In the wet sands of Italy’s beaches, an epic battle of the sexes is playing out. The combatants are flatworms and though their bodies are simple, their sexual habits are anything but. Take the common species Macrostomum lignano. After sex, if a female has been filled with unwanted sperm from an undesirable partner, it can double over, put its mouth over its genital opening, and suck the sperm back out. It’s an easy fix that gives the female control over who fertilises her eggs.
But the sperm can put up a fight. It has a very different shape to the tadpole-like figure of human sperm. At its front, it has a sinuous ‘feeler’ that is uses to anchor itself into the walls of the female genitals. Further down, two large backwards-pointing bristles also help to hold the sperm in place. These barbed sperm can’t be sucked out easily. Even after the worm tries, you can often see several sperm sticking out of the genital opening.
There is another twist to this tale. For this flatworm, every individual is both male and female – they are hermaphrodites. When two of them mate, their male organs (the stylets) penetrate each other’s female organs (the antrums) at the same time. They form a little sexual ring, often spinning as they mate. Watch them go at it here.
The courtship rituals of the spider Harpactea sadistica start innocently enough, with a dance and a hug. The male spider taps the female gently with his front legs and embraces her. But from that point onwards, things for the female go rapidly downhill. The male bites her and she becomes passive, allowing him to manoeuvre her into position. Like all spiders, his genitals are found next to his head, on a pair of appendages called the pedipalps. But unusually, his penis ends in a needle-sharp tip called an embolus.
The embolus sits at the end of a loop called the conductor. The male hooks one of these loops around the opposite embolus to steady it. Then, by rotating the anchored needle, he drives the point straight through the female’s underside and ejaculates directly into her body cavity. On average, he does this six times, moving slowly downwards and alternating between his two penises. The entire cringeworthy sequence lasts about 15 minutes and throughout it, the male spider never penetrates the female’s actual genital opening.
The species was discovered in Israel last year by Milan Rezac from the Crop Rsearch Institute in the Czech Republic. He named it well. H.sadistica practices a style of sex that’s understandably known as “traumatic insemination“. It’s disturbingly common among insects and other invertebrates, and is most famously practiced by bedbugs. But this is the first time that the behaviour’s been seen among the chelicerates – the group of animals that includes spiders, scorpions and mites.
You can see it happening in the videos below. In the first, the male spider bites and incapacitates the female. In the second, he hooks the conductor of one pedipalp around the embolus of another and, with rotating motions, drives it into the female. These videos aren’t pretty – you’ve been warned.