Crab spiders (Family Thomisidae) are often encountered in gardens and on flowering plants, prompting concern about their potential danger to people. Their unusual appearance and hunting style can cause alarm for observers. These small arachnids get their name from their crab-like shape and sideways movement. They are active predators frequently seen waiting on flowers, leading to questions about whether they pose a threat to human health.
Defining the Danger: Venom vs. Poison
The distinction between venomous and poisonous is important when discussing spiders like the crab spider. Crab spiders are technically venomous because they produce and inject toxins to subdue prey, typically small insects such as bees and flies. Their venom contains neurotoxic components strong enough to immobilize victims much larger than themselves.
Crab spiders are not poisonous, as their toxins are not passively absorbed through touch or ingestion. For humans, their venom poses no significant health risk and they are not considered medically significant. The venom is specifically adapted to the physiology of small arthropods.
Identifying Common Crab Spiders
Crab spiders are identified primarily by their physical structure, which gives them their common name. They possess two pairs of front legs that are noticeably longer and more robust than the rear pairs, held out in a crab-like stance. This unique leg arrangement allows them to move sideways and backward, a behavior characteristic of the family Thomisidae.
These spiders are ambush hunters that do not spin typical capture webs, instead relying on camouflage and patience. Many species, such as the Goldenrod Crab Spider (Misumena vatia), can change their color over several days to match the flowers they wait on. Their coloration can range from bright white, yellow, or green, helping them blend seamlessly into their surroundings.
What to Do If Bitten: Symptoms and First Aid
A bite from a crab spider is extremely rare because these spiders are timid and non-aggressive, preferring to flee from a threat. Bites usually only occur when the spider is accidentally trapped or pressed against human skin. If a bite does happen, the reaction is typically minimal and short-lived.
Symptoms generally include slight localized pain or a brief stinging sensation, often compared to a mild bee sting or mosquito bite. The bite site may show minor redness, a small raised area, and slight localized swelling. These mild symptoms usually subside completely within a few hours to a day without complications.
First aid for a crab spider bite is simple and focused on comfort and hygiene. Immediately clean the area with mild soap and water to prevent secondary infection. Applying a cold compress or ice pack can help reduce any minor swelling or discomfort. Over-the-counter pain relievers can also be used if needed. Medical attention may be necessary if an individual experiences signs of an allergic reaction, such as difficulty breathing or hives. Seeking a medical opinion is also recommended if the bite area shows signs of a secondary bacterial infection, such as increasing redness or pus.