Apart for the two old museum specimens of Scenopinus vitripennis, most of the examined material were collected relatively recently by the authors and their close associates. Except for the reared specimens, majority of the examples have been collected indoors (see notes) directly to vials and killed by freezing, ethyl acetate or potassium cyanide prior to mounting them on entomological pins.

Label data of newly collected specimens are given verbatim using the following symbols: / end of a line and beginning of the next; // end of label and beginning of the next (from top to bottom on the same pin). The specimens are deposited in the following collections and are indicated with the given acronym in the text:

AHC Private collection of Antti Haarto, Mynämäki, Finland

JPC Private collection of Jaakko Pohjoismäki, Joensuu, Finland

MIZ Museum and Institute of Zoology, Polish Academy of Sciences, Warszawa, Poland

MZH Finnish Museum of Natural History, Zoological Museum, University of Helsinki, Helsinki, Finland

MZT Zoological Museum of the University of Turku, Turku, Finland

SMNS Staatliches Museum für Naturkunde Stuttgart, Germany

Male terminalia were dissected and prepared for examination essentially as described by O’Hara (2002). The dissected terminalia are preserved in glycerol in a small plastic vial pinned together with the specimen. Location abbreviations refer to the geographical provinces (e.g., Kb = Karelia borealis, explained when mentioned for the first time, see https://laji.fi/theme/emk for more details) and coordinates (NNNN:EEEE) on the labels are mostly given in the old national Finnish map grid coordinate system (YKJ; see Ollikainen and Ollikainen 2004), which is still in common use for biological sampling. As the first number of the longitude coordinate is always 3, this is often left out in the collection labels to save space. Sampling coordinates in decimal degrees as well as additional notes or commentaries about the specimen are given in brackets.

The classification follows Herting and Dely-Draskovits (1993). The morphological terminology used in this study follows Cumming and Wood (2017), except for the features of male terminalia, where Winterton and Gaimari (2017) is used.

The images were taken with a Leica Z6APO stereomicroscope and a Leica DFC450c (5MPix) camera, MSV266 motorised focus and using the Leica Application Suite 4.6.0 software for Z-axis stacking. Images were cropped, colour- and contrast-enhanced but not manipulated otherwise.

Cytochrome oxidase subunit 1 (COI) DNA barcoding was performed as a part of the Tachinidae project of Finnish Barcode of Life initiative (FinBoL). The 5´-terminal part of COI was amplified using the routine barcoding primers LepF1 and LepR1 (Hebert et al. 2004). The sample identifiers in the barcode of life database (BOLD) are given for each barcoded specimen.

Sequence comparisons were performed using MUSCLE alignment (Edgar 2004) and Bayesian inference phylogenetic tree generated using MrBayes 3.2. (Ronquist et al. 2012), applying GTR substitution model with gamma-distributed rate variation across sites and a proportion of invariable sites, and 1,000,000 MCMC generations. The tree was visualised using FigTree 1.4.4. (Rambaut 2009).

Scenopinus jerei sp. nov. was originally confused with Scenopinus vitripennis (Haarto 2000; Kahanpää and Winqvist 2005). The mistake occurred because Scenopinus jerei sp. nov. easily keys out as S. vitripennis using the key in Kelsey (1969), due to the dark halteres and relatively smooth female frons. As pointed out by Kelsey (1969), S. vitripennis was treated as a synonym of S. glabrifrons by many authors prior to Trojan (1956), who established it as a valid species based on female characters. In retrospect this conclusion is problematic, as the original type specimen of Meigen was a male and is presumed lost. At the time of Kelsey’s work (1969), the type was the only reported male specimen of the species. Interestingly, there is one male identified in 1964 by Kelsey as S. vitripennis in SMNS. It is unknown to us why Kelsey did not include this specimen in his work on world Scenopinidae. The specimen is only 3.4 mm long, dark, has entirely grey-brown legs and, apart for some dirt and broken antennae, is in good condition (Fig. 1E). The colour of the legs, including the pale coxae, and general appearance of the specimen does not agree with our concept of Scenopinus jerei sp. nov. and neither does the examined female specimen in the ZIM collection (Fig. 1F). Notably, the halteres of these specimens are grey or reddish brown, much paler than the thorax, whereas those of Scenopinus jerei sp. nov. are black or as dark as the thorax, similar to the situation in S. niger.

