Instead of penises, male spiders have evolved a way to transfer sperm using a modified part of their leg-like appendages called pedipalps. These paired body parts are attached to their heads – which makes them look like antennae or pincers – and in addition to sex, arachnids use them to create sperm plugs, capture prey, and maneuver food into their mouths. Now, for the first time, researchers have discovered neurons and a nerve in the male copulatory organ of spiders. The findings, published in Biology Letters this week, mean that males are receiving sensory input during sex. And they may be using this feedback to adjust the quality of their ejaculation. 

Researchers used to think that the part of the pedipalps dedicated to sperm transfer – called the palpal organ (pictured below) – lacks nerves, sensory organs, and muscles. This would mean that it’s numb, which doesn’t really make all that much sense if they must be maneuvered precisely into female genitalia. So, a trio led by Elisabeth Lipke from the University of Greifswald studied male and female Tasmanian cave spiders (Hickmania troglodytes) using a combination of tissue analyses, X-ray micro-CT, and computer modeling. 

They found a nerve that runs all the way to the embolus, or tip of the palpal organ, as well as two clusters of neurons within the bulb of the copulatory organ. The first cluster is located at one end of the region where seminal fluid is contained, and the other is situated in the embolus at the tip. The nerve and both neuron clusters are in yellow in the diagram below.

^{3D model representation of the male copulatory organ: neurons and nerve (yellow), bulbus gland (purple), embolus gland (orange), and the seminal-fluid-containing spermophor (green). E Lipke, JU Hammel, and P Michalik/Biol. Lett.}

Additionally, they also identified two glands in the palpal organ: the bulbus gland (purple) and the embolus gland (orange). The latter is directly innervated, and it probably helps to modulate the rapid adjustments in the male’s secretions.

So, no, the spider’s male genitalia aren’t numb. A sensitive palpal organ – thanks to sensory reception and a directly innervated gland – could play a very important role in securing paternity and in deciding the amount of investment the male wants to make in terms of the quality and volume of his ejaculates. After all, in theory, males should invest more sperm in high-quality females, but they also want to take advantage of any further mating opportunities. To "put it simply," Lipke and Michalik tell Live Science, "males of this spider species are likely able to outsmart females."