Palpal bulb

Palpal bulbs are only fully developed in adult male spiders and are not completely visible until after the final moult.

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").

By contrast, members of the Entelegynae have evolved extremely elaborate palpal bulbs, with multiple complexly shaped sclerites.

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.

[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.

[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".

[19] 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.

The early diverging Mesothelae have moderately complex palpal bulbs, in which the same three basic sclerites (hardened parts) are present as in the most derived Entelegynae.

By contrast, many members of the Entelegynae have extremely elaborate palpal bulbs, with multiple complexly shaped sclerites.

Mesothelae Mygalomorphae Haplogynes and other non-entelegynes Entelegynae Two explanations have been proposed for the pattern of palpal bulb complexity.

[22] The fact that all the major parts of entelegyne palpal bulbs are already found in mesotheles argues against this view.

[24] Various explanations have been proposed for the evolution of the complex structure of the palpal organs found in most groups of spiders.

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.

[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 of parthenogenetic species, like Triaeris stenaspis, in which males are completely unknown, would have simple genital structures.