Burmella paucivenosa, Godunko & Martynov & Staniczek, 2021

Godunko, Roman J., Martynov, Alexander V. & Staniczek, Arnold H., 2021, First fossil record of the mayfly family Vietnamellidae (Insecta, Ephemeroptera) from Burmese Amber confirms its Oriental origin and gives new insights into its evolution, ZooKeys 1036, pp. 99-120 : 99

Burmella paucivenosa sp. nov. Figures 1 , 2 , 3 , 4 , 5 , 6 , Table 1

Material examined.

Holotype. Male imago in Mid-Cretaceous Burmese amber, SMNS collection, inventory number BU-179. Well preserved specimen visible in lateral aspect. Due to fragility, the piece of amber is additionally embedded in translucent resin to seal the specimen from oxygen and prevent mechanical damage. Body and both pairs of wings completely preserved (Figs 1 , 4 - 5 ); most part of right and left foretibiae and both foretarsi missing; most part of caudal filaments missing. For measurements see Table 1 .

Derivation of name.

The species epithet combines Latin “paucus”, few, and “venosus”, veined, referring to the reduced wing venation of the hind wing.

Diagnosis.

Male imago: body length 5.75 mm; forewings with 3-4 marginal intercalaries connected with longitudinal veins, two free marginal intercalaries, no cross veins in anal field; hind wings strongly rounded, small, as long as 0.14 × of forewing length, three cross veins between C-Sc, three cross veins between Sc-RA, one cross vein between RA-RSa, one cross vein between RA-RSp, RS not forked; penis lobes relatively simple, obliquely truncate apically, nearly tube-like; strong apical tooth on outer margin.

Description.

Colouration relatively pale, yellowish-brown to dark brown; eyes and mesonotum darkest, dark brown to blackish; abdominal segments partly translucent; traces of dark brown maculation along of lateral margins of terga (Figs 1 , 2 , 4 , 6 ).

Head. Compound eyes well-developed, large, widely rounded, medially contiguous; upper portion of compound eyes translucent and slightly yellowish apically, brownish-black basally; border between dorsal and ventral portions of compound eyes well distinguishable; lower portion of compound eyes brownish-black (Figs 1 , 2A-C ). Facets of compound eyes hexagonal. Ocelli poorly preserved, relatively small, without conspicuous colouration. Facial keel relatively small. Antennae slightly longer than head.

Thorax. General colouration yellowish-brown to brownish-black. Prothorax narrow, light brown. Mesonotal suture transverse, distinctly expressed; medioparapsidal suture relatively straight; lateroparapsidal suture distinctly curved laterally; scutellum not modified; no preserved natural colouration of pigmented area of mesonotum. Mesosternum with brownish basisternum and slightly paler furcasternum; basisternum elongated; furcasternal protuberances distinctly separated. Lateral sides of mesothorax light brown to brown, with blackish maculation. Metathorax brown to dark brown, blackish maculation dorsally (Fig. 2C, D ).

Wings. Forewings hyaline, translucent, relatively narrow; venation well recognizable, light brown to dark brown; veins darker proximally and slightly paler distally; relatively small number of cross veins, especially in medial, cubital, and anal fields; no jagged edge along of ventral margin of forewings. Pterostigma with 3-4 simple veins. Vein sections between C and RA slightly frosted-brown distally; veins C and Sc brown to dark brown, visible all over their length; RS forked near base, after 0.14 of its length; iRS well-developed, connected with RSp by 5 cross veins, not approximated to RSa1; MA fork slightly asymmetrical, forked after 0.60-0.62 of its length; MA1 and MA2 connected with iMA by 2-3 cross veins; MP asymmetrical, forked after 0.25 of its length, MP1 and MP2 basally connected by a single cross vein; iMP relatively short, connected with MP1 and MP2 by single cross veins from each side; CuP smoothly curved toward wing base, basally connected with CuA by cross vein cua-cup, CuP connected with A1 by cross vein cup-a1, cua-cup located distally from cup-a1; in cubital field two secondary bifurcate veins iCu1+2 and iCu3+4 arising from CuA (i.e. four veins iCu1-iCu4 each reaching basitornal margin of forewing); basal end of CuP closely approximated to CuA base; A1 closely approximated to A2; no cross veins in anal field. Several intercalaries (iRSa, iRSa2, iMA, iMP) connected to longitudinal veins by crossveins; two small, basally free marginal intercalaries in R and MP fields; no free intercalary veins in cubital and anal fields (Figs 1 , 4 ).

