Tuesday, July 14, 2009

Traumatic Insemination in aptly named 'Harpactea sadistica' spider

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. This has been described as a "traumatic insemination strategy," in which insemination wounds are created by male genitalia in areas outside the genital orifice of females. It is practiced because it gives the first male a reproductive advantage by bypassing structures in the females' genitalia.

These tactics have been observed in insects such as mites, bedbugs, and flies, but this study was the first time that it was documented in spiders. Typically, spider males deliver their genetic package via sperm that manually inserted using a pair of appendages called 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 fertilize an egg.

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. The male possesses a pair of emboli, appendages modified for piercing females.

The way in which the male H. sadistica inseminates the female is choreographed and complex. The male taps the female, subdues her, and wraps himself around her to properly position the sex organs. He then alternates between the two organs, piercing and injecting the sperm on one side, then the other. The physical marks left are 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. This is yet another example of the co-evolutionary arm's race we see between many male and female species, in which males evolve to more efficiently inseminate the female and displace other males' seminal fluid, and females evolve to be able to control who inseminates her precious eggs.

Dr. Rezac foresees the race continuing. He suggests 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 observed in some spiders and butterflies.

Rezac, M. 2009. The spider Harpactea sadistica: co-evolution of traumatic insemination and complex female genital morphology in spiders. Proceedings of the Royal Society: Biological Sciences 276(1668):2697-701.

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