Palpal bulb

The two palpal bulbs – also known as palpal organs and genital bulbs – are the copulatory organs of a male spider. They are borne on the last segment of the pedipalps (the front "limbs" of a spider), giving the spider an appearance often described as like wearing boxing gloves. The palpal bulb does not actually produce sperm, being used only to transfer it to the female. Palpal bulbs are only fully developed in adult male spiders and are not completely visible until after the final moult. In the majority of species of spider, the bulbs have complex shapes and are important in identification.

The palpal bulb of a mature male spider is borne on the last segment of the pedipalp. This segment usually has touch-sensitive hairs (setae) with nerves leading to them. The bulb itself is entirely without nerves, and hence without sensory organs and muscles, since these depend on nerves for their functioning, although some spiders have one or two muscles external to the bulb and connected to it by tendons. The bulb contains a tube or duct, usually coiled, open near the tip of the bulb and closed at the other end, in which sperm is stored before being used to inseminate a female. The closed end may be expanded, forming a "fundus". The tube usually opens via a narrow tip, the "embolus".

The palpal bulbs are only fully developed in adult male spiders. They develop within the end segment of the palp (the tarsus), and are only completely visible after the final moult. In some species, apart from carrying the palpal bulb, the tarsus is relatively unchanged. In most species, the tarsus changes shape and forms a hollow structure which surrounds and protects the bulb. It is then called a "cymbium". The structure of the palpal bulb varies widely. Most species have a bulb made up of three groups of hardened parts (sclerites), separated from the rest of the palp and one another by elastic sacs called "haematodochae" (also spelt "hematodochae"). Normally, the haematodochae are collapsed and more-or-less hidden between the groups of sclerites, but they can be expanded by haemolymph being pumped in, thus causing the sclerites to move and separate. In some groups of spiders (e.g. Segestrioides species) the bulb is reduced to a single pear-shaped structure. By contrast, members of the Entelegynae have evolved extremely elaborate palpal bulbs, with multiple complexly shaped sclerites.

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Wikipedia