Pedipalps are the second pair of appendages found near the mouthparts of all arachnids, situated just behind the chelicerae, or fangs. While they are present in both sexes, the pedipalps of mature male spiders are dramatically modified to serve a reproductive function. This modification transforms the tips into complex, bulbous structures that are visually distinct, often leading to the appearance of “boxing gloves” on the male’s face. These highly specialized organs are central to the male spider’s unique and indirect method of sperm transfer.
General Anatomy and Location
The pedipalps are attached to the cephalothorax, the fused head and thorax region of the spider, positioned between the chelicerae and the first pair of walking legs. Like the legs, each pedipalp is a jointed structure composed of six segments: the coxa, trochanter, femur, patella, tibia, and tarsus. The coxae, the segments closest to the body, often have extensions called maxillae that assist in processing food.
The terminal segment, the tarsus, houses the specialized anatomy of the mature male. This segment is enlarged to contain the palpal bulb, a complex, hard-plated structure that develops fully only after the final molt. In many species, the tarsus changes shape to form a protective, spoon-shaped structure called the cymbium, which cradles the delicate bulb.
The palpal bulb is a non-muscular, non-sensory organ containing a coiled duct for sperm storage. This internal reservoir ends in a narrow, often whip-like or leaf-like tip known as the embolus. The intricate shape of the palpal bulb and its components varies greatly between species, making it a feature used for spider classification and identification.
The Mechanics of Sperm Transfer
The testes are located in the abdomen, but the copulatory organs are on the pedipalps, requiring an indirect transfer method. Before mating, the male performs sperm induction by spinning a small, specialized “sperm web.”
The male deposits a drop of sperm fluid onto the silk sheet from an opening on the underside of his abdomen. He then uses the palpal bulbs to draw the sperm up into the internal reservoir. This loading process can take minutes to several hours, and the mechanism is thought to involve capillary action or pressure changes.
Once the pedipalps are charged, the male is ready to seek a mate. During copulation, the male inserts the embolus of one or both pedipalps into the female’s genital structure, a hardened plate on the abdomen called the epigynum. For most species, the male inserts one pedipalp into one of the female’s two copulatory openings.
The actual injection of sperm is a hydraulic process, as the palpal bulb lacks internal muscles. Increased hemolymph pressure in the surrounding soft tissue causes the bulb to inflate and the embolus to twist or hook into the epigynum. This hydraulic inflation acts like a syringe, forcing the stored sperm into the female’s receptacles. Mating can be brief or extended, and the male must refill his pedipalps if he mates again.
Auxiliary Roles Beyond Reproduction
While reproduction is the most distinct use of the male pedipalps, they still retain their original functions as multi-purpose appendages. The pedipalps are highly sensitive organs that act as tactile extensions, helping the spider sense objects and navigate its environment. They are equipped with sensitive hairs that allow the spider to detect touch and even ground vibrations.
The pedipalps also contain chemical detectors, functioning as organs for taste and smell to supplement the spider’s other sensory input. Spiders use them for grooming, often seen cleaning their chelicerae and legs with these appendages. The palps can also assist in handling prey, manipulating food toward the mouthparts, and in some species, even helping to shape the silk web.