Gymnocheta Robineau-Desvoidy, 1830

Pohjoismaeki, Jaakko & Bergstroem, Christer, 2021, Review of the Nordic Gymnocheta Robineau-Desvoidy (Diptera, Tachinidae) with report of two species new to Europe, ZooKeys 1053, pp. 145-184 : 145

Genus Gymnocheta Robineau-Desvoidy, 1830

Gymnocheta Robineau-Desvoidy, 1830: 371 (also subsequently spelled Gymnochaeta , unjustified emendation). Type species: Tachina viridis Fallén, 1810 (as Tachina viridis Meigen), by monotypy (see also O’Hara et al. 2009: 142).

Redescription

(European species). Male (Figs 1 , 2 , 4 , 5 ).

Colouration (Fig. 1 ): The colour described here is based on freshly collected specimens. In older specimens the black or dark brown colour is often changed to brown, red brown, or beige brown. Head to a varying extent covered with microtomentum, which affects how clearly the metallic green ground colour shines through, the latter characteristic of the genal dilation, fronto-orbital plate, and occiput but sometimes hard to recognise, as the interpretation depends on the direction of the incidence of light: the specimens should be viewed from different angles. Facial plate light brown to black with or without a metallic green spot at lower margin. Parafacial black but in older specimens narrowly reddish-brown along the ptilinal fissure. Genal groove reddish brown. Frontal vitta dark brown to black. Occiput and postgena with white to greyish white hairs. Antenna matt dark brown or black due to the greyish microtomentum, pedicel sometimes lighter apically, arista black. Palpus black or dark brown in older bleached specimens. Clypeus (sometimes with a metallic green tint) and prementum black, labella from dark brown to beige brown.

Thorax and abdomen with different shades of metallic green but often also partly blue, purple or red. In our European species G. magna and G. viridis show a bright metallic ground colour, G. lucida an olive green and G. zhelochovtsevi a dark green colour, the shine depending on the direction of the light. Scutum, when viewed from the side and slightly from behind, with four black longitudinal stripes of microtomentum, changing from grey to purple depending on the direction of the incident light; presutural area with the medial stripes narrow and widely separated, the lateral stripes are wedge-shaped and reach the level of posthumeral seta. In aged specimens, caught late in the season, the microtomentum can be worn off and give the specimens a polished appearance on scutum. Proepisternum black and densely covered with microtomentum or with the metallic green colour slightly subshiny. Legs normally extensively black or dark brown, which can change to lighter brown in aged specimens; fore coxa in anterodorsal region and sometimes also femora with remnants of metallic green or blue tint, covered with light grey microtomentum. Wing veins black and/or brownish black, wing membrane with a brownish tinge. Wing membrane around crossvein r-m sometimes narrowly infuscate. Tegula and basicosta dark brown to black. Halteres brown but stalk and knob partly blackish. Calypters greyish white, edge for the most part beige but inner edge distinctly infuscate, with a white fringe.