The original description of Scenopinus vitripennis by Meigen (1824:115) is brief but provides sufficient information: “Schwartz; Beine gelbroth [sic]; Schwinger braun; Kopf unten weiß; Flügel glasshelle [sic]. Niger; pedibus rufis; halteribus fuscis; clava subtus alba; alis hyalinis.” [Black; legs yellow-red; halteres brown, underside of the head white; wings hyaline.]

Meigen’s description of the colouration of the legs as well as the underside of the head make it clear that S. vitripennis is not conspecific with our Scenopinus jerei sp. nov. However, these features are also not evident in the small, dark male specimen in the SMNS collection either. It may be that Kelsey disregarded this specimen from his work for the same reason. In fact, the male of S. vitripennis was later redescribed and illustrated by Krivosheina (1981). The shape of the antennae, terminal sternites as well as the visible genitalia features in Krivosheina’s illustrations differ markedly from those of Scenopinus jerei sp. nov. However, the comparison with S. glabrifrons is not very detailed in Krivosheina’s work. We also note that the female specimen in MIZ, collected and identified by Trojan, is very similar to S. glabrifrons in general appearance, including the flagellomere, which is cylindrical and parallel sided, compared to the basally more robust, apically narrowing flagellomere Scenopinus jerei sp. nov.

For some reason, the male genitalia of Scenopinidae have been traditionally dissected only partially, the only visible parts being the proximal parts of the aedeagus as well as the terminal segments, which makes the comparison of the published illustrations prone to interpretation errors. When fully dissected, the aedeagus has very distinct, species-specific features (Fig. 4), which could be better utilised for species identification. Unfortunately, S. vitripennis is very rare in collections, and clearly more work is needed to fully understand the extent of interspecific variation between it and S. glabrifrons. These future efforts should also focus on applying modern sequencing methods that allow the genotyping of old museum specimens (Staats et al. 2013; Prosser et al. 2016; Nakahama 2021).

To further validate our interpretation of the new species status of Scenopinus jerei sp. nov., we also checked the potential candidates among the known species of Scenopinus outside Europe. Since the revision of the world Scenopinidae by Kelsey (1969), relatively few Scenopinus species have been described (Pape and Thompson 2021), most of which can be excluded by their exotic location. However, as several Eastern Palearctic species in other families extend their range to Finland (e.g., Cannings and Kahanpää 2013; Stuke et al. 2020; Pohjoismäki and Bergström 2021), we wanted to rule out the following species from the Russian Far East:

• Scenopinus mariensis Kelsey, 1981. This species is close to S. lesinensis but has notable colour patterning on the thorax.
• Scenopinus sibiricus Krivosheina, 1982. Wings dark brown, tibiae black. Eyes separated by a frontal stripe in males. The species is morphologically close to the Nearctic Scenopinus aquelonius Kelsey, 1969, S. breviterminus Kelsey, 1969, and S. undulafrons Kelsey, 1969.
• Scenopinus ussuriensis Krivosheina, 1981. Illustrations of the antenna shape and male genitalia are dissimilar to those of Scenopinus jerei sp. nov.
• Scenopinus zhelochovtsevi Krivosheina, 1982. Legs uniform in colour. Illustrations of the antenna shape and male genitalia are dissimilar to those of Scenopinus jerei sp. nov.

No potential candidates were found among the known Nearctic species.

