Crab spiders (family Thomisidae) are arachnids recognized for their flattened, crab-like body shape and unique leg posture. They do not construct sticky silken webs to capture meals. Instead, these predators rely on stealth and surprise, specializing as hunters found primarily on flowers and foliage where they await passing insects.
Primary Prey and Hunting Grounds
The primary diet of crab spiders consists of insects that visit flowers, establishing them as effective predators of pollinators. Their menu includes flies, smaller butterflies, wasps, and bees. They can successfully capture and consume prey, such as bumblebees, which are several times larger than the spider itself, due to the speed of their attack and the potency of their venom. They are most recognized for positioning themselves on the petals and centers of brightly colored flowers, though some species also hunt on tree bark or among leaf litter.
The Ambush Strategy
Crab spiders are sit-and-wait predators. Their characteristic hunting posture involves holding the first two pairs of legs—which are noticeably longer and stronger—outstretched and poised for immediate action. This stance gives them their common name and allows them to move laterally and backward, mimicking the movement of a crab for subtle repositioning.
When an unsuspecting insect lands or crawls within striking distance, the spider executes a lightning-fast pounce, using its powerful forelegs to seize the victim. The attack is completed by injecting a fast-acting, paralyzing venom that quickly subdues the prey. The venom is highly toxic to insects but poses no threat to humans.
Unique Adaptations for Capturing Prey
The efficacy of the crab spider’s ambush technique depends on its ability to blend seamlessly into the environment. Many species possess fixed cryptic colors, but select groups, such as the Goldenrod Crab Spider (Misumena vatia), demonstrate reversible color change. This adaptation, known as chromatic adaptation, allows the spider to shift its body coloration between white and yellow to perfectly match the flower it occupies.
The color change is a physiological process achieved by actively secreting or reabsorbing liquid yellow pigments, known as ommochromes, from specialized glands in the epidermal cells. The transition from white to yellow can be lengthy, sometimes requiring up to 25 days to fully complete the color match. Conversely, reverting from yellow back to white is generally quicker, often completed in five to six days, ensuring the predator remains virtually undetectable to both prey and larger visual predators.