A new fossil species of the genus Bibio, with an update on bibionid flies from Baltic and Rovno amber (Diptera, Bibionidae)

Species of Bibionidae from Baltic amber are reevaluated based on newly discovered material, and a key to the species is given. Bibio succineus sp. nov. is described based on one male specimen, this is the first Bibio named from Baltic amber. The males of Hesperinus electrus Skartveit, 2009 and Penthetria montanaregis Skartveit, 2009 are redescribed. A single, autoclave treated specimen of Penthetria sp. is described but not formally named. Plecia tenuicornis Skartveit, 2009 is found to be a synonym of Plecia hoffeinsorum Skartveit, 2009, this species is recorded for the first time from Rovno amber, and both sexes of the species redescribed. Additional specimens of Plecia clavifemur Skartveit, 2009 and Dilophus crassicornis Skartveit, 2009 are described. Two female specimens probably belonging to the species discussed as Dilophus sp. by Skartveit (2009) are described, but not formally named.


Introduction
Bibionid flies are a very abundant group in European Tertiary insect fossil deposits (e.g., Skartveit and Nel 2017), where they frequently make up a large percentage of the total insect specimens. On the other hand, bibionid specimens are relatively scarce among amber fossils, though a number of species have been described from European amber fossils (Gee et al. 2001;Skartveit 2009). Outside Europe, bibionid flies are known from Cretaceous Canadian (Peterson 1975), Miocene Dominican (Waller et al. 2000;Skartveit and Bechly 2013) and Chiapas (Hardy 1971) amber.
Rovno amber comes from mines in Rivne Oblast, Ukraine (Perkovsky et al. 2010). The deposits have been dated to the Lower Oligocene, suggesting a younger age than Baltic amber (Perkovsky et al. 2010), though more recently it has been redated to the late Eocene (Perkovsky and Makarkin 2019). While the fauna in Rovno amber is rather similar to the one in Baltic amber, suggesting they are of the same age and origin (e.g., Dlussky and Rasnitsyn 2009;Szwedo and Sontag 2013), some differences have been noted between the two palaeofaunas (Perkovsky et al. 2007), with about 100 taxa found in Rovno amber not recorded from Baltic amber (Perkovsky et al. 2010). Rovno and Baltic amber are very similar in chemical composition; however, analyses of stable carbon and hydrogen isotopes in Rovno, Bitterfeld and Baltic amber suggest that Rovno amber is of similar age to Baltic amber, but originated in a more southerly location (Mänd et al. 2018). Bibionid flies have not previously been recorded from Rovno amber.
I (Skartveit 2009) reviewed a reasonably large material of bibionids from Baltic amber. Subsequently, some additional specimens have surfaced which can shed some light on the Baltic amber bibionid fauna; they are described here.  12 Thorax without spines on pronotum and mesonotum (Fig. 25). Anterior tibia without mesal spines and apical spine circlet (Fig. 26) Skartveit, 2009 15 Antenna short and stout, flagellum 6-7-segmented (Fig. 29). Protibial mesal spines long and erect ( Fig. 30) (Fig. 9)  Note. The species was described based on a single, male specimen (Skartveit 2009). One additional male specimen has turned up which shows details in wing venation and terminalia better than the type.
Discussion. The original description was found to contain some errors, e.g., the flagellum has eight, not seven segments, and is shorter than stated in the description. The poorly developed mouthparts suggest that this species did not feed in the adult stage, this may be a general trait for Hesperinus species as all seem to have very small mouthparts. The genus Hesperinus has frequently been referred to a separate family, the Hesperinidae (e.g., Krivosheina 1997). However, molecular evidence suggests that the genus belongs in the Bibionidae (Ševčik et al. 2016) and this is also supported by fossil material, where Hesperinus and Penthetria species look far more similar than in the modern species. Hence, I prefer to place Hesperinus and Penthetria together in the subfamily Hesperininae in Bibionidae (Skartveit and Ansorge 2020). Skartveit, 2009: 13 Figs 2, 15-17 Note. The species was described based on a single, male specimen (Skartveit 2009), however this specimen was to a large extent covered by white emulsion (Verlumung). The present specimen clarifies some aspects of the species' morphology.
