Weird These flatworms plunge their penises into their own heads to inject themselves with sperm


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The flatworm Macrostomum hystrix isn’t exciting to look at. Its diet of microalgae doesn’t raise any eyebrows, and you probably wouldn’t even notice one if you came across it in its native habitat. But in the bedroom, these flatworms take kink to a whole new level: when they can’t find a partner, they will stab themselves in the head with their needle-like penises and inject sperm to self-fertilize.

In principle, it is what scientists call hypodermic insemination: the practice of forcefully depositing sperm outside a female genital tract—and yes, it’s as usually as rough as it sounds. Bed bugs are infamous for it, as this type of insemination (also called traumatic insemination) causes major damage to the female and reduces her fitness, though a number of species have been demonstrated to reproduce in this way, including flatworms.

Since most flatworm species are hermaphrodites (possessing both male and female sex parts), the decision of who gets to inseminate whom is settled in a rather special way: penis fencing. Two individuals will, quite literally, use their male copulatory parts (called stylets) in bizarrely beautiful combat until one is able to plunge his sperm-delivering organ into the other’s body. The then-labeled “male” in this exchange earnestly injects his sperm, then disengages, leaving the now-female with the burden of bearing the pair’s young.

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