Palpal bulb

Male Cheiracanthium mildei showing the enlarged and darker palpal bulbs at the end of the pedipalps

The two palpal bulbs or palpal organs 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.

Structure

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 muscles and sensory organs, since these depend on nerves for their functioning.[1] 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".[2][3][4]

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".[2] 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 three 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.[5] 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.[6]

Left palpal bulb of a male Thaida chepu: 1 – from the right (side nearest the mouth); 2 – from the underside; 3 – from the left (side away from the mouth)
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
See text for further explanation.

Spider specialists (arachnologists) have developed a detailed terminology to describe entelegyne palpal bulbs. Starting from the end nearest the head:[5][7][8][9]

Other sclerites or "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).[11]

Function

Like most arachnids, spiders have internal fertilization by indirect sperm transfer. The tubular testes of a male spider, which produce sperm, are located in the abdomen.[12] Sperm is exuded from the gonopore (genital opening) of the male and deposited on the top surface of a small "sperm web", constructed for this sole purpose. The male moves to the underside of the sperm web and takes up the sperm into the sperm ducts of the palpal bulbs, either through the base of the web or around its side.[13][12] Various mechanisms have been proposed to explain how the sperm is taken up. Capillary action and gravity are two possibilities.[13] Where the sperm duct has rigid walls, removal of the liquid via the epithelium surrounding them may suck sperm into the duct. The reverse mechanism may explain how the sperm is ejected. In other species with more flexible duct walls, changes in pressure of the surrounding haemolymph may be involved.[3]

In most spiders, during copulation, only the end of the bulb – the embolus – is inserted into a female pore before the sperm is ejaculated. In a minority of spiders with simple palps, most or all of the bulb is inserted.[13] Since the palpal bulbs lack sensory organs, the male faces difficulties in ensuring the correct positioning of the palpal bulbs relative to the female, difficulties which have been described as like "those of a person attempting to adjust a complex, delicate mechanism in the dark, using an elongate, elaborately formed fingernail".[14] In many species, a multi-step process is involved. Various structures on the palp and the palpal bulb create a "preliminary lock" on the female, creating a stable support point for further, more precise orientation. The expansion of the haematochodae then causes the various sclerites to brace with one another. The precise details differ from species to species. In Agelenopsis, the embolus at the tip of the bulb first engages with the female, after which the hematodochae expand and cause the conductor to mesh with the female, before finally the embolus enters the female copulatory pore.[15] In Araneus, the median apophysis first hooks onto part of the female epigyne, positioned by the conductor, before inflation of the hematodocha causes the tegulum to rotate, pushing the embolus into the copulatory pore.[16]

Evolution and phylogenetic significance

The early diverging Mesothelae have moderately complex palpal bulbs. Many members of the Mygalomorphae and the Haplogynae have less complex palpal bulbs; in some cases (e.g. Segestrioides species) the bulb is reduced to a single pear-shaped structure. By contrast, members of the Entelegynae have extremely elaborate palpal bulbs, with multiple complexly shaped sclerites.[6][7] Two explanations have been proposed. The ancestors of all modern spiders may have had moderately complex palpal bulbs, with simpler ones evolving in the Mygalomorphae and Haplogynae and more complex ones evolving in the Entelegynae.[6] Alternatively, ancestral spiders may have had simple palpal bulbs, with parallel evolution of more complex bulbs in the Mesothelae and the Entelegynae.[17]

Simple palpal bulb of Unicorn catleyi, a member of the Haplogynae group of spiders
Spider phylogeny and palpal bulb complexity
Aranae (spiders)

Mesothelae
Opisthothelae

Mygalomorphae
Araneomorphae

Haplogynae

Entelegynae







Members of the earlier diverging Mesothelae (green) mostly have moderately complex palpal bulbs; members of the derived Mygalomorphae and Haplogynae (yellow) have simpler palpal bulbs; Entelegynae species (blue) have the most complex palpal bulbs.

Various explanations have been proposed for the evolution of the complex structure of the palpal organs found in most groups of spiders. One is the "lock-and-key" theory. The epigyne of the female spider also has a complex shape, and studies of pairs killed instantaneously during copulation show a precise fit between the male and female structures. Hence the shapes of both the palpal organs and the epigynes may have evolved to ensure that only individuals of the same species can mate. However, this theory predicts that a species long separated from others (e.g. by being isolated on an island or in a cave) would have less complex copulatory structures, and this has not been observed.[18]

Another explanation is "cryptic female choice". Since the correct alignment of male and female structures is difficult, female genitalia may have evolved to ensure that only those males with "good quality" copulatory structures are able to mate, thus increasing the likelihood of having male offspring also able to mate successfully.[19]

Notes and references

  1. ↑ Eberhard & Huber (2010), p. 253
  2. 1 2 Roberts (1995), p. 17
  3. 1 2 Eberhard & Huber (2010), p. 250
  4. ↑ Ruppert, Fox & Barnes (2004), p. 580–581
  5. 1 2 Roberts (1995), p. 18
  6. 1 2 3 Eberhard & Huber (2010), pp. 250–251
  7. 1 2 Foelix (2011), pp. 226–229
  8. ↑ Ramírez (2014), pp. 210–239
  9. ↑ Michalik, P.; Ramírez, M.J. (2013), "First description of the male of Thaida chepu Platnick, 1987 (Araneae, Austrochilidae) with micro-computed tomography of the palpal organ", ZooKeys 352: 117–125, doi:10.3897/zookeys.352.6021
  10. ↑ Unlike Foelix (2011, p. 228) and Ramírez (2014, p. 239), Roberts (1995, p. 18) places the conductor with the embolus rather than with the median apophysis.
  11. ↑ Eberhard & Huber (2010), p. 249
  12. 1 2 Ruppert, Fox & Barnes (2004), p. 581
  13. 1 2 3 Roberts (1995), p. 22.
  14. ↑ Eberhard & Huber (2010), p. 254
  15. ↑ Eberhard & Huber (2010), pp. 254–255
  16. ↑ Foelix (2011), p. 229
  17. ↑ Foelix (2011), pp. 335
  18. ↑ Eberhard & Huber (2010), pp. 260–261
  19. ↑ Eberhard & Huber (2010), pp. 263–265

Bibliography

External links

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