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Palpal bulb

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MaleCheiracanthium mildeishowing the enlarged and darker palpal bulbs at the end of the pedipalps

The twopalpal bulbs– also known aspalpal organsandgenital bulbs– are thecopulatory organsof a malespider.They are borne on the last segment of thepedipalps(the front "limbs" of a spider), giving the spider an appearance often described as like wearing bo xing gloves. The palpal bulb does not actually producesperm,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 finalmoult.In the majority of species of spider, the bulbs have complex shapes and are important in identification.

Structure[edit]

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,[1]although some spiders have one or two muscles external to the bulb and connected to it by tendons.[2]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".[3][4][5]

The palpal bulbs are only fully developed in adult male spiders. They develop within the end segment of the palp (thetarsus), 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".[3]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 byhaemolymphbeing pumped in, thus causing the sclerites to move and separate.[6]In some groups of spiders (e.g.Segestrioidesspecies) the bulb is reduced to a single pear-shaped structure. By contrast, members of theEntelegynaehave evolved extremely elaborate palpal bulbs, with multiple complexly shaped sclerites.[7]

Left palpal bulb of a maleThaida 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.

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:[6][8][9][10]

  • thecymbiumis the modified tarsus of the palp (Cbin the images)
  • thebasal haematodochaorproximal haematodochaseparates the cymbium from the following group of sclerites (bHin image 3)
  • thesubtegulumis the main sclerite in the first group (Stin image 1)
  • themedian haematodochaseparates the subtegulum from the next group of sclerites (mHin images 1 and 2)
  • thetegulumis the main sclerite in the second group (Tein the images), along with themedian apophysis(mAin images 1 and 2) and theconductor(where present)[11]
  • thedistal haematodochaseparates the tegulum from the final group of sclerites (not shown in the images); inLycosidaeand certain other taxa,[12]: 7–8 apalea,a part of the distal tegulum that is not fully sclerotized,[13]: 17 is formed out of the wall of the distal haematodocha[12]: 7–8 
  • theembolusis the main part of the third and final group of sclerites (Ein 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. arehomologous).[14]This applies in particular to the "median apophysis". Inmesothelespiders, whose palpal bulbs have the same basic structure as those of entelegyne spiders, the namecontrategulumhas been used in preference to "median apophysis" for the sclerite immediately following (distal to) the tegulum.[15]

Diagram of palpal bulb components

The palpal bulb is expanded and moved during courtship and copulation. In mesothele spiders, such asLiphistiusandHeptathela,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 inmygalomorphs.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 asArgyroneta aquaticaandDeinopisspecies, which retain one muscle.[2]

Function[edit]

Like mostarachnids,spiders have internal fertilization by indirect sperm transfer. The tubulartestesof a male spider, which produce sperm, are located in theabdomen.[16]Sperm is exuded from thegonopore(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.[17][16]Various mechanisms have been proposed to explain how the sperm is taken up. Capillary action and gravity are two possibilities.[17]Where the sperm duct has rigid walls, removal of the liquid via theepitheliumsurrounding 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.[4]

In most spiders (in particular mesotheles and entelegynes) only the end of the bulb – the embolus – is inserted into a female pore during copulation before the sperm is ejaculated. In a minority of spiders with simple palps (mygalomorphs and haplogynes), most or all of the bulb is inserted.[17]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".[18]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. InAgelenopsis,the embolus at the tip of the bulb first engages with the female, after which the haematodochae expand and cause the conductor to mesh with the female, before finally the embolus enters the female copulatory pore.[19]InAraneus,the median apophysis first hooks onto part of the female epigyne, positioned by the conductor, before inflation of the haematodocha causes the tegulum to rotate, pushing the embolus into the copulatory pore.[20]

Evolution and phylogenetic significance[edit]

Palpal bulbs are believed to have evolved from the claws of the male pedipalp. Prior to the male's final moult, the bulb develops from cells at the base of where the claw would be. The transformation of claws to bulbs occurred before the evolution of all living spiders.[2]There is no fossil evidence of the stages by which this transformation occurred.

The early divergingMesothelaehave moderately complex palpal bulbs, in which the same three basic sclerites (hardened parts) are present as in the most derivedEntelegynae.[21]Manymygalomorphandhaplogynespiders, along with other non-entelegyne araneomorphs, have less complex palpal bulbs; in some cases (e.g.Segestrioidesspecies) the bulb is reduced to a single pear-shaped structure. By contrast, many members of theEntelegynaehave extremely elaborate palpal bulbs, with multiple complexly shaped sclerites.[7][8]Thomisus spectabilishave disc shaped palpal bulbs.