Head (Figs 1 , 2 ): Head in profile angularly protruding at level of antennal insertion, width of parafacial at this level ca. 0.6-0.8 × the horizontal eye diameter. Frons in dorsal view at narrowest point, 0.3-0.7 × the width of an eye. Frontal vitta gradually tapering toward ocellar tubercle in G. magna and G. viridis , tapering toward middle (sometimes narrowest here) and then parallel-sided in G. lucida or wide and parallel-sided in G. zhelochovtsevi . Fronto-orbital plate with a row of 10-14 medioclinate strong frontal setae and some additional setulae, uppermost setula tiny and sometimes slightly reclinate; 3-5 setae extend on upper part of parafacial, reaching the middle of the pedicel with the row curving laterally, and here sometimes accompanied by some setulae; frontal plate outside the frontal row of setae with sparsely short and tiny setulae. Height of face 0.8-0.9 × the length of frons. Gena in profile at narrowest point, 0.3-0.4 × as high as vertical eye diameter. Lower anterior area of genal groove in front of genal dilation bare or rarely with some minute setulae. Vibrissa normally well developed and slightly shorter than height of face, inserted at the level of the lower facial margin. In some specimens of G. lucida there is no distinct vibrissa but two or three equally strong supravibrissal setae. Facial plate slightly convex between the antennae and except for G. magna hardly visible in profile; lower facial margin protruding particularly in the middle but except for G. magna only vaguely visible in profile. Chaetotaxy variable: Facial ridge with 1-4 strong and 0-3 additional thinner supravibrissal setae on lowest quarter (length of them at least equal to narrowest width of parafacial) and 2-5 thin and short setulae. Ridge below vibrissa with 3-7 strong subvibrissal setae continuous with the genal setae, longest 0.5-0.6 × the length of vibrissa. Ocellar tubercle with the ocelli forming a slightly pointed or in some specimens an almost equilateral triangle, two strong lateroproclinate ocellar setae inserted between anterior ocellus and posterior ocelli, accompanied by a tuft of thin setulae (sometimes one or two pairs of additional shorter setae may be present). Inner vertical setae crossed, at least subequal in length to the ocellar setae. Outer vertical setae less developed and subequal with the adjacent postocular setae, except for G. zhelochovtsevi where they are almost subequal with the inner vertical. Postocular setae long and thin, apically pronouncedly bending forward over the eyes except for G. zhelochovtsevi . Occiput evenly convex, normally with a pair of postocellar setae, rarely missing; in upper part with 2(3) somewhat irregular rows of short black setae behind the postocular row of setae. Palpus slightly clavate at tip, subequal to the length of the antennae, with black setulae. Antenna: Scape erect. Pedicel subtriangular and with one elongate seta. First flagellomere relatively short, in profile usually subrectangular or slightly widened towards apex, but sometimes thick and extended or flat and truncate; maximal width in profile 0.8-1.3 × as wide as parafacial at narrowest point and 1.4-1.8 × as long as pedicel. Arista at first sight bare but at higher magnification pubescence, widened in its proximal 1/4 to almost 2/3, and gradually tapering to apex. Eyes densely covered with long hairs, white with a yellowish tint, hairs slightly shorter in G. zhelochovtsevi .

Thorax (Fig. 1 ): Proepisternum bare. Prosternum bare or with tiny setulae present. Postpronotal lobe with 4(5) strong setae, the three basal arranged in an almost right angle, a slightly weaker anterior seta anterior or anterolateral to the inner basal seta. Chaetotaxy of scutum variable: 3(2)+3 acrostichal, 3(2)+4 dorsocentral and 1+3 intra-alar setae. Posthumeral seta inside and in front of the presutural intra-alar seta, both close to edge of postpronotum; 1+4 supra-alar setae, presutural supra-alar inserted in the middle between transverse suture and postpronotum; first postsutural supra-alar seta at least subequal with the notopleural setae, shorter than first postsutural intra-alar seta; two notopleural setae, two strong postalar setae. Ground vestiture on scutum (consisting of thin setulae) dense and erect, longest setulae subequal to the shortest setae. Katepisternum with three setae (2+1), rarely four, present. Anepimeron with 2(3) strong setae subequal to the strongest katepisternal setae. Katepimeron bare or with up to five tiny setulae in anterior 1/2. Scutellum also with a rather variable chaetotaxy; normally with four or five pairs of strong marginal setae, sometimes mixed with some shorter and weaker marginal setae/setulae, almost horizontal with the plane of scutellum, apical setae missing, one subapical pair, sometimes very close to apex, parallel or slightly diverging, 2(3) lateral pairs and one basal pair; apart from this often with a tiny pre-basal seta at least present on one side; 2-4 strong suberect preapical discal setae, together with some setulae forming a row in front of the marginal setae, the strongest pair in the middle sometimes subequal to the lateral setae, mixed with numerous tiny setulae.

