The first stage of the evolution of Rhabdomastix (Diptera, Limoniidae) and the taxonomic implications of genus

Abstract A new subgenus Myanmamastix subgen. n. and four new species of Rhabdomastix from Cenomanian Burmese amber were described: Rhabdomastix (Myanmamastix) asiatica subgen. and sp. n., Rhabdomastix (Myanmamastix) cretacica subgen. and sp. n., Rhabdomastix (Myanmamastix) krzeminskae subgen. and sp. n., Rhabdomastix (Myanmamastix) myanmae subgen. and sp. n. The species Rhabdomastix jarzembowskii known from Burmese amber was reclassified to the new subgenus Myanmamastix subgen. n. An analysis of the morphological structures and taxonomical differences between the representatives of Cretaceous Rhabdomastix was carried out. The manuscript presents results of original research, including the first 3D reconstruction of a Dipteran preserved as an inclusion in Burmese amber using computer microtomography.http://www.zoobank.org/urn:lsid:zoobank.org:pub:79C82852-2502-4A2F-B684-CCCE10D6C525


Introduction
Mesozoic Burmese amber (98.79 ± 0.62 Ma (Shi et al. 2012)) occurs in deposits in the state of Kachin in Myanmar (Burma) near Shwebo, Thayetmyo, Pakokku and Pegu provinces. This resin is found in the presence of thin-layer sandstone, silt, clay shale, micritic limestone and coal (Poinar & Brown 2003;Ross et al. 2010), in the form of lenses of various sizes or in the form of structures resembling small stalactites with circular cross-sections generally about 1 cm in diameter, though some larger pieces are known (Cruickshank & Ko 2003).
Intensive research on Burmese amber conducted in the twenty-first century has brought surprising discoveries. This fossil resin is characterized by an exceptional species diversity, including a large number and broad taxonomic diversity of insects preserved as inclusions. So far, representatives of over 290 families of invertebrates, mainly insects, arachnids, crustaceans and other arthropods, have been described from inclusions in Burmese amber (Ross 2020). The amber contains inclusions of flies from the family Culicidae Meigen 1818 along with representatives of many other insect groups, including species from the orders Embioidea Hagen, 1862, Strepsiptera Kirby, 1813, Zoraptera Silvestri, 1913, Archeognatha Börner, 1904, Zygnetoma Börner, 1904, Hemiptera Linnaeus, 1758and Lepidoptera Linnaeus, 1758(Engel & Grimaldi 2002, 2006Borkent & Grimaldi 2004;Grimaldi et al. 2005;Ross 2020). However, the Diptera are one of the most numerous in terms of the number of species and specimens, as well as the one of the best known groups of insects represented in Burmese amber.
The representatives of the genus Rhabdomastix Skuse, 1890 described herein are medium-sized limoniids with 16-segmented antennae, 4-segmented palpus maxilaris, without ocelli or tibial spurs. They are characterized, like the other representatives of Limoniidae Speiser 1909 (Diptera, Nematocera), by termination of the subcostal vein into the costa. Wing venation also shows four radial veins and three medial veins well developed, while vein r-r (R 2 ) is usually atrophied and d-cell is closed. Hypopygium of males is not large with massive gonocoxites and two pairs of gonostyles, while the ovipositor of females is not very elongate with two pairs of saberlike valves and one pair of cerci (Skuse 1890;Starý 2003;Szwedo et al. 2020).
In contemporary fauna the genus Rhabdomastix is represented by over 130 species and subspecies, which occur mainly in the Holarctic, with 68 species and subspecies, known across the region of these 40 species are represented in the Palearctic region, while 29 occur in the Nearctic region. Only a single species, Rhabdomastix (Rhabdomastix) borealis Alexander 1924 is known from both the Paleartcic and Nearctic regions (Oosterbroek 2021). Beyond the Holarctic only three species of the genus Rhabdomastix occur in the Afrotropic region, 42 occur in the Neotropic region, 15 in the Oriental region, and 23 species in the Australasian region (Figures 1 and 2) (Oosterbroek 2021).
