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Spider sex violent but effective

30 April 09 13:59 GMT

A violent but evolutionarily effective mating strategy has been spotted in spiders from Israel.

Males of the aptly-named Harpactea sadistica species pierce the abdomen of females, fertilising their eggs directly in the ovaries.

The so-called traumatic insemination gives the first male to inseminate a reproductive advantage by bypassing structures in the females' genitalia.

The findings are reported in Proceedings of the Royal Society B.

Insects including mites and bedbugs have been spotted using a similar strategy, but this is the first time that it has been seen in spiders.

Typically, spider males deliver their genetic package via sperm that is deposited into a small web and manually inserted using a pair of appendages on their undersides known as pedipalps.

The sperm are then held in a receptacle between the ovipore and ovary known as a spermatheca until an egg is released.

However, the spermatheca is a "last in, first out" structure, so that if any further males inseminate a female, the last mate's sperm is the first in line to fertilise an egg.

Direct route

Milan Rezic, an entomologist at the Crop Research Institute in Prague, has spotted a spider circumventing this problem by delivering sperm directly to the ovaries via holes that the males bore directly in the females' abdomens.

Naming the species H. sadistica, Dr Rezac noted that the species has specialised sex organs at the ends of its pedipalps, with one part specialised for gripping and another, hypodermic needle-like structure for injecting sperm.

Like many spider mating rituals, H. sadistica's approach follows an elaborate pattern, with the male tapping the female, subduing her, and wrapping himself around her to properly position the sex organs.

He then alternates between the two, piercing and injecting the sperm on one side, then the other, forming two neat rows of holes in her abdomen.

An analysis of the females of the species has shown that relative to other spiders, their spermathecae are atrophied, or shrunken.

In an apparent case of co-evolution, they seem to be slowly shrinking into nonexistence now that their purpose is being bypassed by the males' more direct approach.

"In insects there is a co-evolutionary development of female physiological responses to the male sperm that gives her at least some control of fertilisation," said William Eberhard, an expert in the mating habits of insects and spiders at the Smithsonian Tropical Research Institute.

"Something similar might occur here."

Dr Rezac suggests equally that a means to avoid the injury caused by the males might drive the evolution of secondary genitalia nearer to the ovaries, which have been seen in some spiders and butterflies.

"The evolution of these features has been heretofore difficult to explain," he said.

"Perhaps the secondary genital structures of butterflies and spiders could have originated via traumatic insemination."

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