Many people wonder if all spiders truly possess eight legs. This common question often arises from encountering various multi-limbed creatures in nature. Understanding spider anatomy clarifies their typical leg count and distinguishes them from other arthropods.
Spider Anatomy and Leg Count
True spiders, belonging to the Order Araneae, are defined by several distinct anatomical features, most notably their eight legs. These eight legs are arranged in four pairs, all of which attach to the cephalothorax, the fused head and thorax region of the spider’s body. A slim waist, called the pedicel, connects the cephalothorax to the second main body part, the abdomen.
Each of a spider’s legs consists of seven segments: the coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus. The tarsus, the tip of the leg, bears claws, typically two or three, which aid in movement and web manipulation. Spiders extend their limbs primarily through hydraulic pressure, using hemolymph (spider “blood”) to push their legs outwards, as they lack extensor muscles in the distal joints. This unique hydraulic system allows for efficient movement and explains why dead spiders often have their legs curled inwards.
Common Misconceptions and Look-Alikes
The widespread question about spider leg count often stems from encountering other creatures that resemble spiders but are anatomically different. Many arachnids, a class that includes spiders, do indeed have eight legs, but they are not all spiders. For instance, harvestmen (Order Opiliones), commonly known as “daddy longlegs,” possess eight legs but differ significantly from true spiders. Unlike spiders, harvestmen have a single, fused body segment rather than the distinct cephalothorax and abdomen of spiders, giving them an oval appearance.
Mites and ticks are also arachnids and typically have eight legs in their adult and nymph stages, though their larval stage may have only six legs. These creatures are generally much smaller than spiders and have a single, unsegmented body, which further distinguishes them. Insects, in contrast to arachnids, are characterized by having six legs, three body segments (head, thorax, and abdomen), and usually antennae and wings, none of which are found in spiders. These anatomical differences help differentiate spiders from other multi-legged organisms.
When Spiders Appear to Have Fewer Legs
Even though all true spiders are born with eight legs, an individual spider might sometimes appear to have fewer. A common reason for this is the loss of a leg due to injury or as a defense mechanism called autotomy. Like some other arthropods, spiders can voluntarily detach a limb to escape a predator, and younger spiders can regenerate lost legs over successive molts. However, once a spider reaches adulthood, it typically loses the ability to fully regenerate a lost limb.
Another factor creating the illusion of fewer legs is the presence and function of pedipalps. Spiders have a pair of pedipalps located near their mouthparts, in front of their walking legs. While jointed and somewhat leg-like in appearance, pedipalps are not used for walking like their main legs. They serve various functions, including handling food, sensing the environment, and in male spiders, transferring sperm during mating. When a spider holds its pedipalps or even one of its true legs in an unusual position, it can momentarily give the impression of an abnormal leg count.