Male spiders don't have a penis -- all their sex is digital. After ejaculating onto a tiny piece of webbing, a male sucks his sperm into a chamber at the tip of one of the short limbs on his head. Once he convinces a female to accept him, he'll push that appendage inside her genital opening and (hopefully) make some babies.
The chamber that stores sperm is part of a structure called the palpal organ, which evolved from the claw that sits at the end of a spider leg. In some species of spider, the palpal organ is incredibly elaborate, with joints and sockets that interlock with a female's genitals. But biologists had never found neurons inside them, or any evidence that they had sensory structures on these organs. They assumed that all spiders had sex that was devoid of sensation.
A new study published in Biology Letters today shows that at least one spider can feel what he's doing. When Elisabeth Lipke and her colleagues at Ernst-Moritz-Arndt-University took a close look at the relatively simple and tubelike palpal organ in male Tasmanian cave spiders, they found a group of neurons inside.
Not just random neurons either. Some were attached to the organ's exoskeleton, right at the point where forces on the tip of the organ would stretch the cuticle. That suggests that these males can feel what's going on as they probe the female during sex. What's more, the scientists also found a previously unknown pair of glands inside the palpal organ: they propose that males might use these to modify their semen, depending on what they feel inside the female.