The common name “crab spider” refers to any spider belonging to the family Thomisidae, a diverse group of arachnids found globally. Unlike many other spiders, these species do not build webs to capture their prey, instead adopting a “sit-and-wait” approach as ambush predators. They typically hide on flowers, leaves, or bark, patiently waiting for an unsuspecting insect to come within striking range.
Size Variability Across Species and Sexes
The size of a crab spider is highly variable and depends significantly on the species and the sex of the individual. The vast majority of crab spiders are quite small, with the smallest North American species having a body length of only about 3 millimeters. However, the largest species, particularly those found in tropical regions, can reach a body length of up to 10 to 12 millimeters, excluding their legs.
The most noticeable factor influencing size is sexual dimorphism, which is the physical difference between males and females of the same species. Female crab spiders are almost universally larger and substantially bulkier than their male counterparts. This size disparity can be dramatic, with the female of a species often being many times heavier than the male.
For example, the widespread Goldenrod Crab Spider (Misumena vatia) exhibits this pattern clearly. An adult female of this species typically measures between 6 and 10 millimeters in body length. Meanwhile, the adult male is often only 3 to 5 millimeters long, sometimes being only a quarter of the female’s size.
This difference in size is largely due to the female’s need to produce a large number of eggs, requiring a greater investment in body mass and energy. Females also reach their adult size by growing for a longer duration and undergoing more molts than males.
Distinguishing Physical Characteristics
Beyond size, the crab spider is instantly recognizable by its unique, crab-like body shape and leg arrangement. Their bodies are often flattened and wide, which assists them in blending seamlessly against flat surfaces like bark or flower petals. This morphology allows them to maximize their camouflage while waiting for prey.
The most defining characteristic is the configuration of their eight legs. The first two pairs of legs are noticeably longer, thicker, and significantly more robust than the rear two pairs. These powerful front legs are held out to the sides in a lateral, or crab-like, posture, ready to snap shut on passing insects.
This specialized leg structure gives the spiders the ability to scuttle sideways or even backward with ease, movements that resemble a crab rather than the typical forward walk of most spiders. Furthermore, many crab spiders are masters of color change, a crucial hunting strategy. Species like the Goldenrod Crab Spider can slowly change their body color from white to yellow, and sometimes even pink, to perfectly match the color of the flower they are ambushing prey from.
This color adjustment is achieved by manipulating liquid pigments within the hypodermis, the layer beneath the exoskeleton. By matching the surrounding environment, the spider becomes virtually invisible to pollinating insects like bees and flies, which they then ambush using their enlarged front limbs.
Interaction with Humans and Safety
Given the crab spider’s hunting prowess and use of venom, a common concern is the potential threat they pose to people. Crab spiders are not considered medically significant to humans, meaning their venom is not dangerous. Their primary function is to quickly subdue insect prey, which can be much larger than the spider itself.
Bites on humans are extremely rare because these spiders are timid and non-aggressive, preferring to flee or remain motionless when disturbed. A defensive bite would only occur if the spider were trapped or severely provoked, such as being accidentally squeezed against the skin.
In the rare event a bite does occur, the fangs of most species are too small and delicate to pierce human skin effectively. If penetration does happen, the result is typically minor, localized irritation, slight swelling, or a fleeting stinging sensation, comparable to a pinprick or a mosquito bite. Crab spiders are actually beneficial in gardens, where they help control populations of insect pests.