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The order Diptera includes, as the name implies, insects with only one pair of wings. The body structure of diptera can be reduced to two main types: mosquitoes with multi-lobed antennae and flies with three-lobed antennae. This difference was noticed a long time ago: diptera were divided into long-whiskered Nematocera and short-whiskered Brachycera in the first half of the XIX century. But as often happens, the names are conditional: the antennae of mosquitoes are not necessarily long (you can remember midges), and the antennae of flies are not necessarily short, they are many times longer than the insect's head. In the antennae of flies, there are always no more than 8-10 segments (usually much less), and this option is considered the initial one for Brachycera. Usually the fly's tendril is complicated: the third segment is much larger than the following, which either simply decrease towards the top, or form an arista – a thin, non-segmented bristle.
Flies are evolutionarily more advanced than mosquitoes, and it is logical to expect that they appeared on Earth later. The oldest diptera are known from the Anisian deposits of the Middle Triassic of Europe, they are about 245 million years old. At the same time, a variety of mosquitoes and a single fly were found in one location – in the famous French lagerstette Grès à Voltzia in the Vosges Mountains. The Gallia fly was assigned to the modern ragionidae family based on the venation of a wing with a closed cubital cell (this is the main feature of the Brachycera wing). Rhagionids include the most primitive living flies and are considered basal Brachycera. In the fossil state, they are numerous and diverse in the Mesozoic deposits of Eurasia, starting from the Middle Jurassic, but in the Triassic Gallia remained the only find for a long time.

Paleoentomologists from the A.A. Borisyak Paleontological Institute of the Russian Academy of Sciences (Moscow) and the Steinhardt Museum of Natural History (Tel Aviv) re-examined previously collected material from the French location and found several more specimens of these small flies (wing length 2-3 mm) in it. Although their wing is quite fly-like, with a closed cubital cell, the antennae turned out to be mosquito-like, consisting of 16 almost identical segments. The systematic position of Gallia had to be revised, since the third segment of its tendril is not special, as in ragionids, and the number of segments itself is characteristic of Nematocera. It turns out that "normal" flies have not yet been found in the Triassic, but only "long–whiskered short-whiskers" are known - a rare transitional form between more primitive and more advanced groups of diptera insects.

The work was supported by the Russian Science Foundation, grant 21-14-00284.

© E.D. Lukashevich, M.B. Mostovsky

Publication: Lukashevich E.D., Mostovski M.B. The imitation game: in search for Brachycera in the Triassic // Diversity. 2023. V. 15. Art. 989. https://doi.org/10.3390/d15090989

Source of information and photo: A.A. Borisyak Paleontological Institute of the Russian Academy of Sciences

In the photo: prints of Middle Triassic flies Gallia alsatica Krzemiński et Krzemińska, 2003: a, b, d – general view of various specimens (?females); c, e, f – body fragments: c, e – heads with antennae, f – chest with wings; scale segment: a, b, d – 1 mm, c, e, f – 0.5 mm

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