In the fossil record, the genus Rhabdomastix is represented by 19 species described mainly from inclusions in Eocene Baltic amber (11 species) (Meunier 1906;Alexander 1931;Podenas 2006) and Oligocene imprints in sediments (7 species) (Scudder 1894;Cockerell 1920Cockerell , 1927Statz 1944). To date only one species has been described from the Cretaceous, based on inclusion in Cenomanian Burmese amber, this is Rhabdomastix jarzembowskii Krzemiński, 2004, the oldest representative of the genus. Despite Diptera being among the most numerous insects represented in Burmese amber, to date, only 13 species within 11 genera of Limoniidae have been described from inclusions in Burmese amber and only the one species of Rhabdomastix (Table I) (Ross 2020).
In Burmese amber we can find very well preserved specimens of insects, allowing for study of important taxonomic features at the same level as research of contemporary entomofauna. New findings of Rhabdomastix preserved as inclusions in Cretaceous Burmese amber shed new light on the early evolution of these insects.
The genus Rhabdomastix is recognized as a taxon with one of the most difficult internal classification systems. This is due to the slight degree of differentiation of the hypopygium, a structure that is usually highly distinctive and specialized for individual taxa at different levels within the family Limoniidae (Skuse 1890). However, study of the newly discovered fossil material highlights additional diagnostic features that give valuable insight on the classification within the genus Rhabdomastix, consistent for contemporary and fossil species.
The system of classification provided by Alexander (1914) included three subgenera of Rhabdomastix: Rhabdomastix s. str., Sacandaga Alexander, 1911, andPaleogonomyia Meunier, 1899 and was based on the ratio of antennae length to body length. According to this system antennae longer than the body length Figure 1. Worldwide distribution of subgenera and species of Rhabdomastix in recent fauna (data according to Oosterbroek (2021)). characterized the representatives of subgenus Rhabdomastix, much shorter antennae than the body distinguished the subgenus Sacandaga, and only slightly shorter than the body -Paleogonomyia. This classification was used also by Savchenko in the Catalog of Palearctic Diptera (Savchenko et al. 1992). The Catalog of fossilized flies of the world (Insecta: Diptera) include 15 species, which were classified to the genus Rhabdomastix, Paleogonomyia, Sacandaga and 6 species unclassified to any subgenus (Evenhuis 1994;Starý 2003 Krzemiński, 2004 as belonging to the newly described subgenus.

Material and methods
The material examined included four specimens of fossil Diptera of the genus Rhabdomastix Skuse, 1890, preserved (Poinar & Brown 2003;Shi et al. 2012;Smith & Ross 2017). Burmese specimens were acquired in 2016, prior to the armed conflict and the escalation of the ethnic strife in the area (Szwedo et al. 2020).
All the studied specimens, erected as a new species, described, or figured are deposited in an accessible, permanent repository in public institutions: Institute of Systematics and Animal Evolution of the Polish Academy of Sciences in Krakow (ISEA PAS) (3 specimens) and Nanjing Institute of Geology and Palaeoentomology, Chinese Academy of Science (CAS) (1 specimen) ( Table I).
The research was carried out using a Nikon SMZ 1500 stereomicroscope equipped with a Nikon DS-Fi1 camera, and the measurements were made using the NIS-Elements D 3.0 software. The drawings were made by W. J.-S. The specimen No. MP/3630 was visualised, using micro-computed tomography. MicroCT data were collected with Zeiss Xradia MicroXCT-200 imaging system equipped with a 90 kV/8 W tungsten X-ray source in the Laboratory of Microtomography, Institute of Paleobiology, Polish Academy of Sciences, Warsaw. The scans were performed using the following parameters: voltage: 133 kV, power: 4 W, exposure time: 30.00 s, voxel size: 3.85 µm. Radial projections were reconstructed with XMReconstructor software provided with the Xradia system. The 3D images of specimens and XMT sections were obtained with Avizo 7.1 Fire Edition software.
The wing venation nomenclature follows McAlpine et al. 1981, while the nomenclature of the male and female terminalia is in accord with later works (Krzemiński 2002;Podenas 2006). The classification system proposed by Starý 2003, covering both modern and extinct species, was adopted. The length of vein M 3 was measured from the point of its connection with the m-m cross-vein to the wing edge, and the length of the d-cell from its rear edge (from the wing base) to the cross-vein connection point of m-m with vein M 3 . Measurements of individual parts of the body were given only when the measured morphological structures were not damaged.

