Crab spiders (family Thomisidae) are common in gardens and fields worldwide. Despite their unusual appearance, crab spiders are not considered dangerous to humans. This article clarifies the facts about their mild toxicity and behavior, providing accurate information about the level of threat they pose.
Crab Spiders: Venomous but Not Medically Significant
Crab spiders are technically venomous, meaning they inject toxins through a bite. Like nearly all spiders, they possess venom glands and fangs, but their venom is designed to quickly immobilize small insect prey.
The chemical composition of this venom has a negligible effect on mammals, including humans. Their fangs are quite small, and many species cannot pierce human skin at all. A bite is an extremely rare event, as they are not aggressive and only bite in self-defense if accidentally squeezed.
Appearance and Hunting Behavior
Crab spiders are named for their unique physical characteristics and movement, including their propensity to walk sideways like a crab. Their first two pairs of legs are noticeably longer and stronger than the others, and these are often held extended outward, resembling a crab’s claws. Most species are small, ranging from 4 to 10 millimeters in body size, with wide, somewhat flattened bodies.
They are ambush predators and do not spin webs to catch prey. They are frequently found perched on flowers or foliage, using camouflage to blend in with their surroundings. Some species, such as the Goldenrod Crab Spider (Misumena vatia), can actively change their body color from yellow to white to match the flower they are hunting on.
What to Expect from a Bite
If a crab spider bite occurs, the typical human reaction is minimal, often described as similar to a pinprick or a mild bee sting. The venom is not strong enough to cause systemic effects or widespread injury. The most common symptoms are localized to the bite site and include transient discomfort or a brief stinging sensation.
A slight, localized reaction may develop, such as redness, swelling, or a small amount of irritation. These symptoms usually subside within a few hours to a day without any treatment. Basic first aid involves cleaning the area with soap and water and applying a cold compress to help manage any slight swelling. While extremely uncommon, if a person experiences signs of a severe allergic reaction, such as difficulty breathing or hives, medical attention should be sought immediately.