Simple palpal bulb ofUnicorn catleyi,a haplogyne spider
Spider phylogeny and palpal bulb complexity
Aranae (spiders)

Mesothelae

Opisthothelae

Mygalomorphae

Araneomorphae

Haplogynes and other non-entelegynes

Entelegynae

Members of the earlier diverging Mesothelae (green) have moderately complex palpal bulbs with muscular movement; the derived mygalomorphs, haplogynes and other non-entelegyne araneomorphs (yellow) have simpler palpal bulbs with muscular movement; Entelegynae species (blue) have the most complex palpal bulbs, most with only hydraulic movement.

Two explanations have been proposed for the pattern of palpal bulb complexity. The ancestors of all modern spiders may have had moderately complex palpal bulbs, with simpler ones evolving in the Mygalomorphae and non-entelegyne araneomorphs and more complex ones evolving in the Entelegynae.[7]Alternatively, ancestral spiders may have had simple palpal bulbs, with parallel evolution of more complex bulbs in the Mesothelae and the Entelegynae.[22]The fact that all the major parts of entelegyne palpal bulbs are already found in mesotheles argues against this view.[23]

The evolutionary change from muscular to hydraulic movement of palpal bulbs is hypothesised to have been driven by the more complex movements this made possible. With only two muscles available, the degree of movement is limited; for example the maximum reported rotation is 300° with muscle movement as opposed to 540° with hydraulic movement. Complex folding of haematodochae in the un-expanded bulb can result in rotations and tilts as the haematodochae expand with bulb's inflation.[24]

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. Theepigyneof 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.[25]

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.[26]The more complex movements possible with "advanced" palpal bulbs may provide signals that can be used by females to accept or reject males, during both courtship and copulation.[24]It might be expected that the females ofparthenogeneticspecies, likeTriaeris stenaspis,in which males are completely unknown, would have simple genital structures. It has been described as a "mystery" as to why they in fact have "anomalously complex" ones.[27]

Notes and references[edit]

  1. ^Eberhard & Huber (2010),p. 253
  2. ^abcHuber (2004)
  3. ^abRoberts (1995),p. 17
  4. ^abEberhard & Huber (2010),p. 250
  5. ^Ruppert, Fox & Barnes (2004),p. 580–581
  6. ^abRoberts (1995),p. 18
  7. ^abcEberhard & Huber (2010),pp. 250–251
  8. ^abFoelix (2011),pp. 226–229
  9. ^Ramírez (2014),pp. 210–239
  10. ^Michalik, P.; Ramírez, M.J. (2013), "First description of the male ofThaida chepuPlatnick, 1987 (Araneae, Austrochilidae) with micro-computed tomography of the palpal organ ",ZooKeys(352): 117–125,doi:10.3897/zookeys.352.6021,PMC3837400,PMID24294094
  11. ^UnlikeFoelix (2011,p. 228) andRamírez (2014,p. 239),Roberts (1995,p. 18) places the conductor with the embolus rather than with the median apophysis.
  12. ^abEdwards, G.B. (2004)."Revision of the jumping spiders of the genusPhidippus(Araneae: Salticidae) "(PDF).Occasional Papers of the Florida State Collection of Arthropods.11.ISSN0885-5943.Retrieved29 March2023.
  13. ^Saaristo, Michael I. (2010). "Arachnida". In Gerlach, Justin; Marusik, Yuri (eds.).Arachnida and Myriapoda of the Seychelles islands.Manchester: Siri Scientific Press. pp. 7–358.ISBN978-0-9558636-8-4.
  14. ^Eberhard & Huber (2010),p. 249
  15. ^Haupt (2004),pp. 37, 91–93
  16. ^abRuppert, Fox & Barnes (2004),p. 581
  17. ^abcRoberts (1995),p. 22.
  18. ^Eberhard & Huber (2010),p. 254
  19. ^Eberhard & Huber (2010),pp. 254–255
  20. ^Foelix (2011),p. 229
  21. ^Haupt (2004),pp. 91–92
  22. ^Foelix (2011),pp. 335
  23. ^Haupt (2004),p. 37
  24. ^abHuber (2004),pp. 370–371
  25. ^Eberhard & Huber (2010),pp. 260–261
  26. ^Eberhard & Huber (2010),pp. 263–265
  27. ^Platnick, N.I.; Dupérré, N.; Ubick, D. & Fannes, W. (2012),"Got males? The Enigma tic goblin spider genusTriaeris(Araneae, Oonopidae) ",American Museum Novitates(3756): 1–36,doi:10.1206/3756.2,hdl:2246/6369,S2CID83923612

Bibliography[edit]

External links[edit]

  • Media related toPedipalpsat Wikimedia Commons
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