Two hermaphrodite Siphopteron slugs — only a few millimeters long — mate. The heads have a shieldlike structure, and two winglike flaps curl around their sides. Mating starts with intertwining when each partner sticks out a penis. Then the slugs insert the penis into an opening on the right side of the partner and then drive sharp stylets into each other’s foreheads.

Credit: © Rolanda Lange, Johanna Werminghausen, Nils Anthes

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