Head (Fig. 16): Length 0.75 mm, width 1.05 mm (N = 1). Only just holoptic, complex eyes in contact only 2-3 ommatidia, meeting at distance similar to diameter of ocellar tubercle anterior to tubercle. Complex eye nearly bare, with very short, fine and sparse intraocular hairs. Ocellar tubercle small but fairly prominent, with few strong, short, dark brown setae on posterior face. Rostrum not protruding. Palp not possible to see in this specimen. Antenna: flagellum slightly conical, 8-segmented, 0.58 mm long, 0.09 mm wide, segments subquadrate when seen laterally.
Terminalia (Fig. 17): Width of hypopygium 0.71 mm (N = 1). Epandrium bilobate, lobes rounded with dense, rather long, dark brown setae. Gonocoxite with robust,  dark brown setae, Gonostylus length 0.39 mm, rather long, straight, apically a little expanded, on posterior margin with about 8 strong, dark brown setae. (note: the apparent shape of the gonostylus is rather different from the holotype of Penthetria montanaregis, but this is probably due to preservation in different angles. The apparent shape of the gonostylus in male Bibionidae is extremely dependent on perspective). Ejaculatory apodeme rather wide.
The species differs from Penthetria montanaregis in the following aspects: smaller, wing length about 4.5 mm, wing narrower, more than 3 times as long as wide (in bibionids, females generally have wider wings than conspecific males) with reduced anal lobe, R 2+3 placed more distally so that the segment R 2-5 is almost as long as R 4+5 (less than half as long in P. montanaregis), fork of M strongly asymmetrical (nearly symmetrical in P.montanaregis), CuA 2 apically strongly curved basad (moderately curved basad in P. montanaregis). It differs from female Penthetria integroneura Skartveit, 2009 most conspicuously by the presence of R 2+3 and by the more strongly curved CuA-veins, also by the presence of strong setae dorsally on the thorax and apparently by the head shape, though the latter is likely affected by autoclave treatment.
Description. Male unknown. Female (N = 1): Total length 5.0 mm. The specimen is of a uniform, dark colour, likely affected by the autoclaving.
Head: length 0.58 mm. Apparent shape probably affected by autoclaving, outline of complex eye not possible to see. Flagellum 0.42 mm long, 0.07 mm wide, 7-segmented, shape of flagellomeres obviously affected by autoclaving. Palp relatively long, outer segments appear to be very slender, but this is likely an artefact caused by autoclaving.
Thorax: Length 1.18 mm. Dorsally with some relatively long and strong, erect setae, details otherwise not possible to make out. Haltere light brown.   Legs: relatively long, femorae slightly clavate, tibiae apparently very slender (possibly affected by autoclaving). The legs are clad with relatively long, brown pile, on tibiae about as long as the tibia's width. Tibial spurs fine and sharp. Segment measurements, all inn mm: fore femur length 1.37, width 0.16, fore tibia length 1.32, width 0.12, mid femur length 1.23, hind femur length 1.69, width 0.18, hind tibia length 1.71, width 0.13, hind first tarsomere length 0.63, width 0.07. Wing (Fig. 18): Length 4.6 mm, width 1.5 mm, length/width = 3.1. Brown fumose, relatively slender, veins brown throughout, R-veins considerably more robust than M-and CuA-veins. Costa relatively strong with rather long setules which at least basally are longer than the width of the vein, extending a little past halfway between apices of R 4+5 and M 1 . Humeral vein present, oblique. Pterostigma relatively small, oval, brown. Subcosta fine but distinctive, straight. R 2+3 oblique, almost straight, a little less than half as long as R 4+5 , which is gently curved. Rs straight, nearly three times as long as R-M, which is vertical. R 2-5 (basally of R 2+3 ) almost as long as R 4+5 (distally of R 2+3 ). M basally connected to CuA. M-veins fine, M and M 2 form an approximately straight line with M 1 diverging forward from this. CuA 1 apically slightly, CuA 2 strongly bent basad. CuP parallel to CuA/CuA 2 , fine but does reach wing margin. Anal lobe strongly reduced, basal-posterior edge of the wing nearly straight-lined. Vein lengths, all in mm: Subcosta 2.48, Basal R 1.46, R 1 1.44, Rs 0.53, R 2-5 1.09, R 2+3 0.58, R 4+5 1.27, R-M 0.19, basal M 1.41, distal M 0.44, M 1 1.85, M 2 1.41, M-CuA 0.14, CuA 1.02, CuA 1 1.99, CuA 2 1.18. The wings have numerous, transverse cracks probably caused by autoclaving, this has not altered the overall shape, though.
Abdomen: Length 3.6 mm, dark, cylindrical, with fine, dark, short pile. Shape of terminalia difficult to make out, probably affected by autoclaving.
