Male Dragonflies Scoop Out a Rival's Sperm Before Mating
Because females store sperm, males evolved a cleanup tool that lets them remove rival sperm, and researchers coat it in gold to see its shape.
Science Vs · Where Have The Insects Gone?? With Dr. Jessica WareA male dragonfly does not just deliver his sperm. He first removes someone else's.
The reason is storage. A female dragonfly can keep sperm inside her body after mating, so a male wants to ensure she uses his sperm to fertilize her eggs, and not sperm left behind by a rival.
His tool for that job sits apart from the organ that first makes the sperm, and scientists call it a secondary penis. In some dragonflies it can be shaped like a small scoop or a ragged claw. During mating, the male works that scoop inside the female to scrape out stored sperm from an earlier mate, and only then does he deposit his own.
It is hard to appreciate the contraption with the naked eye because it is very, very small. Jessica Ware, an entomologist employed by the American Museum of Natural History, has dissected hundreds of them with very tiny instruments to study how the shapes differ from species to species.
To see the details, she puts the specimens into a scanning electron microscope, which forms images with electrons rather than light. To use it, she has to coat each penis in gold and palladium, a step that allows the microscope to image them. When the electron beam hits the metal coating, the electrons bounce off, and that reflected pattern creates a silhouette that shows the small features.
Under that beam, the variety is striking. Some look like scoops, while others look like pine cones or bottle brushes. Ware says the shapes differ from species to species.
Questions people ask
Why would a female dragonfly store sperm from multiple males?
Stored sperm lets her fertilize eggs over time without mating again before every batch. The tradeoff is that males compete after mating, so the female may end up using sperm from the male that was best at removing rivals, not necessarily the first male she chose.
Is sperm removal only found in dragonflies?
No. It is especially well studied in dragonflies and damselflies, but sperm competition happens across insects and other animals. Males in different groups use plugs, chemicals, prolonged guarding, or specialized genitalia to reduce a rival's chance of fathering offspring.
What does a scanning electron microscope actually do?
It scans a sample with a focused beam of electrons and detects the electrons that bounce off or knock loose. Metal coating makes tiny, nonconductive specimens reflect electrons more evenly, producing highly detailed, three-dimensional looking images at magnifications far beyond a light microscope.