Eoexechia gallica, Camier & Nel, 2020

Camier, Marie & Nel, Andre, 2020, The oldest fungus gnat of the tribe Exechiini in the lowermost Eocene Oise amber (Diptera: Mycetophilidae), Zootaxa 4722 (1), pp. 91-98 : 92-97

Eoexechia gallica sp. n.

( Figs 1–3 )

urn:lsid:zoobank.org:act:A785CF8A-E9CE-48FA-9CC3-E251D9941A35

Material. Holotype MNHN.F. A71245 (PA3071, male, together with a second specimen of the same species in main part hidden by a large but incomplete adult Trichoptera), stored in the Palaeontological Institute, MNHN.

Remark. During the study, a cracking happened around the head and thorax, rendering the observation more complicated. Thus some thoracic structures, first clearly visible, became less discernible.

Diagnosis. As for the genus.

Description. Body 2.07 mm long; head not elongated, 0.42 mm wide.

Head. Large compound eyes covering almost all of length of head in lateral view, with a curvature near antennal base, eye 0.22 mm high; lateral ocelli near eye margin, mid ocellus present; frons with several dark setae laterally in front of ocelli, and several strong setae above upper eye margin, occipital furrow present, frontal furrow very short, confined to back of head; face short, with a few setae; frontal tubercle bilobate and broad; clypeus ovate, covered with rather long setae; antenna unmodified, rather long but slightly shorter than head and thorax together; flagellum cylindrical with 14 short flagellomeres (F2 about as wide as long, then gradually longer towards tip where F14 is two times as long as wide); antennal ratio (= F2/F1) larger than 2/3; palp with four visible segments, sensory pit not situated basally and shorter than 0.5 length of palpomere 1; palpomere 2 with setae; palpomere 3 enlarged; premental apodeme with one process;

Thorax. Compact, 0.73 mm long, highly arched as in most Exechiini , scutum with small decumbent bristles intermingled among small setae, not arranged in distinct lines, dorsal surface of scutellum densely covered with dark setae, subapical bristles slightly longer than scutellum; basisternum 1 with setae; proepimeron with setae; anepimeron without setae, with ventral part ending before ventral side; anepisternum bare, taller than wide; mediotergite with setae; ventral edge of preepisternum 2 not round, covering base of coxa; posterior basalare without setae; metepisternum with some small setae on anterodorsal part; metepimeron without setae; metanotum with setae behind halter; mediotergite with setae; prescutum and notum well produced beyond upper rim of anepisternum; notum with large setae in rows; distal median plate without ventral and dorsal setae; basicosta with large bristles;

Wing. 1.93 mm long, hyaline; membrane covered with microtrichia in regular longitudinal rows, but without setae; humeral vein with setae; vein sc quite long, with apex distinct, ending in R1, costa distinctly produced beyond R4+5; vein R1 short, ending in costa beyond half way from base of dmp to tip of wing; vein ta short, not interrupted by a longitudinal fold, with setae; stem of M-fork distinct, short, Cu+M4-fork branching before branching of Mfork, M1 with row of trichia; M2 distinct and reaching wing margin, with row of trichia; CuA+M4 stem with row of small trichia; M4 present, with row of trichia; CuA present, with row of trichia; CuAP strong, apically weaker, not reaching level of point of furcation of M; vein A1 weak, short; A2 phantom-like;

Legs. Covered with dark trichia and setae, all coxae with several black setae apically, femora clothed with numerous dark trichia, fore tibia shorter than femur; mid femur with apical bristles; mid tibia with large setae and trichia in distinct rows; hind coxa with several bristles at basis; hind femur arched (width/length> 0.2); hind tibia with setae less than twice width of tibia, and with apical brush produced across tibia; trichia on hind leg in distinct rows; dorsal surface of hind tibia with a triangular depression; hind spurs longer than half length of basitarsus; one spur on fore tibia, two spurs on both mid and hind tibia; tarsal claws small; empodium present, rather large and as long as claws;

Abdomen. Abdominal segments with dark brown markings along posterior margins, sternites narrow, longitudinal medial concave fold(s) not discernible.

Male terminalia. Large, 0.26 mm long, but only visible laterally and dorsally in part; terminal bristles on cerci present; basal bristles on T9 absent; T9 with setae on its mid surface, only ventral branch of gonostylus clearly visible, rounded and very large, with a row of long bristles around its margin; medial and internal branches partly visible.

Etymology. Named after the Latin name for France. Discussion. We performed a phylogenetic analysis based on the morphological phylogenetic matrix of Rindal & Søli (2006), adding our fossil. The outgroups are the same as in this paper, viz. four genera of Mycetophilidae outside the subfamily Mycetophilinae , Leia Meigen, 1818 , Docosia Winnertz, 1863 , Boletina Staeger, 1840 and Dziedzickia Johannsen, 1909 . We used the software Nexus to complete the matrix of Rindal & Søli (2006) with Eoexechia gen. n., and win-Paup4b10 with the HSearch option with 200 replicates. We obtained 16 equally most parsimonious cladograms (Length = 260 steps, CI = 0.2962, RC = 0.1897) (for strict consensus cladogram see Fig. 4 ).