Discussion. The specimen of this species is obviously affected by autoclave treatment, particularly so in the head where the overall shape appears changed, the outlines of the complex eyes are not possible to make out, and the shapes of the antennal and palp segments are strongly disrupted. The autoclaving appears also to have altered the appearance of the terminalia, and possibly tho-rax and legs to some extent. However, the wing characters appear to be uncompromised and should be sufficient to recognise the species, at the very least to differentiate it from the other species of Penthetria known from Baltic amber. Identifying Penthetria species based on female specimens is very difficult in recent species and this is probably so in fossil species, too, hence this specimen is not given a formal name at this stage. Additional material, females. JS-Baltic-002, in piece of amber 20×15×4 mm, JS-Baltic-003, in piece of amber 18×13×6 mm. These specimens do not reveal any characters not seen in the type material, but their morphometric data is given below.
The species was described based on seven male specimens preserved together in one piece of amber. The present specimens do show the male terminalia better than the type material, hence this is redecribed here, otherwise the external morphology is adequately described in the original descriptions of Plecia hoffeinsorum and Plecia tenuicornis (Skartveit 2009: 17-22). The female of Plecia hoffeinsorum was described from a compression fossil from Grube Messel (Skartveit and Wedmann 2016). The two species Plecia hoffeinsorum and Plecia tenuicornis were originally thought to have rather different male terminalia (Skartveit 2009, figs 34, 35, 46, 47), however studying additional specimens suggests that the two "forms" are actually the same structure with the gonostylus fixed in different positions. As usual with Bibionidae the angle of view is quite critical when studying these structures.
Comparison to other species. Four species of Bibio have been described from the Eocene/Oligocene of Isle of Wight (Krzeminski et al. 2019). Of these, Bibio gurnetensis Cockerell, 1917 andBibio oblitus Cockerell, 1921 both have crossvein R-M much longer than the basal Rs, this character suggests the species belong in the genus Dilophus, though, not having seen the original material, I will not make a formal transfer here. Bibio extremus Cockerell, 1921 differs from the present species in that the basal part of Rs is quite steep and that crossvein M-Cu meets M 2 considerably distally of furcation. Bibio oligocenus Cockerell, 1917, which despite the name was described from the late Eocene, differs from the present species by being larger (wing length 8 mm) with a conspicuously darkened costal cell in the wing.
Etymology. The epithet is derived from Latin succinum, amber, referring to the preservation of the type specimen. It is the first species of the genus Bibio described from amber fossils. Diagnosis. A medium-sized Bibio, body length about 7.5 mm. Body and legs entirely black, densely pilose, pile on thorax and abdomen pale, black on legs. Antennal flagellum 8-9-segmented. Haltere pale brown. Wing light brownish fumose in male, brown fumose in female, pterostigma pale and indistinctive, radial sector about four times as long as R-M. Fore tibia with spur a little less than half as long as spine. Hind tarsus not enlarged.
Head (Fig. 25): Length 1.37 mm, width not possible to measure. Complex eye with rather dense, pale, fine, medium-length intraocular pile. Ocellar tubercle not prominent, with short, dark setae. Rostrum not prominent. Antenna: flagellum 8-segmented, 0.53 mm long, 0.13 mm wide. Two distal palp segments slender (more basal segments not possible to see).
Abdomen: Black, clothed with rather short, fine, pale pile.
Wing: Brown fumose, membrane without microtrichia. Costa and R-veins brown, more posterior veins colourless. Basal radial sector about five times as long as crossvein R-M Legs: black, rather stout. Length of fore femur 1.5 mm, width 0.5 mm, length of fore tibia 1.4 mm, of hind tibia 2.2 mm. Fore tibia (Fig. 27) with spine about three times     as long as spur, this is stout, straight and pointed. The tibia is clothed with quite long, fine setae and has a field of rounded coeloconical sensillae in the middle of the anterior face. The tarsi are quite slender.
Abdomen: cylindrical, no details possible to see. Terminalia: no details possible to see. Wing: Length 3.44-3.75 mm (N = 2). Hyaline, slightly brownish, veins fine and brown. Pterostigma brown. Costa extends to half-way between apices of R 4+5 and M 1 .
