Scientists Finally Solve The Mystery Of How Millipedes Do It

Not a rave, but in fact a mating pair of Pseudopolydesmus millipedes, Stephanie Ware/Field Museum

Rachael Funnell 14 Feb 2020, 15:08

Ever found yourself up late at night, tossing and turning and asking yourself, how exactly do millipedes do it? Same. Thankfully, with a little bit of help from complex UV imaging, researchers from the Field Museum in Chicago can now show you.

There are over 13,000 known species of millipedes – which belong to the arthropod subphylum Myriapoda, which in Greek means "countless feet” – but for this study, published in the journal Arthropod Structure and Development, researchers focused on Pseudopolydesmus, a group of half-inch-long brown millipedes from North America.

"One of the problems with millipedes is that they do a lot of things while they are dug into the ground, and if you take them out, you will disturb them and they'll stop what they're doing," said Petra Sierwald, associate curator at the Field Museum in Chicago and author on the paper, in a statement. Fortunately, their chosen species’ lack of inhibitions meant "They will even mate in the lab in the Petri dish under the light." Nice.

However, “countless feet” really means countless feet and, even with exhibitionists for subjects, catching sight of what’s occurring under a skirt of limbs proved quite the obstacle. To get around this, the team got to work employing a wealth of lighting and imaging techniques, taking dozens of photographs at varying distances before using a computer program to try and knit them together and reveal the minuscule details (no offence, Pseudopolydesmus). As luck would have it, millipedes' genitals glow under UV light, making it far easier to tell the different tissues apart, and the resulting gallery is a neon rave of microscopic millipede genitalia.

Pseudopolydesmus
A Pseudopolydesmus millipede, viewed under UV light. Stephanie Ware/Field Museum

To get a more intimate understanding of millipede sex mechanics, Xavier Zahnle at the University of California Davis, the paper's first author, conducted micro-CT scanning. This method could capture “slices” of images of both single millipedes and those locked together in the throes of passion without having to dissect the young lovers.

One curious thing about most millipedes, including Pseudopolydesmus, is that their testes are located on the second pair of legs, but the gonopods – which actually insert the sperm into the female – are way back on the seventh body ring. The images revealed that post-ejaculation (which is blue in color, FYI), the millipedes had to reach to coat their gonopods before inserting them into the female. "She has two openings, one on each side between her second pair of legs," said Sierwald. "We had no idea for this entire group, which part is inserted and where it is inserted in the female."

female pseudopolydesmus
A Pseudopolydesmus vulva, under UV light. Stephanie Ware/Field Museum

Researchers already knew that after mating the female’s genitalia is sealed with a viscous secretion, but it was thought this was done by the males to prevent further mating with competing individuals. However, the CT scans revealed that this sealant actually comes from the female, the exact purpose of which is still unknown and something Sierwald notes is an interesting avenue for further study.

While this research sheds light on the mechanics of millipede sex, Sierwald hopes the project will also reveal how all millipedes are related, helping us to better understand their evolution through shared mating mechanics. With 16 orders of millipedes across the globe, when it comes to understanding the iceberg that is millipede sex and evolution, Pseudopolydesmus is just the tip.

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