s skin. Understanding their appearance and preferred habitats can help in avoiding these defensive bites.
The most recognized dangerous spider is the Black Widow (Latrodectus species). The female is identifiable by its shiny black body and the distinct red or orange hourglass marking on the underside of its abdomen. The venom of the black widow is neurotoxic, meaning it attacks the nervous system. These spiders build irregular, messy webs often found in dark, sheltered outdoor areas like woodpiles, sheds, under decks, or in crawl spaces.
The Brown Recluse spider (Loxosceles reclusa) is recognized by a dark, violin-shaped marking on its cephalothorax, with the neck of the “violin” pointing toward the abdomen. Unlike most spiders, the brown recluse has only six eyes arranged in three pairs, rather than the usual eight. Their venom is cytotoxic, which can cause local tissue damage, sometimes leading to a necrotic lesion.
Recluse spiders prefer quiet, undisturbed areas, often indoors in places like attics, basements, closets, or inside stored clothing and rarely used shoes. The adult body size is relatively small, about a quarter to a half-inch long. Identifying these two species by their markings is the most reliable way to assess a potential threat.
What to Do After a Bite
Most spider bites result in only minor symptoms, often resembling a localized insect bite with mild pain, redness, and swelling that resolves quickly. If a bite is suspected, the immediate first aid response should focus on cleaning the area to prevent secondary infection. The wound should be thoroughly washed with mild soap and water.
Applying a cool cloth or an ice pack wrapped in a cloth to the site for about 15 minutes each hour can help reduce swelling and pain. If the bite is on an extremity, elevating the limb can also assist in minimizing swelling. Over-the-counter pain relievers and antihistamines may be used to manage discomfort and itching.
It is important to seek professional medical attention if the bite is known or suspected to be from a black widow or brown recluse spider. Immediate care is also necessary if severe symptoms develop, such as intense muscle cramping, severe abdominal pain, nausea, or vomiting, which can be signs of neurotoxic venom. A worsening local reaction, including spreading redness, streaks, or the development of an open, painful ulcer with a dark center, warrants a medical evaluation. Any signs of a systemic reaction, such as fever, chills, difficulty breathing, or swelling of the face or throat, are considered a medical emergency.