Millipede Mating Pair

A mating pair of Pseudopolydesmus millipedes, seen below UV gentle in order that the buildings of their genitals would present up higher. Credit score: Stephanie Ware, Discipline Museum

Excessive-tech imaging below UV gentle reveals which components go the place when millipedes mate.

Scientists have a reasonably good deal with on how the birds and the bees work, nevertheless it involves mating, nearly all millipedes have been a thriller–till now. For the primary time, researchers have puzzled out how these tiny creatures’ advanced genitalia work, because of new imaging methods and blacklights that make the completely different tissues glow. The findings are revealed in a brand new paper within the journal Arthropod Construction and Growth.

“That is the primary time we’ve been capable of perceive these millipedes’ mechanism of insertion, how the female and male organs work together with one another. Earlier than this, we had no concept how he would really get the sperm into her,” says Petra Sierwald, an affiliate curator on the Discipline Museum in Chicago and one of many paper’s authors.

Millipedes are centipedes’ chill vegetarian cousins. They’re in a unique animal group from bugs and spiders, however they’re a part of the identical large umbrella of arthropods. They’ve dozens of legs, however they’re sluggish movers–they eat decaying crops, in order that they don’t should be speedy to catch a meal.

“Millipedes are mulch munchers,” says Sierwald. “They’re in waste administration, they usually’re superb at it.”

Millipede Vulva

A Pseudopolydesmus vulva, below UV gentle. Credit score: Stephanie Ware, Discipline Museum

There are over 13,000 completely different species of millipedes recognized to science (with many extra found yearly), they usually every have their very own distinctive manner of mating. The genus that Sierwald and her colleagues centered on on this research, Pseudopolydesmus, is made up of half-inch-long brown millipedes from North America. They’re not too thrilling to take a look at, however their habits makes them stand out to scientists: they’re unusually desirous to mate, even in laboratory circumstances.

“One of many issues with millipedes is that they do numerous issues whereas they’re dug into the bottom, and when you take them out, you’ll disturb them they usually’ll cease what they’re doing,” says Sierwald. Not Pseudopolydesmus, although. “They may even mate within the lab within the Petri dish below the sunshine.”

However whereas Pseudopolydesmus are the exhibitionists of the millipede world, it’s nonetheless arduous to see precisely what’s happening down there–there are numerous legs in the best way.

“This paper has been a very long time within the making. It began out when I discovered a pair of this genus, a female and male, in copula, kind of connected. Their legs had been obscuring every part,” says Sierwald.

To unravel this drawback, Sierwald and her colleague, Discipline Museum co-author Stephanie Ware, experimented with completely different lighting and imaging methods. Ware took dozens of pictures of the tiny topics at barely various distances, after which used a pc program to stack the photographs collectively and mix them so that each one the miniscule particulars had been in focus. so She took pictures in each pure and ultraviolet gentle circumstances because the millipedes’ genitals glow below UV gentle, making it simpler to inform the completely different tissues aside. The ensuing blacklight pictures appear like a rave, albeit one made up of microscopic millipede genitalia.

Millipede Under Blacklight

A Pseudopolydesmus millipede, seen below UV gentle. Credit score: Stephanie Ware, Discipline Museum

To get additional details about the construction of the millipedes’ genitals, Xavier Zahnle on the College of California Davis, the paper’s first creator, performed micro-CT scanning. “You’ll be able to put these tiny millipedes into take a look at tubes and do CT scanning on them, and also you don’t should dissect the specimen, so it stays intact,” explains Sierwald. “That’s the actually cool factor. The CT scan takes photographs of slices, and Xavier labored out the complete advanced software program program to investigate the slices and put them again collectively.”

All of those photographs of the millipedes’ genitalia, each on their very own and locked collectively within the act of mating, helped the researchers work out how the method really works for Pseudopolydesmus.

In most millipedes, together with Pseudopolydesmus, the male’s testes are situated within the physique beginning behind his second pair of legs. However his gonopods, the specialised pair of legs used to insert sperm into the feminine, are manner again on his legs of the seventh physique ring. “He has to ejaculate after which dig his seventh-ring gonopods into that ejaculate,” she explains. “It’s a blue-ish liquid.”

As soon as the male has gotten his gonopods lined in blue ejaculate, he’s able to insert them right into a feminine’s vulvae. “She has two openings, one on either side between her second pair of legs,” says Sierwald. The microscopic photographs confirmed the tiny fleshy a part of the male’s gonopods that truly enter the vulvae. “We had no concept for this whole group, which half is inserted and the place it’s inserted within the feminine,” says Sierwald. Tiny claws on the top a part of the male gonopod hook behind ridges on the feminine vulva.

After mating, the feminine’s vulvae get sealed up with a gooey secretion, trapping the sperm inside. Later, when she lays her eggs, they get coated with the saved sperm on their manner out of her physique. The brand new imaging on this research additionally helps present how that course of works.

“Earlier than this research, we had no concept actually the place the secretions got here from. I at all times thought it got here from the male, as a result of I assumed the male needed to seal off the feminine in order that she couldn’t mate once more,” says Sierwald. “However now, having seen the glands inside the feminine’s vulvae by the CT-scanning, I feel most of that secretion comes from the feminine. I don’t know whether or not that’s her manner of defending her vulvae or preserving the sperm. These are attention-grabbing fields for additional research.”

Along with giving us a greater understanding of the mechanics of millipede intercourse, Sierwald hopes the undertaking will allow scientists to raised perceive the relationships between completely different millipede species, which might make clear how they advanced.

“This research can be necessary for understanding how millipedes on this genus are associated and the way they’re distributed,” says Sierwald. “They’ll inform us in regards to the geologic historical past of North America. As mountain ranges and rivers shaped, teams of millipedes would get minimize off from one another and grow to be new species.” And, she notes, Pseuopolydesmus is simply the tip of the iceberg.

“There are 16 orders of millipedes on the planet, and for many of them, we’ve solely faint concepts what the vulvae appear like.”

Reference: “Genital morphology and the mechanics of copulation within the millipede genus Pseudopolydesmus (Diplopoda: Polydesmida: Polydesmidae)” by Xavier J.Zahnle, Petra Sierwald, Stephanie Ware and Jason E. Bond, 28 January 2020, Arthropod Construction & Growth.
DOI: 10.1016/j.asd.2020.100913

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