Two female specimens, belonging to the Dilophus febrilis-group, with 9 flagellomeres, so not fitting any of the previously described species which have 6-7 (Dilophus crassicornis) or 12 (Dilophus pseudofebrilis and Dilophus succineus) flagellomeres. They are likely to be conspecific with the poorly preserved specimen treated as Dilophus sp. by Skartveit (2009: 38). As the other species of Dilophus from Baltic amber are based on male specimens I find it not advisable to formally name this species at the present stage of knowledge. Males of this species, should they appear, should be recognizable by characters of the antenna and fore tibia.
Total length 4.55-5.68 mm. Body and legs entirely dark brown. Head (Fig. 33): Length 0.68-0.80 mm. Occiput, frons and gena all with strong, erect, relatively dense setae. Complex eye about half as long as head, somewhat protruding, with short and fine, rather dense intraocular pile. Ocellar triangle rather tall. Flagellum 9-segmented, 0.35-0.40 mm long, 0.06-0.08 mm wide. Palp shorter than antenna, with last segment conical, about 2.5 times as long as wide, bearing relatively long setae.
Wing (Fig. 32): Rather crumbled in the newly acquired specimens. The specimen studied by Skartveit (2009) with wing as in Fig. 32, wing length 4.4 mm. Almost hyaline, costa and R-veins dark brown, more posterior veins lighter brown but still distinctive. All veins are fine, no conspicuous thickenings. Costa with biseriate, rather dense, pale setulae, basally about as long as the width of the costal cell, decreasing in length apicad. Costa extends to about half-way between apices of R 4+5 and M 1 . Pterostigma oval, brown, distinctive. Humeral vein present, subcosta fine, straight, running well separated from R 1 in entire length. Basal R and R 1 dorsally with uniserial, fine, dark setulae which are about as long as the width of the vein and separated by about as much as their length, veins otherwise bare. Radial sector about one-third the length of crossvein R-M. R 4+5 gently curved. Area between R 4+5 and M 1 about 1.5 times as wide as fork of M. M-veins apically straight. Crossvein M-CuA meets M well basad of furcation. CuA with rather long stem and short fork, CuA 2 apically a little curved basad. CuP running approximately parallel to CuA, rather indistinctive. A 1 apparent on stem of wing only.
Abdomen: Strongly swollen in specimen at hand, membraneous areas stretched. This is presumably because it is egg-filled. Tergites and sternites clad with short, dark setae.

Discussion
Bibio succineus is the first Bibio species formally named from amber fossils. This is a bit peculiar since the genus is common to abundant in Tertiary compression fossils from Europe (e.g., Skartveit and Pika 2014;Skartveit and Nel 2017) and other bibionid genera are known from numerous amber specimens. It may have to do with habitat preferences. In the recent, European fauna, 32 species are known in the genus Bibio (Skartveit 2013), of these only four (Bibio clavipes, B. handlirschi, B. nigriventris and B. varipes) are normally found in closed forest habitats (Skartveit, personal observation). On the other hand, species of the genus Dilophus, which are not that rare in amber fossils, are also not commonly found in forests in the recent fauna. Most Bibio species are fairly large-sized (most are > 6 mm), and amber fossil samples are strongly biased towards small-sized specimens, this may also be part of the explanation.
While there are many similarities between the faunas of Baltic and Rovno amber (e.g., Szwedo and Sontag 2013), there may also be some differences (Perkovsky et al. 2010). Bibionid flies are often widespread, and they also appear to be quite persistent in time (e.g., Wedmann and Skartveit 2020;Skartveit and Wedmann 2021), thus it is not surprising that a species described from Baltic amber also turns up in Rovno amber, even if this may have been deposited further south than Baltic amber (Mänd et al. 2018).
Although amber fossils may be excellently preserved with anatomical structures visible in great detail, in most specimens some traits are not visible because of opaque emulsions (Verlumung), because they are covered by other body parts or because they have been deformed (e.g., crumbled wings). For abundant taxa there may be a large number of specimens available to pick from, but for less abundant taxa such as Bibionidae the taxonomy may have to rely upon less-than-perfect specimens. When this is the case, finding new specimens of already described species offers an opportunity to gradually improve the knowledge of the taxon. This is so with all fossil materials, with the possible exception of limited outcrops which are no longer available for sampling, any fresh set of specimens found offers an opportunity to improve upon the taxonomy of any group, and any fossil classification should be viewed as preliminary, pending the discovery of new material.
Presently, a large fraction of the Baltic amber material available has been treated with an autoclave to improve the transparency and general appearance of the amber (Hoffeins 2012). Unfortunately, this may alter the appearance of the specimens in fairly unpredictable ways, and may destroy much taxonomically relevant information (Hoffeins 2012).