Male spiders can feel sex through what would be the equivalent of their penis, scientists have discovered.
Previously, it had been thought the spider pedipalps – or "palpal organ" – was numb.
However, a team of scientists from the University of Greifswald in Germany has found this is not the case. To mate, male spiders use the palpal organ to transfer seminal fluid.
In their study published in the Royal Society journal Biology Letters, the scientists said the palpal organ developed from the tarsal claws and no nerves, sensory organs or muscles had been detected in the palpal bulb, "suggesting the spider male copulatory organ is numb and sensorily blind ... despite the fact that males need to manoeuvre these 'numb' structures precisely into the female genitalia".
But when researchers looked at the Tasmanian cave spider Hickmania troglodytes – examining both male and females through X-ray micro computed tomography to create a 3D reconstruction of the male palpal organ – findings showed the presence of a nerve and two clusters of neurons within the male palpal bulb, suggesting male spiders feel "stress and strain" during sex.
"Our data provides the first evidence of neuronal tissue in the spider male copulatory organ," the authors wrote. "Thus, this spider's male genitalia are likely not numb and sensory input could play an important role in copulation and consequently in male mating investment and/or securing paternity."
Most studies suggest female spiders are in control of mating processes and the authors believe having a sensitive palpal organ could be an advantage because it could be used to alter the male's behaviour "in order to stimulate the female for the male's own benefit".
Another suggestion is that it could help the male assess the female and adjust its investment (quality of ejaculates) – both are speculative, however. They conclude the findings suggest the presence of a currently unknown mechanism for securing paternity among spiders.