How Millipedes’ Legs Become Sex Members

Talk about getting a leg up—helminthomorph millipedes have one or two pairs of legs that also serve as sex appendages, called gonopods, that clasp the female while transferring sperm.

Now, scientists have figured out how these legs turn into gonopods.

Using a laser-scanning microscope, scientists have observed a “dramatic transformation” that occurs when the invertebrate molts, shedding its skin and adding more segments to its body.

Dramatic is right—the millipede apparently rearranges its internal anatomy to accommodate these new sexual appendages, even moving around its nerve cord and digestive tract, the team at Italy’s University of Padova discovered. The researchers collected wild specimens from two millipede species, Nopoiulus kochii and Oxidus gracilis, for their research in the lab.

Oxidus gracilis, one of the millipede species with legs as sexual appendages. Photograph by Giuseppe Fusco.

Explains study co-author Leandro Drago: “Conversion of legs into specialized gonopods via non-systemic metamorphosis is not a simple matter of changing the appearance of the legs and adding spines. (Read more details in the provisional abstract in the journal Frontiers of Zoology.)

“The exoskeleton and muscle reorganization of segment VII during sexual development forces the nerve cord to become displaced and a causes a reduction in the volume of digestive tract through this segment,” Drago said in a statement.

What’s more, such an abrupt reshuffling also does a number on the male millipede’s figure—the arduous transition requires a lot of energy, and it’s likely why he appears to lose weight when sexually mature. In fact, in an effort to prevent starvation, some males will even revert back to their immature stage—without the gonopods—to stave off starvation.


A closeup of a millipede’s gonopod. Photograph courtesy Giuseppe Fusco.

Millipedes have another claim to fame when it comes to sex appendages—they probably have the oldest fossil genitals, which date back 450 million years, National Geographic News reported in 2003. (See: “‘Probably the Oldest’ Penis Found in Spider Fossil.”)

Check out more weird coverage on National Geographic News.






Meet the Author
Christine Dell'Amore, environment writer/editor for National Geographic News, has reported from six continents, including Antarctica. She has also written for Smithsonian magazine and the Washington Post. Christine holds a masters degree in journalism with a specialty in environmental reporting from the University of Colorado at Boulder. Her book, South Pole, was published in 2012.