Last summer, evolutionary biologist, Sanle Xavier (Xavier Zahnle) delivered to fellow researchers centipedes visual pleasure. Graduate student at the University of California at Davis took digital pictures of the male sexual organ (or gonopod) millipedes Pseudopolydesmus serratus from the inside. Sanle carefully studied the pictures, looking for the answer to the old question — how males millipedes deliver sperm.
Sex of centipedes — it is dark, the hidden rows of legs. Female genitalia millipedes — steam the vulva, are located in the third segment from the head. When the female is ready to mate, she pushes out of the vulva for my legs. The male embraces her with his numerous legs, gear vulvar their gonopodium, each with a length of just a millimeter, and inserts them inside the hairy extremity. Since 1931, researchers centipedes suspect that the sperm is deposited directly into the female on the channel of gonopod called seminiferous channel. In a review article of the genus Pseudopolydesmus from 2019 Zanla and his colleagues supported this idea. But to test this experimentally so far failed.
To take a closer look at Sanle together with Servald Petra (Petra Sierwald), assistant curator in the Department of arachnids and myriapods at the Field Museum of natural history in Chicago, and other colleagues turned to the most modern imaging — up to scanning electron microscopy and UV fluorescence imaging. Applying these methods to samples from the collections of the Field Museum, including a couple of P. Serratus, which is preserved during copulation, the team reconstructed how the mating millipedes.
Then they looked inside of animals using the technique of visualization called microcomputer tomography or micro-CT. The sample is irradiated with x-rays for a series of images in a grey scale where areas look brighter than they are denser. A computer program then compiles the images into a single unit pixels, and the operator using the software paints various structures within the block, creating a three-dimensional replica. This allows researchers to study digital models of specimens “from all sides”, says Sanli. “Seen at a glance that where is, where is and where is,” he says.
So, micro-CT revealed amazing detail, the male centipede delivers the sperm. The images showed that the seed channel bypasses the fibrous tip and dumps the cum in the camera. Only after the sperm go through the process. “The channel describes a complete circle, it’s unbelievable, — says Sanle is not what we described in the article 2019. We made a mistake”. He and his colleagues published their findings in the journal “Structure and development arthropod” (Arthropod Structure and Development) at the beginning of this year.
Not involved in the study of aquatic entomologist from the Spanish University of Granada Javier Alba-Tercedor (Javier Alba-Tercedor) called the work Sanle “a good example of what can be achieved by using microcomputer tomography”. Alba-Tercedor, winner of prizes for his work on micro-CT, said that the most difficult part of the process is staining on the images of the various structures. But once this stage is ready, the data can be used by other researchers for further study.
Servald believes micro-CT scan of a turning point in the study of centipedes. When she began to study millipedes in 1997, the technology of micro-CT in entomology is almost not used. Her original plan for the study of vulva centipedes dropped at cover failed because it turned out that “damned millipedes devour their exuviae”. Without micro-CT had to kill a lot of centipedes and dismember them, she says. Now, the opening no longer, and this means that more animals will survive for future studies. “It’s really cool,” she says, adding that micro-CT “is really a big step forward” in understanding how to work the sex organs.
Servald hopes that sooner or later imaging techniques will help researchers determine the types of centipedes, to understand their geographic distribution and to learn more how the different taxa are related to each other. She suspects that the allocation of mating centipedes can have a commercial or pharmaceutical application.
The entomologist and curator of the Museum of natural science in the Danish Aarhus Thomas Simonsen (Thomas Simonsen) the study was not involved, but considers it “excellent”. He agreed with Servald that the study of centipedes will help advanced research methods. He notes that along with DNA research it is important to have a detailed knowledge of the morphology, in order to “understand the diversity and evolution of arthropods”, especially in those taxonomic groups where phylogenetic relationships are poorly understood.
Sanle is currently focused on males millipedes. He says that millipedes are a variety of forms and functions. According to him, it’s not just “lots of legs” and a large variety “from the flat back to the big cylinders and the tiny bristles”. Thanks to the rapid development of new technology with micro-CT Sanle was in “good company” — this technique uses more and more researchers arthropods. Now he wants to know, would there be the same loop of seed channel in other species: “I want to explore them all.”