Acronyms
ISEA PAS -Institute of Systematics and Animal Evolution of the Polish Academy of Sciences in Krakow; CAS -Nanjing Institute of Geology and Palaeoentomology, Chinese Academy of Science; BMNH -British Museum of Natural History.

Systematic palaeontology
Vein Sc terminating before half the length of Rs; cross-vein sc-r positioned not more than 2x its length from the tip of Sc not more than  Diagnosis. Antennae only slightly longer than the length of the head and thorax combined; vein Sc terminating beyond half the length of the wing; just before or beyond half the length of Rs; cross-vein rr (R 2 ) completely atrophied; cross-vein sc-r present; bifurcation of Rb before half, at half or beyond half the length of the wing; Rs very short, always shorter than half the length of Mb; d-cell pentagonal, large, elongate, longer or as long as M 3 ; R 3 more or less oblique, length of R 3 one-quarter to one-half the length of R 4 ; distal part of M 1+2 and M 3 slightly arched; inner gonostylus longer than outer gonostylus.
Etymology. The subgenus name is derived from the name of the country of Myanmar (Burma), in which there are deposits of Burmese amber. Gender: feminine.
Description. Body 2.49-3.48 mm long, dark brown, wing without color pattern, pterostigma absent. Head: width of head 0.24-0.31 mm; antenna with cylindrical pedicel, slightly narrowed at the base and widened in the distal part, scape short, as long as wide, or twice as wide; pedicel and scape with sparse setae less or more elongate; flagellomeres comparable length, the last flagellomere shorter than the penultimate one; setae of various length, from as long as 1/2 the length of flagellomere to longer than the length of the flagellomere bearing them; the last flagellomere with two setae situated at its tip. Antenna 1.10-1.29 mm long: pedicel 0.06-0.10 mm; scape 0.06-0.10 mm; flagellomeres: 1/0.06-0.10 mm; 2/0.07-0.08 mm; 3/0.06-0.08 mm; 4/0.07-0.08 mm; 5/0.07-0.08 mm; 6/0.07-0.08 mm; 7/0.07-0.10 mm; 8/0.07-0.10 mm; 9/0.07-0.10 mm; 10/0.07-0.08 mm; 11/0.06-0.07 mm; 12/0.06-0.08 mm; 13/0.06-0.07 mm; 14/0.03-0.05 mm). Palpus 0.19-0.33 mm long (1/0.04-0.06 mm; 2/0.05-0.10 mm; 3/0.05-0.07 mm; 4/0.05-0.11 mm); palpomeres cylindrical, elongate, narrow, first palpomere 3-3.5 times as long as wide, second 2.5-4 times as long as wide, third palpomere 2.5-3 times as long as wide, the last palpomere longer than the penultimate one, 5-5.5 times as long as wide. Each palpomere with a few to a dozen elongate 1156 I. Kania-Kłosok et al. setae, as long as the length of the palpomeres bearing them or longer. Thorax: wing 2.68-3.21 mm long, 0.55-0.83 mm wide, 3.5-4.5 times as long as wide, cross-vein (h) positioned at one-fifth the length of the wing from its base; cross-vein sc-r positioned not far from the tip of Sc, approximately 2-5 times its own length from the tip of Sc; the distance between the tips of Sc and R 1 of comparable length or 0.3 times its length longer than the distance between the tips of R 1 and R 3 ; R 3 short, equal to 0.3 times the distance between the tips of R 1 and R 3 . M 1+2 from straight to slightly arched, 0.25-0.3 times longer than d-cell; M 4 short, shorter than length of d-cell; vein Mb equal to 2.5-5 times the length of M 3 ; Cu from the point of connection with cross-vein m-cu to the edge of wing straight or slightly bent toward the wing margin. The distance between the tips of M 1+2 and M 3 approximately as long as the distance between the tips of M 3 and M 4 or 1.5 times longer; the distance between the tips of M 3 and M 4 shorter than the distance between the tips of M 4 and Cu; the distance between the tips of M 4 and Cu and the tips of Cu and A 1 shorter, of comparable length or longer; the distance between the tips of A 1 and A 2 and the tips of M 4 and A 1 shorter, of comparable length or longer. Haltere: 0.28-0.54 mm long, stem very narrow, only slightly longer than knob. Abdomen: hypopygium 0.27-0.35 mm long; gonocoxite elongate, massive; outer gonostylus narrow, blunt or sharply terminated, strongly sclerotized, in some representatives with a small appendix in the apical part.