Legs (Fig. 1 ): Claws and pulvilli on fore legs equal to or slightly longer than tarsal segment 5, the latter ca. 2 × as long as tarsal segment 4. Chaetotaxy variable and it is sometimes difficult to distinguish between seta and setula (seta here accepted if its length is subequal with the width of tibia at middle): Fore tibia with 4-7 anterodorsal setae, 2-5 posterodorsal setae in an irregular row and two or three posterior setae; preapical anterodorsal seta well developed, subequal with the preapical dorsal and preapical posterior setae.

Wing (Fig. 1 ): Wing veins apart from costal vein and vein R4+5 bare. Usually two costal spines (rarely one or three), the strongest lower spine 1.5-3 × as long as the surrounding costal setulae. Fourth and fifth costal section 1.8-2.4 (n = 6) × as long as sixth costal sector. Vein R4+5 at node with ventral and dorsal setulae. Bend of vein M acute, length of appendix 0.3-0.5 × the shortest distance from bend to wing edge. Cell r4+5 open at wing edge or rarely almost closed (according to Zimin normally short-stalked or closed in G. mesnili ). Apical section of vein M concave reaching wing margin in front of wingtip.

Abdomen (Figs 1 , 4 ): Domed, ground-vestiture erect or at least semierect on tergites 3 and 4, also ventrally. Syntergite 1+2 with middorsal depression reaching to the end of the segment. Chaetotaxy variable: Tergite 2 without a pair of median marginal setae, with 2(1)-3 lateral setae on each side. Tergite 3 with 2-4 pairs of unequally often irregularly set strong median discal setae, with a pair of median marginal setae and (1)2-3 lateral setae on each side. Tergite 4 with 2-3(4) pairs of unequally and likewise irregularly set strong median discal setae, in dorsal view with a full row of 10-14 marginal setae. Tergites 3 and 4 frequently with one median discal seta missing or set more laterally. Tergite 5 with two or three irregular rows of unequally strong discal setae and a row of medium strong marginal setae.

Male terminalia (Figs 1E , 4 , 5 ): Tergite 6 completely reduced. Sternite 6 strongly asymmetrical, with the broader left arm articulating with segment 7+8. Segment 7+8 large and convex, slightly larger than epandrium, separated from tergite 5 by a wide membrane. Epandrium in profile with a subquadrate appearance, with numerous setulae (the longest placed dorsally). Sternite 5 (Fig. 1E , 4E ) in ventral view with distinct posterolateral lobes and a broad medial cleft that anteriorly narrows to a small U-shaped indentation; length of cleft (measured from the tip of the posterolateral lobes, including the U-shaped cleft) 1.8-2.5 × its width (measured at the anteriormost incision). Lobes with widespread setulae, rounded or triangular at apex; along the inner edge towards the cleft distinctly concave, from inner dorsomedial surface showing a long narrow or backwards expanding process; U-shaped median incision in posterior medioventral region with a pair of densely spinulose wart-like lobes; basal plate bare (without setulae) ca. 0.40-0.45 × the length of the sternite itself, and 2.2-2.7 × as wide as long, anterior margin distinctly concave; transversal membranous stripe wide and narrow, almost touching the U-shaped indentation. Tergite 6 completely reduced. Sternite 6 strongly asymmetrical, with the broader left arm articulating with syntergosternite 7+8, the latter large and convex slightly larger than epandrium. Epandrium arched trough-shaped, in lateral view with a subtriangular appearance, with numerous setulae (the longest placed dorsally). Syncercus in lateral view (Fig. 5A ) smoothly curved at apex; in caudal view (Fig. 5B ) triangular, narrowed to a blunt or rounded apex, with long basal lobes; 1.7-2.1 × as long as maximal width (length measured from base of membranous median suture to apex), slightly incurved at middle and close to apex; numerous black setae/setulae laterally give G. lucida a characteristic appearance. Surstylus in lateral view (Fig. 5A ) slightly curved ( G. lucida ) or arch-like bent ( G. magna , G. viridis , and G. zhelochovtsevi ), at least proximally along posterior margin with some tiny setulae, otherwise with numerous spread sensory pores; separated from epandrium by a very narrow membranous succession. Pregonite lance-like in caudal view; lobe-like in lateral view (Fig. 5C ) with a hook-like projection apically and some sensorial hairs along its posterior margin. Postgonite quite narrow, broadly membranous apically and therefore with a hook-like appearance.

