page_title: Palpal bulb
section_title: Structure
hierarchical_section_title: Palpal bulb / Structure
caption_attribution_description: English: Left male palp of a male of Thaida chepu (Araneomorphae: Austrochilidae) from a wet lowland mixed forest at Lago Huillinco (Chiloé, Chile). Extended focal plane images of male palp in prolateral (1), ventral (2) and retrolateral (3) view. Key: bH – basal hematodocha Cb – cymbium E – embolus HSt – hook of subtegulum mA – median apophysis mH – median hematodocha PSt – process of subtegulum St – subtegulum Te – tegulum
context_page_description: 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, 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.
context_section_description: 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.
  Spider specialists (arachnologists) have developed a detailed terminology to describe the kind of palpal bulb found in most spiders. Starting from the end nearest the head:
  the cymbium is the modified tarsus of the palp (Cb in the images)
  the basal haematodocha or proximal haematodocha separates the cymbium from the following group of sclerites (bH in image 3)
  the subtegulum is the main sclerite in the first group (St in image 1)
  the median haematodocha separates the subtegulum from the next group of sclerites (mH in images 1 and 2)
  the tegulum is the main sclerite in the second group (Te in the images), along with the median apophysis (mA in images 1 and 2) and the conductor (where present)
  the distal haematodocha separates the tegulum from the final group of sclerites (not shown in the images)
  the embolus is the main part of the third and final group of sclerites (E in the images)
  Other sclerites and "protrusions" (apophyses) may also be present. The diversity of entelegyne palpal bulbs makes it difficult to be sure that structures given the same name actually have the same origin (i.e. are homologous). This applies in particular to the "median apophysis". In mesothele spiders, whose palpal bulbs have the same basic structure as those of entelegyne spiders, the name contrategulum has been used in preference to "median apophysis" for the sclerite immediately following (distal to) the tegulum.
  The palpal bulb is expanded and moved during courtship and copulation. In mesothele spiders, such as Liphistius and Heptathela, there are two muscles, originating lower in the pedipalp, that attach by tendons to parts of the bulb and help to move it, the soft haematodochae allowing both movement and expansion. The same two muscles are also present in mygalomorphs. In araneomorph spiders there is a trend towards loss of muscles and greater development of the basal haematodocha. Most non-entelegyne araneomorphs have both muscles; most entelegynes have neither, although there are some, such as Argyroneta aquatica and Deinopis species, which retain one muscle.
caption: Left palpal bulb of a male Thaida chepu: 1 – from the right (prolateral side nearest the mouth); 2 – from the ventral side, i.e. underside; 3 – from the left (retrolateral side, furthest from the mouth)
  bH – basal haematodocha; Cb – cymbium; E – embolus; HSt – hook of subtegulum; mA – median apophysis; mH – median haematodocha; PSt – process of subtegulum; St – subtegulum; Te – tegulum
  See text for further explanation.
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