Comparison. In representatives of subgenus Myanmamastix subgen. n. the inner gonostylus is much longer than the outer gonostylus, while in representatives of the other subgenera within the genus Rhabdomastix the outer gonostylus is longer than the inner gonostylus. Contrary to the subgenus Lurdia, where cross-vein sc-r is absent (11(a)), in representatives of the subgenus Myanmamastix subgen. n. this vein is well developed (11(b) and 12(a-e)). In contrast to the representatives of other subgenera within the genus Rhabdomastix, in Myanmamastix subgen. n. the d-cell is elongate, 2.5 times as long as wide, as long, or longer, than the length of M 3 . In the species classified to the subgenus Lurdia (11(a)), as well as representatives of the subgenus Myanmamastix subgen. n. (11(b)) the d-cell is pentagonal (Starý 2003), while in representatives of the subgenus Rhabdomastix the d-cell is hexagonal (11(c)) (Starý 2004). Antennae of representatives of the subgenus Myanmamastix subgen. n. are usually only slightly longer than the length of the head and thorax combined, while the subgenus Rhabdomastix the antennae can vary from slightly longer than the head to much longer than the body.

New combination
Rhabdomastix (Myanmamastix) jarzembowskii Krzemiński, 2004 comb. n. Rhabdomastix (Paleogonomyia) jarzembowskii Krzemiński, 2004: Krzemiński, p. 124(12d Remarks. The species, was first described and classified by Krzemiński in 2004 to the subgenus Paleogonomyia Meunier, 1899. Howewer, features such as a complete atrophy of cross-vein r-r (R 2 ), presence of cross-vein sc-r and a very short vein Rs, always shorter than half the length of Mb, presence of a large, elongate, pentagonal d-cell equal in length to vein M 3 , slightly arched veins M 1+2 and M 3 or inner gonostylus longer than characteristic, strongly sclerotized outer gonostylus all indicate inclusion of this species in the subgenus Myanmamastix subgen n. and allow for its designation as the type species for the subgenus.
Rhabdomastix (Myanmamastix) asiatica subgen. and sp. n. (Figs 3; 12a) Diagnosis. Vein Sc terminating just beyond half the length of Rs; cross-vein sc-r terminating 3 times its own length from the tip of Sc; fork of Rb before half the length of wing from wing base; R 3 approximately one-third the length of R 4 ; R 4 equal to the length of R 2+3+4 ; Rs longer than R 2+3+4 ; R 1 very short, terminate at about 0.3 of the length of R 2+3+4 ; M 3 equal to the length of the d-cell, very strongly bent toward thewing margin; d-cell 2.5 times as long as wide; the distance between the tips of Sc and R 1 is slightly shorter than the distance between the tips of R 1 and R 3 . Etymology. The name of the new species is derived from the name of the continent "Asia", on which the specimen was found.
Description. Body (Figure 3(a)) dark brown, wing without color pattern, pterostigma absent, haltere elongate (Figure 3 (b)). Thorax: wing (Figure 3 (c-e)) 3.21 mm long, 0.83 mm wide, 4.5 times as long as wide, cross-vein (h) positioned at one-fifth the length of the wing from its base in; R 3 short, its length approximately 0.3 times the distance between the tips of R 1 and R 3 . Vein M 1+2 arched, 1.5 times longer than d-cell; length of Mb equal 4.5 times the length of M 3 ; Cu from the point of connection with cross-vein m-cu to the edge of wing straight. The distance between the tips of M 1+2 and M 3 1.5 times the distance between the tips of M 3 and M 4 ; the distance between the tips of M 4 and Cu 2 times longer than the distance between the tips of M 3 and M 4 ; the distance between tips of M 4 and Cu slightly shorter than the distance between the tips of Cu and A 1 ; distance between tips of M 4 and A 1 1.5 times the distance between the tips of A 1 and A 2 . Veins A 1 and A 2 slightly arched (12(a)). Haltere: (Figure 3 (b)) 0.38 mm long, sterm narrow and relatively short, about 1.5 times longer than the knob. Rs which is longer than vein R 2+3+4 , and has vein R 1 terminating at about one-third the length of R 2+3+4 , while in R. (M.) myanmae subgen. and sp. n. vein Rs is shorter than R 2+3+4 , and R 1 terminates just before half the length of R 2+3+4 . In R. (M.) krzeminskae subgen. and sp. n. cross-vein sc-r terminates 5 times its own length from the tip of Sc, while in R. (M.) asiatica subgen. and sp. n. sc-r terminates 3 times its length from the tip of Sc. Moreover, in R. (M.) asiatica subgen. and sp. n. the distance between the tips of Sc and R 1 is shorter than the distance between the tips of R 1 and R 3 , while in R. (M.) krzeminskae subgen. and sp. n. this distance is longer. Finally R. (M.) jarzembowskii Krzemiński, 2004 comb. n. vein Sc terminates before half the length of Rs. R 3 is one-quarter the length of R 4 , and R 4 is longer than R 2+3+4 . While in R. (M.) asiatica subgen. and sp. n. Sc terminates near half the length of Rs. R 3 is one-third the length of R 4 and R 4 is equal to R 2+3+4 .