Hind wings hyaline, translucent, strongly rounded, small, as long as 0.14 of forewing length; venation light brown to brown; venation significantly simplified, with strong reduction of number of longitudinal and cross veins; ventral margin of hind wings without jagged edge. Few cross veins between C-Sc (3 veins), Sc-RA (3 veins), RA-RSa (one vein), and RA-RSp (one vein); no triads of RS, MA and MP; MA connected with R; MP approaching CuA; no secondary branches of cubital veins; no free marginal intercalaries; costal process rounded apically, markedly protruding above anterior wing margin, situated at nearly middle of hind wing length (Fig. 5A, B ).

Legs well preserved, except of tarsi missing in both forelegs; margins of preserved leg segments without visible strong spines or setae. For measurements of leg segments see Table 1 .

Right foreleg: length ratio of femur/tibia = 1/2.02; left foreleg: length ratio of femur/tibia = 1/1.96. Right middle leg completely preserved: length ratio of femur/tibia/tarsus = 1/0.71/0.21; length ratio of tarsomeres: 1/1.25/1.25/1.38/1.75 (5> 4> 3 = 2> 1). Left middle leg much shorter than right one, probably re-grown after previous injury, therefore with changed proportions of tarsomeres. Right and left hind legs completely preserved; right hind leg: length ratio of femur/tibia/tarsus = 1/0.91/0.51; length ratio of tarsomeres: 1/1.29/1.43/1.71/2.00 (5> 4> 3> 2> 1). Left hind leg: length ratio of femur/tibia/tarsus = 1/0.66/0.41; length ratio of tarsomeres: 1/1.40/1.40/2.00/2.20 (5> 4> 3 = 2> 1). Patellotibial suture present on middle and hind legs, absent on forelegs. First tarsomere of middle and hind legs fused with tibia. Claws ephemeropteroid on preserved middle and hind legs, with outer claw hooked and inner claw blunt (Figs 1A, B , 3A-D ).

Abdominal segments completely preserved, partly translucent, relatively pale, yellow to brown, with intensively brown maculation on terga laterally and sterna posteriorly. Vestigial gill sockets, not finger-like, recognizable on segments II-VI, poorly visible on segment VII due to influx of resin and cracks. Abdominal segments without large and prominent posterolateral projections; abdominal segments VIII-IX not elongated compared to previous segments. Abdominal sterna slightly paler than terga. Cerci brown, partly preserved; no trace of paracercus (Figs 1 , 4 , 6A, B ).

Genitalia well preserved, light brown to brown, darker maculation on forceps. Styliger plate angulate, mediocaudally deeply incised; median projection large, widely rounded apically, markedly protruding above anterior margin of styliger. Basal segment I of forceps short, with rounded inner margin, slightly wider than long; segment II of forceps strongly elongated, slender distal segments III and IV much shorter, approximately of equal length; segment IV expanding apically; length ratio of forceps segments II-IV: 1.00/0.20/0.18 (Fig. 6C, D ). Penis lobes widely separated by V-shaped cleft, relatively simple, obliquely truncate apically, nearly tube-like; structure of left penis lobe poorly visible; inner side of right penis lobe probably partly damaged or lost (i.e. looks semicircular from ventral side); strong apical tooth on outer margin; titillators not distinguishable (Fig. 6C, D ).

Affinities.

