With the intense X-ray light from DESY's particle accelerator PETRA III, researchers have investigated an unusual find: a 50-million-year-old insect larva from the era of the Palaeogene. The results offer a unique insight into the development of the extinct insect, as the team reports in the journal Arthropod Systematics & Phylogeny.

When the biologist Hans Pohl from the Friedrich Schiller University in Jena tracked down an insect fossil trapped in amber on eBay, the joy of discovery was great: it was a special specimen, a 50-million-year-old larva of an extinct twisted-wing insect from the order of Strepsiptera. But in order to be able to investigate it in detail, he needed the help of materials researchers from the Helmholtz Centre in Geesthacht, which operates a beamline at DESY's X-ray source PETRA III.

Strepsiptera are parasites that infest other insects, such as bees and wasps, but also silverfish. “In most of the approximately 600 known species, the females remain in their host throughout their lives,” says Pohl. “Only the males leave it for the wedding flight, but then live only a few hours.” But there are exceptions: In species that infest silverfish, the wingless females also leave their host.

Some time ago, the Jena researchers succeeded in making a remarkable discovery: they were able to identify a nearly 100-million-year-old fossil of a twisted-wing larva in its first developmental stage, enclosed in an amber. With their size of just 0.2 millimetres, such larvae are among the smallest multi-cellular organisms known.

Pohl then made a lucky find on the Internet when a trader offered a Baltic amber with an insect enclosure via eBay. “I immediately noticed that it was a unique fossil,” the biologist recalls. After a long negotiation, he was able to secure the piece for a price of 1000 euros. The 4.4 millimetre animal was enclosed and sealed by tree resin approximately 50 million years ago. It is the first known larvae fossil of a twisted-wing insect that has developed past the first larvae stage.

The problem: “Under the light microscope, essential details could not be seen,” explains Pohl. “It was not possible to see whether the larva had antennae or what the mouth parts and eyes were like and whether it was a female or a male larva.” His team therefore focused on the object using a high-resolution X-ray method - microtomography with synchrotron radiation.

To do this, they had to move to a large scientific facility – the PETRA III storage ring at DESY in Hamburg. Here, the Helmholtz Centre Geesthacht (HZG) operates the measuring station P05, which usually deals with materials research. “The method is similar to a CT scanner in a hospital, producing three-dimensional images of the inside of a body,” says HZG researcher Jörg Hammel.

However, while conventional X-ray tubes might be sufficient for many tasks of medical imaging in hospitals, the two-kilometre-long particle accelerator PETRA III delivers a much stronger and sharper X-ray beam. “The X-ray radiation from PETRA III is extremely intense,” explains Hammel. “This allows us to take very sharp pictures even of very small samples.” During image acquisition, the sample is rotated in the beam so that it can be photographed from all sides. Each individual image looks like a normal X-ray image. The computer then combines the many images to form a 3D image.

Of the twisted-wing larva, the method delivered pin sharp images with a resolution of 1.3 micrometers. “All the important details can be seen on it,” Hans Pohl says. “Among other things, we were able to see that it was probably in its third larval stage and that it was very probably a female larva of Mengea, a genus that is extinct today.” What is special, the female larva had left its host – a behaviour known only from the silverfish parasites among today's Strepsiptera species. “Some evidence suggests that the host may have been a cockroach,” reports Pohl. “To be certain, however, we would have to track down and analyse more fossils.”

It is quite possible that these analyses will then again be conducted at PETRA III. “In recent years, the HZG beamline seems to have become an insider tip among biologists,” says Jörg Hammel. “Apparently, we did become so good at analysing amber fossils that we are receiving more and more enquiries from experts.”

 

Reference:
The first fossil free-living late instar larva of Strepsiptera (Insecta); Hans Pohl, Jörg U. Hammel, Adrian Richter, Rolf G. Beutel; Arthropod Systematics & Phylogeny, 2019; DOI: 10.26049/ASP77-1-2019-06