Eoexechia falls in the tribe Exechiini as sister group of all extant genera. The phylogeny of Rindal & Søli (2006) is not totally respected, especially for the position of the genus Cordyla Meigen, 1803 , previously sister group of other extant Exechiini , while in our analysis it is falling with Tarnania Tuomikoski, 1966 and Pseudexechia Tuomikoski, 1966 .

The frontal furrow very short, confined to the back of the head (character 6, state 2), and the hind tibial apical brush produced across tibia (character 59, state 1) support the clade ( Eoexechia + extant Exechiini ), even if first one is reversed to state ‘0’ in Cordyla .

All the extant Exechiini have a narrow frontal tubercle (character 7, state 1), while it is bilobate and broad in Eoexechia (state ‘0’). Also, they all have a reduced empodium (character 49, state 1), while it is well-developed in Eoexechia (character 49, state 0). These two characters support the position of Eoexechia as sister group of the extant Exechiini . Eoexechia has the dorsal surface of the hind tibia with a triangular depression, a character only present in Exechia and Exechiopsis , after Rindal & Søli (2006). The presence of three ocelli would be a symplesiomorphy present in Allodia , Neallodia , Brachypeza , Boraceomyia , and Eoexechia ( Burdíková et al., 2019).

The analysis of Rindal & Søli (2006) did not include the Exechiini genera Neallodia Edwards, 1932 , Brachyradia Ševčík & Kjaerandsen, 2012 , Synplasta Skuse, 1890 , Myrosia Tuomikoski, 1966 , and Boraceomyia Lane, 1948 . These genera have the character states supporting the (extant Exechiini + Eoexechia ) and the clade (extant Exechiini ), as discussed above ( Ševčík & Kjaerandsen, 2012).

The two molecular phylogenetic analyses of the Exechiini of Rindal et al. (2007) and of Burdíková et al. (2019) gave phylogenetic hypotheses quite different from that of Rindal & Søli (2006). The phylogeny of Rindal et al. (2007) is hardly comparable to that of Rindal & Søli (2006) because Pseudexechia is not included in it, while Synplasta is added. Burdíková et al. (2019) obtained very different results through a maximum likelihood and a Bayesian analyses. The different conclusions in all these papers confirm the difficulties in establishing the relationships in this tribe. Unfortunately, it is not possible to include Eoexechia in these molecular analyses.

If we compare Eoexechia with the other genera of Exechiini , Neallodia flavida Edwards, 1932 , Brachyradia and Anatella Winnertz, 1863 are the only extant taxa with the costal vein distinctly produced beyond the tip of R5 as in Eoexechia . Neallodia flavida has the antennal flagellomeres twice as broad as long, while they are as broad as long in Eoexechia and Brachyradia . Eoexechia differs from Cordyla in the very long antennae. All Exechiini , except Brachypeza Winnertz, 1863 , differ from Eoexechia in the very short ScP, nearly as long as h vein (twice as long as h in Eoexechia ); all Exechiini , except Brachyradia and Brachypeza , have the vein R1 much longer than in Eoexechia . Brachyradia and Neallodia have a shortened M2, unlike Eoexechia .

Remark. The internet site Fossilworks Gateway to the Paleobiology Database (http://fossilworks.org/) cites the three Miocene genera Prophronia Armbruster, 1938 , Protrichonta Armbruster, 1938 , and Proepicyta Armbruster, 1938 as ‘sister taxa’ of the tribe Exechiini . These taxa need to be revised because the original descriptions of these compression fossils are very incomplete ( Armbruster, 1938).

Blagoderov (2000) described the fossil genus and species Exechiites tadushensis from the Paleocene of the Russian Federation, on the basis of an incomplete wing. He attributed this fossil to the subfamily Mycetophilinae but to no precise tribe. The wing venation resembles that of an Exechiini .

Figures

FIGURE 1. Eoexechia gallica sp. n. holotype MNHN.F.A71245. A, habitus; B, wing. Scale bars = 0.5 mm.

FIGURE 2. Eoexechia gallica sp. n. holotype MNHN.F.A71245. A, thorax, anepst anepisternum, ltg laterotergite, preepst 2 preepisternum 2, sctl scutellum; B, coxae, arrow setae on hind coxa. Scale bars = 0.5 mm (A), 0.2 mm (B).

FIGURE 3. Eoexechia gallica sp. n. holotype MNHN.F.A71245. A, head from above, m.o median ocellus; B, genitalia, v.b. ventral branch of gonostylus, m.b. medial branch of gonostylus, i.b. internal branch of gonostylus. Scale bars = 0.5 mm (A), 0.2 (B).

FIGURE 4. Strict consensus cladogram of 16 equally most parsimonious cladograms.