Remarks.
A specimen without a well-preserved head and legs, a darkened field is visible in the place of the

1158
I. Kania-Kłosok et al. head, making it impossible to examine the morphological features of this part of the body.

Rhabdomastix (Myanmamastix) cretacica
subgen. and sp. n. (Figs 4, 5, 12b) Diagnosis. Vein Sc terminating just beyond half the length of Rs; cross-vein sc-r terminating at a distance of 3 times its own length from the tip Sc; fork of Rb positioned approximately at half the length of the wing; R 3 half the length of R 4 , R 3 very short, its length slightly longer than half of R 2+3+4 ; Rs longer than R 2+3+4 ; R 1 very short, terminating about opposite one-third the length of R 2+3+4 ; M 3 much shorter than d-cell, only slightly bent toward the wing margin; d-cell 2.5 times as long as wide; distance between the tips of Sc and R 1 and the tips of R 1 and R 3 of comparable length; on each flagellomere a few elongate, thick setae, longer than the flagellomere bearing them, two of them on each flagellomere arranged almost perpendicular to the surfaces of the individual flagellomeres, the others at an acute angle; last flagellomere elongate, only slightly shorter than penultimate one; outer gonostylus short, narrow, sharply tipped, inner gonostylus broad basally then tapering to a blunt tip. Etymology. The name of the new species is derived from the name of the geological period "Cretaceous"/Latin: crētāceus.
Thorax: (Figure 4(a)) wing (Figures 4(a, e) and 5(a)) 2.68 mm long, 0.82 mm width, 3.5 times as long as wide; cross-vein (h) positioned at one-fifth the length of the wing from its base; R 1 terminating at about opposite one-third the length of d-cell; R 3 short, equal to 0.3 times the distance between the tips of R 1 and R 3 . Vein M 1+2 only slightly longer than the length of d-cell; M 4 almost straight, nearly half the length of d-cell; vein Mb 5 times longer than the length M 3 ; Cu from the point of connection with cross-vein m-cu to the edge of wing slightly bent. The distance between the tips of M 1+2 and M 3 and the tips of M 3 and M 4 of comparable length; distance between the tips of M 3 and M 4 shorter than the distance between the tips of M 4 and Cu; distance between the tips of M 4 and Cu slightly longer than the distance between the tips of Cu and A 1 ; the distance between the tips of A 1 and A 2 slightly shorter than the distance between the tips of M 4 and A 1 . A 1 slightly bent toward the wing margins; A 2 sinusoidal (12(b)). Haltere: (Figure 4(a, e)) 0.38 mm long, stem relatively short, only 1.5 times longer than the knob. Abdomen: hypopygium (Figures 4(a, b) and 5(c, d)) 0.35 mm long, gonocoxite massive, wider at the basal part.

Comparison.
