Researchers, led by Dale Greenwalt from the National Museum of Natural History, said the specimen was unique; its preservation an extremely improbable event. "The insect had to take a blood meal, be blown to the water's surface, and sink to the bottom of a pond or similar structure to be quickly embedded in fine anaerobic sediment, all without disruption of its fragile distended blood-filled abdomen," the researchers said in the Proceedings of the National Academy of Sciences.

Using non-destruction technologies, the team screened the fossil for a range of compounds, including two components of heme, the protein that transports oxygen in red blood cells.

The detection of these components, iron and porphyrin molecules, in the abdomen of the female – only females suck blood – confirmed the iron was from the insect's last meal and not part of the fossilisation process.

The female specimen was one of 36 mosquitoes recovered from the Kishenehn Formation, part of which formed around 46 million years ago during a time known as the middle eocene.

The fossils are unique because they were preserved in shale rather than amber, the material in which most fossil insects have been found, said the researchers.