Aedeagus: Basiphallus with a basal projection and a distinct digitiform epiphallus. Distiphallus in lateral view (Fig. 5C ) with the dorsal sclerite as two narrow separate arms extending from apex of basiphallus, close to the posterior edge of the distiphallus all the way to the anterior tip; i.e., not fused dorsomedial and consequently without a median projection; basis of ventral sclerite prominent, lateroventral region with a row of numerous spines; medioventral ridge in lateral view narrow and weakly sclerotised, in ventral view resembling X, with base directed towards the ventral sclerite and apex to the end of the dorsal sclerite.

Female (Figs 3 , 6 ): Differs from male as follows:

Colouration: Fronto-orbital plate with a more widespread metallic green colour; the metallic ground colour continues down on the upper part of parafacial; in large parts covered by greyish microtomentum in G. zhelochovtsevi .

Head (Fig. 3 ): Frons wider, at its narrowest point 0.8-1.0 × as wide as an eye in dorsal view. Frontal vitta either progressively tapering towards ocellar tubercle and at this level mostly narrower than width of fronto-orbital plate ( G. magna and G. viridis ), narrowest close to middle ( G. lucida ) or with almost parallel edges in anterior 1/2 or more and the narrowing ( G. zhelochovtsevi ). Fronto-orbital plate normally with fewer, 6-11 moderately strong medioclinate frontal setae. Ocellar tubercle with a pair of strong proclinate ocellar setae as in male but with shorter and more sparsely set setulae. Fronto-orbital plate with 2(3) strong proclinate orbital setae, the anterior one stronger, one upper, slightly lateroclinate, orbital seta at level of anterior ocelli. Inner vertical setae strong and crossed. Outer vertical setae strong and at least 0.6 × the length of the inner vertical setae. Postocular setae short and insignificantly bending forward. First flagellomere 1.3-1.8 × as long as pedicel.

Legs: Legs black. Claws and pulvilli on fore legs shorter ca. 0.7-0.8 × as long as tarsal segment 5, the latter 1.5-1.7 × longer than tarsal segment 4.

Abdomen: Tergite 5 trapezoid, along anterior margin ca. 2 × as wide as long.

Female terminalia (Fig. 6 ): Ovipositor short, deeply drawn into the lumen of the sternite 5. Uterus present, 3 equally sized spermathecae. Species are ovolarviparous; dissected females of G. lucida and G. viridis sometimes contained> 200 first instar larvae in different state of development. The first instar larvae are known to search actively for the host.

Tergite 6 reduced into narrow and somewhat elongated hemitergites, widely separated, in the membranous region accompanied by 0-3 tiny setulae posterior to the seventh spiracle and sometimes also one or two setulae in a more apical position, sixth spiracle in lateral margin of tergite 6. Sternite 6 shorter 0.5-0.7 × as long as, but slightly wider (1.1-1.2 ×) than sternite 5, 1.5-2 × as wide as long with 4-8 strong setae along the posterior margin accompanied by numerous setulae in posterior 1/5. Tergite 7 strongly reduced, divided into two widely separated small hemitergites located close to the lingulae, with 0-3 setulae in the adjacent membranous area, left and right seventh spiracles displaced in membrane between tergites 6 and 7, sometimes close to sixth spiracles. Sternite 7 ca. as long but slightly narrower than sternite 6, with numerous tiny setulae most of them restricted to the posterior margin, anterior 4/5 bare. Tergite 8 missing (fully reduced). Sternite 8 strongly reduced, hidden between hypoproct and sternite 7; covered by sensory pores (sensilla trichodea). Hypoproct in ventral view semi-circular to almost triangular, with a middorsal depression, densely covered with setulae, in lateral view (Fig. 6 ) with dorsal side straight or slightly bent towards the cerci, lingulae present. Epiproct strongly reduced (no sclerotising visible) but in caudal view indicated by the presence of 3-10 setulae inserted above the cerci. Cerci with numerous setulae of varying length, in lateral view the strongest setulae at least subequal in length with sternite 7.