A short and strongly arched vein R 4 , barely twice the length of the vein R 3 and a very short vein M 3 , much shorter than the length of the d-cell, readily distinguish R. (M.) cretacica subgen. and sp. n. from all others species classified the subgenus Myanmamastix subgen. n. In contrast to R. (M.) asiatica subgen. and sp. n., where the fork of Rb is positioned more or less at the midlength of the wing, R. (M.) cretacica subgen. and sp. n. has the fork of Rb positioned at less than half the length of the wing. R. (M.) cretacica subgen. and sp. n. crossvein sc-r positioned 3 times its own length from the tip of Sc, while in R. (M.) krzeminskae subgen. and sp. n. this distance is much longer, equal to 5 times the length of the vein sc-r. In further contrast to R. (M.) krzeminskae subgen. and sp. n., R. (M.) cretacica subgen. and sp. n. has R 1 short, ending opposite one third of the length of vein R 2+3+4 , while in R. (M.) krzeminskae subgen. and sp. n. this vein terminates just beyond half the length of vein R 2+3+4 . Moreover, in R. (M.) cretacica subgen. and sp. n. on each flagellomere several elongate, thick setae are visible, longer than the segments bearing them, two of them on each flagellomere arranged almost perpendicular to the surface of the individual flagellomeres, the others arranged at an acute angle, and the last flagellomere is of moderate length, more than half the length of the penultimate one, completely different to R. (M.) krzeminskae subgen. and sp. n., where the setae on the flagellomeres are shorter than the length of segments bearing them, densely spaced, almost perpendicular to the surface of the individual flagellomere, and the last flagellomere is short, only half the length of the penultimate one. In R. (M.) myanmae subgen. and sp. n. the last flagellomere is also of moderate length, only slightly shorter than the penultimate one. Moreover, in R. (M.) myanmae subgen. and sp. n. vein Rs is shorter than R 2+3+4 , while in R. (M.) cretacica subgen. and sp. n. vein Rs is longer than R 2+3+4 . R. (M.) jarzembowskii Krzemiński, 2004 comb. n. shows vein Sc terminating before half the length of Rs, R 3 one-quarter of the length of a vein R 4 , and R 4 longer than R 2+3+4 , while in R. (M.) cretacica subgen. and sp. n. Sc terminates beyond half the length of Rs, R 3 is about half the length of vein R 4 , and R 4 is short and only slightly longer than half the length of R 2+3+4 .
Remarks. The condition of the specimen is good, only the legs are not preserved, the mouthparts are poorly visible.

1160
I. Kania-Kłosok et al. terminating at distance equal to 5 times of its own length from the tip Sc; fork of Rb positioned before half the wing length from its base; R 3 onethird the length of R 4 ; R 4 equal in length to R 2+3+4 ; Rs longer than R 2+3+4 ; R 1 terminating just beyond half the length of R 2+3+4 ; M 3 equal to the length of the d-cell, length of d-cell 2.5 times its width, the distance between the tips of Sc and R 1 longer than the distance between the tips of R 1 and R 3 ; setae on the flagellomeres shorter than length of the flagellomere bearing them, densely spaced, almost perpendicular to the surface of the individual flagellomere; the last flagellomere half the length of the penultimate one; outer gonostylus narrow, strongly sclerotized, dark brown, blunt tipped, inner gonostylus relatively wide, pointed with a small appendage in the distal part.
Etymology. The name of the new species is dedicated to Professor Ewa Krzemińska (Polish Academy of Sciences, Kraków), the eminent specialist of fossil and recent Diptera.
Thorax: (Figure 6(a)): wing (Figures 6(a, e) and 7(a)) 2.71 mm long, 0.64 mm width, 3.5 times as long as wide, cross-vein (h) positioned at one-fifth the length of the wing from its base; d-cell 2.5 times as long as wide. M 1+2 significantly longer than M 3 ; M 4 short, almost straight, shorter than the d-cell; vein Mb slightly longer than 3 times the length of M 3 ; Cu from the point of connection with the cross-vein m-cu to the edge of the wing almost straight; cross-vein m-cu positioned distinctly before half the length of the d-cell from the fork Mb. Distance between the tips of M 1+2 and M 3 and the tips of M 3 and M 4 of comparable length; the distance between the tips of M 3 and M 4 shorter than the distance between the tips of M 4 and Cu; distance between the tips of M 4 and Cu and the tips of Cu and A 1 of comparable length; distance between the tips of A 1 and A 2 significantly shorter than the distance between the tips of M 4 and A 1 . A 1 and A 2 slightly bent toward the wing margin (12 (c)). Haltere: 0.48 mm long, stem very narrow, only slightly longer than the knob. Abdomen: hypopygium (Figures 6 (a,b) and 7(c,d)) 0.27 mm long, gonocoxite relatively elongate, massive.