Burmella paucivenosa sp. nov. exhibits a combination of morphological characters allowing its attribution to Vietnamellidae , namely the presence of strongly rounded hind wings in combination with the presence of short intercalaries distally connected with longitudinal veins. Compared to other representatives of Vietnamellidae , Burmella paucivenosa sp. nov. is characterized by the presence of only two short free marginal intercalaries, while the number of these intercalary veins in all extant species and also in Burmella clypeata sp. nov. is significantly higher.

Within Vietnamellidae , Burmella paucivenosa sp. nov. can be attributed to the newly described genus Burmella gen. nov., as defined in Diagnosis (see above), mainly based on the following diagnostic characters: shape and structure of venation of hind wings, with reduced cross venation and distinct costal process situated centrally; lack of furcation of RS, MA, MP, CuA, and CuP in hind wings (Fig. 5 ); shape of male genitalia with deeply diverted penis lobes (Fig. 6C, D ).

In the latter character, the male imago of Burmella paucivenosa sp. nov. differs from all other known male adults of Vietnamellidae . The genus Vietnamella is characterized by the presence of a club-shaped, elongated penis that is medially fused along its longitudinal axis, with only a small, V- or U-shaped incision apically ( Tshernova 1972: 613, fig. 7; Hu et al. 2017: 385, figs 4C, 5C; Auychinda et al. 2020a: 8, fig. 4G; 2020b: 28, figs 7J, K, 8J, K). In contrast to Vietnamella , the tubular penis of Burmella paucivenosa sp. nov. is medially deeply split, with lobes strongly stretched laterally (Fig. 6C, D ). Obvious differences are also visible in shape and proportions of forceps segments. In Burmella paucivenosa sp. nov. the 4-segmented forceps is strongly elongated and slender, with segment II being the longest, with the same width distally as segment III basally, while distal segment IV is markedly elongated and nearly subequal to segment III (Fig. 6C, D ). On the contrary, in Vietnamella the forceps is only 3-segmented with segments significantly different in shape and proportions: Segment I is the longest one, while shortest segment III is small and rounded, which is typical for many species of Ephemerelloidea (see Kluge 2004).

Figures

Figure 1. Burmella paucivenosa sp. nov., male imago, holotype, general lateral view A left side of body B right side of body. Scale bars: 2 mm.

Figure 2. Burmella paucivenosa sp. nov., male imago, holotype A head, right lateral view B compound eye, left lateral view C head and thorax, left lateral view D mesothorax, left lateral view. Blue line - border between portions of compound eye, da - damaged area, lp - lower portion, LPs and red line - lateroparapsidal suture, MNs and green line - mesonotal suture, MPs and yellow line - medioparapsidal suture, up - upper portion. Scale bars: 0.5 mm (A, C); 0.2 mm (B, D).

Figure 3. Burmella paucivenosa sp. nov., male imago, holotype A, B tarsus of right middle leg C right middle leg D left hind leg. I-V - tarsal segments, pts - patellotibial suture. Scale bars: 0.1 mm (A, B); 0.5 mm (C); 0.2 mm (D).

Figure 4. Burmella paucivenosa sp. nov., male imago, holotype A right and left wings position in amber B right forewing venation and size ratio of fore and hind wings. Scale bars: 1 mm.

Figure 5. Burmella paucivenosa sp. nov., male imago, holotype A right hind wing in amber B right hind wing venation. Scale bars: 0.2 mm.

Figure 6. Burmella paucivenosa sp. nov., male imago, holotype A abdomen, left lateral view B abdominal segments III-VII, lateral view C, D genitalia, ventrolateral view. III-VII - numbers of segments, white arrows mark remnants of gill sockets, pink area - penis lobes, light blue area - gonobasis (styliger plate), brown area styliger segment I, yellow area - styliger segment II, light green area - styliger segment III, purple area - styliger segment IV. Scale bars: 1 mm (A); 0.5 mm (B); 0.2 mm (C, D).