April 30, 2009
Sexually Sadistic Spiders
Researchers have discovered a male spider in the Judean foothills of Israel with an evolutionarily effective yet sadistic sexual perversion.
For most spiders, the mating ritual involves the male ejecting sperm into the female's vulva, where it is stored it in a pouch called the spermatheca. These receptacles give the female control of when her eggs will be fertilized, by moving sperm onto them before she lays. The spermatheca is a "last in, first out" structure which means that the last male sperm has first dibs on fertilizing the egg.
The male species starts his elaborate ritual by biting the female, rendering her incapacitated, and wrapping himself around her in order to properly position the sex organs. He then pierces the body cavity of the female with two needle-like sex organs at the ends of its pedipalps. One part is specialized for gripping and another for directly fertilizing the ovaries by injecting sperm into the abdomen forming two neat holes.
He does this approximately six times, alternating between the two pedipalps, making this whole unsightly process last around 15 minutes. This "Ëtraumatic insemination' gives the first male to inseminate a reproductive advantage by circumventing structures in the females' genitalia.
An analysis of the females of the species has shown that their sex organs are comparatively smaller than those of other female spiders. This atrophy is most likely the result of the genitalia having not been used.
The bypassing of the female genitalia is causing them to shrink into nonexistence, which seems to be a case of co-evolution.
William Eberhard, an expert in the mating habits of insects and spiders at the Smithsonian Tropical Research Institute says, "In insects there is a co-evolutionary development of female physiological responses to the male sperm that gives her at least some control of fertilization."
"Something similar might occur here."
Rezac suggests that just like some spiders and butterflies, a means to avoid the injury caused by the males might drive the evolution of secondary genitalia closer to the ovaries.
This may give a possible explanation to what provided the impetus for such an evolutionary process.
"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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