Comparison. In contrast to the species described by  comb. n. Sc terminates before half the length of Rs. In the species R. (M.) asiatica subgen. and sp. n. the distance between the tips of Sc and R 1 is shorter than the distance between the tips of R 1 and R 3 , and the cross-vein sc-r is positioned three times its own length from the tip of Sc, while in R. (M.) krzeminskae subgen. and sp. n. the distance between the tips of Sc and R 1 is longer than the distance  between the tips of R 1 and R 3 , and the cross-vein scr terminates five times its own length from the tip of Sc. In R. (M.) krzeminskae subgen. and sp. n. the setae on the flagellomeres are shorter than the length of the segments bearing them, are densely spaced, almost perpendicular to the surface of individual flagellomeres, and the last flagellomere is only half the length of the penultimate one, while in both R. (M.) cretacica subgen. and sp. n. and R. (M.) myanmae subgen. and sp. n. the setae are very elongate and thick, longer than the flagellomeres bearing them, and the last flagellomeres of these species are elongate, only slightly shorter than the penultimate ones.
Remarks. The condition of the specimen is good, only the legs are partially not preserved.

Rhabdomastix (Myanmamastix) myanmae
subgen. and sp. n. (Figs 9,10,12e) Diagnosis. Vein Sc terminating opposite approximately three-quarters the length of Rs; cross-vein sc-r terminating at a distance equal to 3 times its own length from the tip Sc; fork of Rb positioned before half the wing length; Rs shorter than R 2+3+4 ; R 1 terminating just before half the length of R 2+3+4 ; M 3 equal in length to d-cell, slightly bent distally, d-cell 3 times as long as wide; distance between the tips of Sc and R 1 and the tips of R 1 and R 3 of comparable length; on each flagellomere several very long and thick setae, longer than the segment bearing them, almost perpendicular to the surface of individual flagellomeres; the last flagellomere, elongate but shorter than the penultimate one, more than half its length; outer gonostylus narrow, strongly sclerotized, dark colored, blunt ending, inner gonostylus relatively wide, pointed. Material examined. Holotype: No. MP/3637 (male) (ISEA PAS).
Etymology. The name of the new species is derived from the name of the country of Myanmar (Burma), where the deposits of Burmese amber are located.
Description. Body (Figure 9(a)) 2.54 mm long, dark brown, wing without color pattern, pterostigma absent, haltere slightly elongate (Figure 9(a)). Head: (Figure 9(a)): head width 0.31 mm; antenna (Figure 9(a, d) and 10(b)) shorter than half the length of the body, slightly longer than the length of the head and thorax combined, pedicel narrowed at the base, slightly widened distally, scape elongate, its length almost 1.5 times its width, distal part of pedicel and scape bearing several elongate setae; flagellomeres elongate, 2.5-3 times as long as wide, more slender towards the tip of flagellum, last flagellomere with two setae at the tip. Antenna 1.10 mm: scape 0.10 mm; pedicel 0.07 mm; flagellomeres: 1/0.09 mm; 2/0.07 mm; 3/ 0.06 mm; 4/0.07 mm; 5/0.07 mm; 6/0.07 mm; 7/ 0.07 mm; 8/0.07 mm; 9/0.07 mm; 10/0.07 mm; 11/ 0.06 mm; 12/0.06 mm; 13/0.06 mm; 14/0.04 mm). Palpus (Figures 9(c) and 10(e)) 0.33 mm long (1/ 0.06 mm; 2/0.1 mm; 3/0.06 mm; 4/0.11 mm); palpomeres elongate, palpomeres 1-3 comparable in length, the last segment slightly longer than the penultimate one. Palpomeres with several very elongate, thick setae, slightly longer or shorter than segment bearing them, the last palpomere with two distinct setae near the apex. Thorax: (Figure 9(a)): wing (Figure 9(a,e-g) and 10(a)) 2.90 mm long, 0.55 mm wide, 4.5 times as long as wide; cross-vein (h) positioned at one-fifth the length of the wing from its base; R 1 terminating distincly beyond half the length of the d-cell; R 3 short, equal to one-third of the distance between the tip of R 1 and the tip of R 3 ; d-cell 3 times as long as wide; vein Mb 4 times as long as M 3 ; Cu from the point of connection with the cross-vein m-cu to the edge of the wing almost straight; cross-vein m-cu positioned before half the length of the d-cell from the fork Mb. The distance between the tips of M 3 and M 4 shorter than the distance between the tips of M 4 and Cu; the distance between the tips of M 4 and Cu and the tips of Cu and A 1 of comparable length; distances between the tips of A 1 and A 2 and the tips of M 4 and A 1 of comparable length. A 1 and A 2 slightly bent toward the edge of the wing (Figure 11, Figure 12(e)).