The European species of Gymnocheta are treated in alphabetical order. All Finnish records are stored in the Finnish species database, www.laji.fi.

Figures

Figure 1. Gymnocheta spp. habitus A habitus Gymnocheta viridis ♂ B-E Tachina viridis Fallen, holotype specimen B dorsal view C head in lateral view. Note the protruding lower facial margin (arrowhead) D detail of the right wing edge. Note the reduced costal spine (arrowhead) E sternite 5 of the holotype F labels associated with the holotype. All photographs by Goeran Liljeberg.

Figure 2. Gymnocheta spp. male heads in lateral (upper panels) and dorsal (lower panels) view A Gymnocheta lucida Zimin B G. magna Zimin C G. viridis (Fallen) D G. zhelochovtsevi Zimin. Note the wide frons and strong outer vertical bristles. Scale bars: 1 mm. All photographs by Goeran Liljeberg.

Figure 3. Gymnocheta spp. female heads in lateral (upper panels) and dorsal (lower panels) view A Gymnocheta lucida Zimin B G. magna Zimin C G. viridis (Fallen) D G. zhelochovtsevi Zimin. Scale bars: 1 mm. All photographs by Goeran Liljeberg.

Figure 4. Fifth sternites of Gymnocheta spp. males A Gymnocheta lucida male abdomen. Note the narrow profile of the posterior edge of the fifth sternite (arrow) B the edge of the fifth sternite in G. viridis is bulky and protruding beyond the tip of the abdomen, clearly recognisable from the side C the posterolateral lobe of G. zhelochovtsevi is somewhat subrectangular as in G. magna, but more rounded in profile. Note the apical hook of the syncercus D in contrast to G. zhelochovtsevi, posterior edge of fifth sternite in G. porphyrophora with well-developed median lobes (arrow) E dissected fifth sternites of Nordic Gymnocheta spp. in comparison: i) G. lucida, ii) G. magna, iii) G. viridis and iv) G. zhelochovtsevi. A distinct dorsomedial process present in G. magna and G. viridis Abbreviations: bpl - basal plate; dmp - dorsomedial process; mcl - median cleft; pll - posterolateral lobe; tms - transversal median stripe; wll - wart-like lobe (or median lobe). Photograph D by V. Neimorovets, rest by J. Pohjoismaeki.

Figure 5. Comparison of Gymnocheta spp. male terminalia A lateral view of the terminalia: i G. lucida ii G. magna iii G. viridis and iv G. zhelochovtsevi - note the apical hook of the syncercus B dorsal view of the terminalia: i G. lucida ii G. magna iii G. viridis and iv G. zhelochovtsevi, same specimen as in Fig. 4 C C aedeagus and gonites of i G. lucida ii G. magna iii G. viridis and iv G. zhelochovtsevi. Abbreviations: bph - basiphallus; dph - distiphallus; prg - pregonite; pog - postgonite. All photographs by J. Pohjoismaeki.

Figure 6. Comparison of Gymnocheta spp. female sternites and terminalia A ventral view of the sternites and terminalia of i G. lucida ii G. magna iii G. viridis and iv G. zhelochovtsevi B detail of the female terminalia of i G. lucida ii G. magna iii G. viridis and iv G. zhelochovtsevi. Note the long cerci in G. zhelochovtsevi. All photographs by J. Pohjoismaeki.