Crab spiders (family Thomisidae) have abandoned the traditional web-spinning strategy for a life of stealth and camouflage. They are found worldwide, inhabiting diverse environments but often favoring flowers and foliage where prey is abundant. Their evolutionary path has led them to become masters of disguise, relying on a distinctive body shape and physiological adaptations to secure their meals. This specialized hunting technique allows them to capture insects much larger than themselves, making them efficient predators in their ecosystems.
Physical Identification and Color Adaptation
The common name “crab spider” comes from their distinctive body plan, featuring a broad, often flattened abdomen and a crab-like posture. The first two pairs of legs are noticeably longer and thicker than the posterior pairs, held laterally, allowing them to scuttle sideways and backward with ease. These powerful forelimbs are central to their predatory success, acting as a trap for unsuspecting insects.
Many Thomisidae species possess an ability to change their body color, a trait especially pronounced in flower-dwelling species like the goldenrod crab spider (Misumena vatia). A spider hunting on a white daisy will appear white, while the same individual moved to a yellow goldenrod can gradually turn yellow. This color change results from the spider’s physiological control over yellow ommochrome pigments located in the outer layer of its body.
To turn yellow, the spider actively secretes pigments into the body’s outer cellular layers, a process that can take up to 25 days. To switch back to white, the spider resorbs the pigment or excretes it, which takes a shorter period, often around six days. This slow, controlled process allows the crab spider to maintain an almost perfect visual match with the petals it occupies, providing camouflage from both predators and prey.
Ambush Hunting Tactics
Crab spiders are ambush predators, employing a “sit-and-wait” strategy. Unlike orb-weavers, they do not spin a silk net for capturing food, instead relying on camouflage on flowers or leaves. They remain motionless, often for hours or even days, with their enlarged front legs held wide and poised to strike.
Their targets are insects visiting flowers for nectar or pollen, including bees, butterflies, moths, and flies. When prey comes within range, the spider rapidly lunges, using its powerful front legs to grasp and restrain the victim. It delivers a paralyzing bite, injecting venom that swiftly incapacitates insects several times the spider’s size.
Once the prey is subdued, the crab spider utilizes external digestion, injecting enzymes that liquefy the insect’s internal tissues. The spider then sucks the nutrient-rich fluids, leaving behind a hollow exoskeleton. Although silk is not used for trapping, crab spiders spin a dragline or safety tether to prevent falls, and females use silk to construct a protective sac for their eggs.
Interaction with Humans
Despite their hunting skills, crab spiders pose virtually no threat to humans. Their venom, while effective against small insects, is not medically significant to people and is too weak to cause more than minor, localized irritation. Bites are rare because these spiders are timid and prefer to flee or remain still when disturbed.
A defensive bite typically only happens if a person accidentally presses a crab spider against their skin, such as when gardening. Any resulting discomfort is often compared to a mild bee sting, presenting as temporary redness or slight swelling. No medical treatment is usually required.
Crab spiders are considered beneficial, especially in garden and agricultural settings, due to their role as natural pest controllers. By preying on a wide variety of insects, they help regulate local populations. Their presence is an indicator of a healthy ecosystem.