Almost all spiders possess venom, a biological tool they use for survival. While the idea of venomous spiders can be unsettling, only a tiny fraction of the over 50,000 known spider species pose any significant threat to humans. Most spiders are either too small to bite humans effectively or their venom is not potent enough to cause harm. This distinction between venomous and dangerous is a key aspect of understanding spider biology.
The Biological Role of Spider Venom
Spider venom is a complex biological mixture produced in specialized glands, typically located in the spider’s cephalothorax or chelicerae. This mixture primarily consists of proteins, peptides, and small molecular compounds like ions, amino acids, and amines. The specific composition varies greatly among different spider species, influencing the venom’s effects.
The primary function of spider venom is to subdue prey. The venom acts rapidly, often paralyzing or killing the prey. Beyond predation, venom also serves as a defensive mechanism against potential threats.
Spider venom is delivered through fangs, which act like hypodermic needles. Many of the active components in venom, such as neurotoxins, target the nervous systems of prey by interfering with ion channels and receptors, leading to paralysis or other disruptive effects. Some venoms also contain enzymes that contribute to the external digestion of the prey.
The Rare Exceptions: Non-Venomous Spiders
While the vast majority of spiders are venomous, a few rare exceptions lack venom glands or functional venom delivery systems. The most notable examples belong to the family Uloboridae, also known as cribellate orb weavers. Their absence of venom glands is a trait that evolved secondarily.
These non-venomous spiders have developed methods for capturing prey. They rely on their specialized silk. Uloborids wrap their prey extensively in many meters of cribellate silk, creating a tightly bound package.
Following the silk-wrapping, these spiders cover their ensnared prey with regurgitated digestive fluids. These fluids contain enzymes that break down the prey’s tissues externally. Research suggests that some of these digestive fluids may contain repurposed toxins.
Spider Venom and Human Interaction
Most spider species are not considered dangerous to humans. Many spiders have fangs too small or fragile to penetrate human skin. Bites usually occur when a spider feels threatened or is accidentally trapped against the skin.
Only about 25 species have venom capable of causing illness in humans. These include widow spiders (Latrodectus), recluse spiders (Loxosceles), Australian funnel-web spiders, and Brazilian wandering spiders. Even with these species, severe reactions are uncommon.
The effect of a spider bite on a human depends on factors like the amount of venom injected, the specific toxins present, and the individual’s sensitivity, age, and health status. Symptoms from most spider bites are mild, limited to localized pain, redness, and swelling at the bite site. While more severe symptoms like muscle cramps, nausea, or tissue damage can occur with bites from certain species, fatalities are extremely rare, estimated at fewer than five deaths worldwide per year.