![Cheiracanthium mildei male](https://upload.wikimedia.org/wikipedia/commons/1/16/Cheiracanthium_mildei_male.jpg)
The two palpal bulbs – also known as palpal organs and genital bulbs – are the
copulatory organ
A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, ...
s of a male
spider
Spiders ( order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk. They are the largest order of arachnids and rank seventh in total species ...
. They are borne on the last segment of the
pedipalp
Pedipalps (commonly shortened to palps or palpi) are the second pair of appendages of chelicerates – a group of arthropods including spiders, scorpions, horseshoe crabs, and sea spiders. The pedipalps are lateral to the chelicerae ("jaws") and ...
s (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
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
, 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 biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...
. 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 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 (
sclerite
A sclerite (Greek , ', meaning "hard") is a hardened body part. In various branches of biology the term is applied to various structures, but not as a rule to vertebrate anatomical features such as bones and teeth. Instead it refers most commonly ...
s), 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
Hemolymph, or haemolymph, is a fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod (invertebrate) body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which ...
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
The Entelegynae or entelegynes are a subgroup of araneomorph spiders, the largest of the two main groups into which the araneomorphs were traditionally divided. Females have a genital plate ( epigynum) and a "flow through" fertilization system; m ...
have evolved extremely elaborate palpal bulbs, with multiple complexly shaped sclerites.
![Thaida chepu male, palp morphology cropped](https://upload.wikimedia.org/wikipedia/commons/c/ce/Thaida_chepu_male%2C_palp_morphology_cropped.jpg)
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)
[Unlike and , places the conductor with the embolus rather than with the median apophysis.]
* 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
''Liphistius'' is a genus of basal trapdoor spiders in the family Liphistiidae. They are found in Japan, China, and Southeast Asia.
Etymology
''Liphistius'' is from the Greek ''leipo'' (lacking) and ''stios'' (equality).
Biology
Female body l ...
'' and ''
Heptathela
''Heptathela'' is a genus of spiders that includes the Kimura spider (''Heptathela kimurai''). They are trapdoor spiders of the family Liphistiidae and are found in Japan, including Okinawa Island, Okinawa and the Ryukyu Islands. Spiders of this ...
'', 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
The diving bell spider or water spider (''Argyroneta aquatica'') is the only species of spider known to live almost entirely under water. It is the only member of the genus ''Argyroneta''. When out of the water, the spider ranges in colour from m ...
'' and ''
Deinopis
''Deinopis'', also known as net-casting spiders, gladiator spiders and ogre-faced spiders, is a genus of net-casting spiders that was first described by W. S. MacLeay in 1839. Its distribution is widely tropical and subtropical. They catch thei ...
'' species, which retain one muscle.
Function
Like most
arachnid
Arachnida () is a class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegaroon ...
s, spiders have internal fertilization by indirect sperm transfer. The tubular
testes
A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testoster ...
of a male spider, which produce sperm, are located in the
abdomen
The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the torso. ...
. Sperm is exuded from the
gonopore
A gonopore, sometimes called a gonadopore, is a genital pore in many invertebrates. Hexapods, including insects have a single common gonopore, except mayflies, which have a pair of gonopores. More specifically, in the unmodified female it is the ...
(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. Various mechanisms have been proposed to explain how the sperm is taken up. Capillary action and gravity are two possibilities. Where the sperm duct has rigid walls, removal of the liquid via the
epithelium
Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...
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.
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. 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". 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
''Agelenopsis'', commonly known as the American grass spiders, is a genus of funnel weavers first described by C.G. Giebel in 1869. They weave sheet webs that have a funnel shelter on one edge. The web is not sticky, but these spiders make up fo ...
'', 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. In ''
Araneus'', 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.
Evolution and phylogenetic significance
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. There is no fossil evidence of the stages by which this transformation occurred.
The early diverging
Mesothelae
The Mesothelae are a suborder of spiders (order Araneae) that includes a single extant family, Liphistiidae, and a number of extinct families. This suborder is thought to form the sister group to all other living spiders, and to retain ancestral ...
have moderately complex palpal bulbs, in which the same three basic sclerites (hardened parts) are present as in the most derived
Entelegynae
The Entelegynae or entelegynes are a subgroup of araneomorph spiders, the largest of the two main groups into which the araneomorphs were traditionally divided. Females have a genital plate ( epigynum) and a "flow through" fertilization system; m ...
. Many
mygalomorph
The Mygalomorphae, or mygalomorphs, are an infraorder of spiders, and comprise one of three major groups of living spiders with over 3000 species, found on all continents except Antarctica. Many members are known as trapdoor spiders due to the ...
and
haplogyne
The Haplogynae or haplogynes are one of the two main groups into which araneomorph spiders have traditionally been divided, the other being the Entelegynae. Morphological phylogenetic studies suggested that the Haplogynae formed a clade; more recen ...
spiders, along with other non-entelegyne araneomorphs, have less complex palpal bulbs; in some cases (e.g. ''
Segestrioides'' species) the bulb is reduced to a single pear-shaped structure. By contrast, many members of the
Entelegynae
The Entelegynae or entelegynes are a subgroup of araneomorph spiders, the largest of the two main groups into which the araneomorphs were traditionally divided. Females have a genital plate ( epigynum) and a "flow through" fertilization system; m ...
have extremely elaborate palpal bulbs, with multiple complexly shaped sclerites. ''
Thomisus spectabilis
''Thomisus spectabilis'', also known as the white crab spider or Australian crab spider, is a small spider found in Australia and far east Asia.
The body length of the female is up to 10 mm, the male 6.2 mm. Including legs, the spider ...
'' have disc shaped palpal bulbs.
![Unicorn catleyi copulatory bulb](https://upload.wikimedia.org/wikipedia/commons/f/fd/Unicorn_catleyi_copulatory_bulb.png)
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. Alternatively, ancestral spiders may have had simple palpal bulbs, with parallel evolution of more complex bulbs in the Mesothelae and the Entelegynae. The fact that all the major parts of entelegyne palpal bulbs are already found in mesotheles argues against this view.
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.
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
The epigyne or epigynum is the external genital structure of female spiders. As the epigyne varies greatly in form in different species, even in closely related ones, it often provides the most distinctive characteristic for recognizing species. ...
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.
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. 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. It might be expected that the females of
parthenogenetic
Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and developmen ...
species, like ''
Triaeris stenaspis
''Triaeris stenaspis'' is a species of spider in the family Oonopidae, with a pantropical distribution. It is also found in Iran and has been introduced into Europe. A very small spider, with a maximum body length of under , it has been shown t ...
'', 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.
Notes and references
Bibliography
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External links
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Spider anatomy