Haltere: (Figure 9(a)) 0.54 mm long, sterm elongate, approximately 1.5 times the length of the knob. Abdomen: hypopygium (Figure 9(a, b) and 10(c,d)) 0.42 mm long, gonocoxite massive, widened basally. While in R. (M.) myanmae subgen. and sp. n. setae are very elongate and thick, longer than the length of segments bearing them, and the last flagellomere is elongate, only slightly shorter than the penultimate one.
Moreover, in R. (M.) myanmae subgen. and sp. n., R. (M.) asiatica subgen. and sp. n. and R. (M.) cretacica subgen. and sp. n. cross-vein sc-r is positioned at a moderate distance from the tip of vein Sc, nearly 3 times its own length, while in R. (M.) krzeminskae subgen. and sp. n. crossvein sc-r is positioned at a considerable distance from the tip of vein Sc, 5 times its own length, and in R. (M.) jarzembowskii Krzemiński, 2004 comb. n., this vein is positioned much closer to the tip of Sc, only twice own length.
Additionally, in R.
(M.) jarzembowskii Krzemiński, 2004 comb. n. and R. (M.) krzeminskae subgen. and sp. n. the distance between the tips of Sc and R 1 is greater than the distance between the tips of R 1 and R 3 , while in R. (M.) myanmae subgen. and sp. n. and R. (M.) cretacica subgen. and sp. n. these distances are of near equal length, and in R. (M.) asiatica subgen. and sp. n. this distance is shorter. Remarks. The condition of the specimen is good, only the legs are not preserved.

Discussion
The analysis of inclusions of Rhabdomastix flies preserved in Burmese amber has allowed for the distinction of a new subgenus, Myanmamastix subgen. n., and the description of four new species within this new subgenus. The new materials have allowed for the definition of clear boundaries regarding the differences in body morphology between taxonomic units at the species or subgeneric level, and an attempt to organize the systematic system within the genus, which has previously been insufficient to accommodate some fossil species. Described from inclusions in Burmese amber the species R. (M.) jarzembowskii Krzemiński, 2004 comb. n., was previously classified into the subgenus Paleogonomyia Meunier, 1899, according to the system used by Savchenko (1976Savchenko ( , 1992. Savchenko followed earlier work by Alexander (1914), whereby the genus Rhabdomastix was divided into three subgenera: Rhabdomastix s. str., Palaeogonomyia and Sacandaga Alexander, 1911, based on the ratio of antennal length to body length. The subgenera Paleogonomyia and Sacandaga were later synonymized with the subgenus Rhabdomastix s. str. by Starý (2003) and the subgenus Lurdia Starý, 2003 was distinguished, leaving the subgeneric placement of R. jarzembowskii uncertain. This classification system covering only two subgenera, Rhabdomastix s. str. and Lurdia, was also adopted in a study of Rhabdomastix flies from Australia (Theischinger et al. 2019). It is only now, in light of the material examined in this work, that a third subgenus, Myanmamastix subgen. n., can be recognised, and the species R. (M.) jarzembowskii Krzemiński, 2004 comb. n. can be placed as the type species.
The study of organisms preserved as inclusions in Mesozoic resins, such as Burmese amber, is important for many reasons. Particuarly, they allow for the   completion of data on the oldest representatives of many insect groups, including the genus Rhabdomastix.
The new discovery of some of the oldest Rhabdomastix species carries taxonomic implications for the subgenera and species within the genus. Based on the analysis of the oldest limoniid inclusions, including representatives of the genus Rhabdomastix preserved in Burmese amber from the Cenomanian period, we can draw conclusions about the possible directions of evolution of this group of insects. The early occurrence of the subgenus Myanmamastix, as much 50 million years before the oldest known species of Rhabdomastix s. str. and Lurdia found in Eocene Baltic amber (Podenas 2006), suggests the subgenus has a basal position within Rhabdomastix, with the other subgenera arising at a later period in the evolution of the genus.