Scenopinus fenestralis is a common, variable, and widespread species. It also lacks clear morphological features, such as strong setae that are used as diagnostic characters for many fly groups, making it difficult to devise generalised descriptions or identification keys for both sexes. For example, the colour of the halteres can vary from dark brown to white and the colour of the legs from pale brown to orange. It is probable that developmental factors and possibly age of the fly play a big role in the morphological variation. Among others, it is known that hoverflies (Diptera: Syrphidae) developing in cool and humid conditions are typically darker than the ones developed in warm and dry conditions (e.g., Ottenheim et al. 1996). Similarly, small specimens developed under insufficient nutrition tend to be darker and morphologically more plain than larger specimens, which could also explain the aberrative habitus of the putative S. vitripennis male specimen in SMNS, mentioned earlier.

This intraspecies variation has contributed to the wealth of synonyms for S. fenestralis, which raised the question if some of the names could correspond to our concept of Scenopinus jerei sp. nov. However, checking all the old, synonymised type specimens dispersed over several collections would have been impossible. Fortunately, the colouration of Scenopinus jerei sp. nov. legs and halteres are so distinct that we were able to validate these diagnostic characters even from the quite brief old descriptions. Pape and Thompson (2021) list the following synonyms for S. fenestralis, whose original descriptions differ from the characters of Scenopinus jerei sp. nov. as indicated:

• Musca tarda Linnaeus, 1761 – White halteres, yellow legs.
• Musca saltitans Scopoli, 1763 – White halteres, red legs.
• Musca spoliata Scopoli, 1763 – Male specimen of the latter.
• Musca senilis Fabricius, 1794 – Legs yellow-red, head white from below.
• Atricha fasciatus Schrank, 1803 – Greyish specimen with milk-white bands on the abdomen. While the abdominal banding sounds like an unusual feature for S. fenestralis, we have observed that the soft white integument can be protruding between the tergites in newly hatched Scenopinus specimens, giving the abdomen a banded appearance (Fig. 6A). Legs entirely olive brown.
• Scenopinus pallipes Say, 1823 – White halteres, yellow legs.
• Scenopinus domesticus Meigen, 1824 – Legs yellow-red, head white from below.
• Scenopinus sulcicollis Meigen, 1824 – Legs yellow-red, head white from below, halteres white.
• Scenopinus scutellatus Macquart, 1843 – Halteres white, scutellum yellow.
• Scenopinus furcinervis Zetterstedt, 1844 – Legs fully yellow.
• Scenopinus fuscinervis Schiner, 1860 – The name is not mentioned in Schiner (1860).
• However, Kelsey attributes the synonymy to Schiner (1862), where S. fuscinervis Zetterstedt is given as a synonym of S. fenestralis. It is obvious that in this context, S. fuscinervis is a misspelling of S. furcinervis. In fact, the spelling is later corrected in Schiner (1864).
• Scenopinus graminicola Zetterstedt, 1859 – Halteres white.
• Scenopinus nigroscutellatus Frey, 1945 – From Azores, halteres white.

Based on this survey, we are confident that the species presented here as Scenopinus jerei sp. nov. is not among the accepted species nor hidden among the synonyms of S. fenestralis. We hope that the species discovery reported here, together with the provisional indentification key we have provided, will encourage more research towards this exciting but poorly known family of flies.

Figure 6. 

Identification of Scenopinidae A male Scenopinus fenestralis. Hanko, Finland, June 22, 2021. Note the three stripes on the abdomen caused by the white integument protruding between the tergites. These should not be confused with white bands of microtomentum on tergites of some Scenopinus species. Photograph by J. Pohjoismäki B illustration of Caenoneura wing. Arrow pointing the petiole on r₅. Drawn after Kelsey (1969) C generic Scenopinus wing. Modified from Winterton and Gaimari (2017) D wing venation in albicinctus-group with R₄ branching from R₅ beyond the middle of cell r₅ (arrow; compare with C) E Wing of Scenopinus griseus (Kröber) with narrow cu cell. Drawn after Narchuk (1988).