Biology

The mystery of millipede mating revealed in landmark imaging study

The mystery of millipede mati...
A mating pair of Pseudopolydesmus millipedes, viewed under UV light so that their anatomical structures are more visible
A mating pair of Pseudopolydesmus millipedes, viewed under UV light so that their anatomical structures are more visible
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A Pseudopolydesmus millipede, viewed under UV light
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A Pseudopolydesmus millipede, viewed under UV light
A Pseudopolydesmus vulva, under UV light
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A Pseudopolydesmus vulva, under UV light
A mating pair of Pseudopolydesmus millipedes, viewed under UV light so that their anatomical structures are more visible
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A mating pair of Pseudopolydesmus millipedes, viewed under UV light so that their anatomical structures are more visible
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A team of US scientists has just solved a long-standing biological mystery – how exactly do millipedes mate? Using a variety of novel imaging methods, including microscopic ultraviolet photography and micro-CT scanning, the research finally figured out how these tiny creatures get it on.

"This is the first time we've been able to understand these millipedes' mechanism of insertion, how the male and female organs interact with each other,” says Petra Sierwald, from Chicago’s Field Museum and one of the study’s authors. “Before this, we had no idea how he would actually get the sperm into her.”

Millipedes can generally be somewhat shy organisms, so getting them to mate in laboratory conditions hasn’t been easy. The new study focused on a type of small, brown North American millipede called Pseudopolydesmus, known for being more than willing to mate, even in the most exhibitionist situations.

"They will even mate in the lab in the Petri dish under the light,” says Sierwald.

A Pseudopolydesmus millipede, viewed under UV light
A Pseudopolydesmus millipede, viewed under UV light

Utilizing a number of modern imaging techniques, the research team discovered some unexpected, and previously unknown, details elucidating the millipedes' mating ritual. First, the researchers needed to understand exactly how a male millipede got his sperm into the female.

Millipedes are one of the few organisms that use specialized limbs called gonopods to transfer sperm into females. In the Pseudopolydesmus, the gonopods are notably quite far from its testes. The new research found the millipede releases its sperm from its testes and then contorts itself to cover its gonopods in the sperm, described as a blue-ish liquid.

The male gonopods then enter the female’s vulva using tiny claws to hook into specific ridges on the vulva. This is the first time researchers have described this kind of lock-and-key relationship in the Pseudopolydesmus millipede.

"She has two openings, one on each side between her second pair of legs," says Sierwald. "We had no idea for this entire group, which part is inserted and where it is inserted in the female.”

Another mystery solved by the new study is the source of a strange secretion that seals the sperm inside the female’s vulva. This process functions to concentrate the sperm until the female lays her eggs, subsequently coating the eggs in sperm as they leave her body.

A Pseudopolydesmus vulva, under UV light
A Pseudopolydesmus vulva, under UV light

"Before this study, we had no idea really where the secretions came from,” explains Sierwald. “I always thought it came from the male, because I thought the male wanted to seal off the female so that she couldn't mate again. But now, having seen the glands inside the female's vulvae through the CT-scanning, I think most of that secretion comes from the female. I don't know whether that is her way of protecting her vulvae or preserving the sperm. Those are interesting fields for further study."

The rigorous imaging study shines a light on a nearly century-old mystery that traditional microscopy simply could not solve. Sierwald says the research is not only important in offering a better understanding of how millipedes are related, but it delivers insights into how different millipede species evolved. And, of course, it answers one burning question many entomologists have been asking for decades – what does a millipede vulva actually look like?

"There are 16 orders of millipedes in the world, and for most of them, we have only faint ideas what the vulvae look like,” says Sierwald.

The new research was published in the journal Arthropod Structure & Development.

Source: Field Museum

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