PSEUDOCHIRONOMINI SAETHER, 1977

Giłka, Wojciech, Zakrzewska, Marta, Lukashevich, Elena D., Vorontsov, Dmitry D., Soszyńska-Maj, Agnieszka, Skibińska, Kornelia & Cranston, Peter S., 2022, Wanted, tracked down and identified: Mesozoic non-biting midges of the subfamily Chironominae (Chironomidae, Diptera), Zoological Journal of the Linnean Society 194, pp. 874-892 : 876-888

TRIBE: PSEUDOCHIRONOMINI SAETHER, 1977

In adult Chironomidae , the lack of the wing vein MCu is the synapomorphy for Chironominae + Orthocladiinae , a clade comprising the two largest, most diverse and probably youngest chironomid subfamilies, considered sister to the remaining Chironomidae combined (e.g. Saether, 2000; Cranston et al., 2012). A character state common to all the Chironominae concerns the gonostylus that is backward-directed and connected rigidly with the gonocoxite in most Chironominae , although able to flex slightly in a few taxa ( Cranston et al., 1989; Tang & Cranston, 2019). Within four Chironominae tribes (Chironomini, Pseudochironomini , Tanytarsini and the recently erected Xiaomyiini ), the character state best separating the tribe Pseudochironomini is the dark comb on the apex of the foreleg tibia, similar to that on mid- and hindlegs (a probable tribal synapomorphy) and the pars ventralis, the structure being, however, weakly developed or absent in most genera.

Following reports on a distinct heteromorphy within the Pseudochironomini , especially with respect to the adult genital morphology, as highlighted recently by Krosch et al. (2020), the significance of diagnostic characters or character combinations of a tribe should be treated with the highest caution. The wing anal lobe usually well-developed (vs. anal lobe weak or absent in the Tanytarsini , but also in some Pseudochironomini genera), the vein RM oblique (vs. RM parallel to M and R 4 + 5 in the Tanytarsini or transverse to M and R + 4 5 in most other Chironominae ) and the foreleg ratio (LR 1 = ta 1 /ti) usually close to, or less than 1 (vs. higher than 1 in most Chironominae ) are the character states in Pseudochironomini that may support the tribe definition. However, these features, and the presence of the pars ventralis, rarely appear as a full set, hence their proper interpretation usually requires a reference to taxa of other Chironominae that may display character combinations or some character states similar to those listed above. Owing to the difficulty in establishing a clear diagnosis based on a set of synapomorphies for the tribe (in the current taxa composition), its monophyly is still being tested but not confirmed ( Cranston et al., 2012; Cranston 2019a, b; Tang & Cranston, 2019; Krosch et al., 2020).

To date, the following six extant genera are included in the tribe Pseudochironomini ( Andersen, 2016; Krosch et al., 2020): Aedokritus Roback, 1958 , Manoa Fittkau, 1963 , Madachironomus Andersen, 2016, Pseudochironomus Malloch, 1915 , Riethia Kieffer, 1917 , and Megacentron Freeman, 1961 , the latter with Megacentron eocenicus Doitteau & Nel, 2007 – the sole Pseudochironomini known from the fossil record (Eocene Oise amber) ( Doitteau & Nel, 2007). Descriptions of two further genera, based on unique characters found in specimens preserved in the Cretaceous ambers, are presented below.

KEY TO THE IDENTIFICATION OF FOSSIL AND EXTANT PSEUDOCHIRONOMINI GENERA (ADULT MALES)

1. Gonostylus directed backward, usually rigidly connected with gonocoxite, with slight ability of flexion inwards at most ( Figs 2E , 3G, H , 7A, B ).................................................................................. Chironominae (2)

- Gonostylus movable and usually folded inward......................... other Chironomidae subfamilies (not keyed)

2. Pars ventralis present ( Fig. 2E, G, H ) and/or foreleg tibia with spur surrounded by darkly pigmented comb similar to those on mid- and hindlegs ( Fig. 6A, B ) ......................................................... Pseudochironomini (3)

- Pars ventralis absent or represented by depressed oval area at most (possibly remnant of pars ventralis), foreleg tibia with bristle(s) or spur at most but comb never present ..other Chironominae tribes (not keyed)

3. Anal point of hypopygium present ( Figs 2E–G , 7A, C, D ) ................................................................................. 4

- Anal point of hypopygium absent, anal tergite with crenate apical extension at most ( Cranston et al., 1989: fig. 10.54e)............................................................................................................................................................ 8

4. True median volsella absent ( Fig. 7A, B ), pseudovolsella as aggregation of linearly-merged tubercles at most ( Fig. 2G, H ); antenna with 14 flagellomeres ( Figs 1D , 5B )................................................................................ 5

- True median volsella present; antenna with 13 flagellomeres (exceptionally 14 flagellomeres present) ( Freeman, 1961: fig. 18a; Oliveira & Messias, 1989: fig. 7; Doitteau & Nel, 2007: figs 48–50; Andersen, 2016: fig. 7; Pinho, 2018: fig. 1e; Pinho et al., 2019: fig. 1d) ......................................................................................... 6

5. Hypopygium with two pairs of volsellae – digitus, pseudovolsella and pars ventralis absent; anal point stout, parallel-sided, with spike-shaped prolongation ( Fig. 7 ); hindleg tibia with strong thorn-like bristles arranged in row and subapical fan ( Fig. 6D–G )............................................................................ Palaeocentron

- Hypopygium with two pairs of volsellae, digitus, pseudovolsella and pars ventralis; anal point narrow, without spike-shaped prolongation ( Fig. 2E–H ); hindleg tibia without thorn-like bristles ........ Mesoacentron

6. Anal point slender or sharp; antenna with 13 flagellomeres (extant species), exceptionally with 14 flagellomeres (single known fossil species, M. eocenicus ) ( Freeman, 1961: fig. 18a; Doitteau & Nel, 2007: figs 48–50)......................................................................................................................................... Megacentron

- Anal point broad, parallel-sided or triangular/subtriangular; antenna with 13 flagellomeres ( Oliveira & Messias, 1989: fig. 4; Andersen, 2016: fig. 6; Pinho, 2018: fig. 1d; Pinho et al., 2019: fig. 1b, c)........................7

7. Wing membrane with shaded areas along radial, medial and cubital veins, but without distinctly outlined spots, anal lobe moderately developed, not protruding; median volsella medially directed, with stem split apically ( Andersen, 2016: figs 7, 8) .......................................................................................... Madachironomus

- Wing membrane with distinct colour spots and/or crossbands, anal lobe well-developed, strongly protruding; median volsella posteriorly directed, with stem single-lobed ( Oliveira & Messias, 1989: figs 2, 7; Pinho, 2018: fig. 1c, e; Pinho et al., 2019: fig. 1a, d) ................................................................................................. Aedokritus

8. Anal lobe of wing large, distinctly protruding; pars ventralis strongly developed ( Paggi & Rodriguez-Garay, 2015: figs 3, 4, 6) ..................................................................................................................... Pseudochironomus

- Anal lobe of wing moderately developed and only slightly protruding at most ( Trivinho-Strixino & Shimabukuro, 2018: fig. 8b), but usually weak or absent; pars ventralis as small protrusion(s) at most or absent ( Andersen & Saether, 1997: fig. 1h; Qi et al., 2017: fig. 6c, d)....................................... Manoa + Riethia

Figures

Figure 1. Mesoacentron kaluginae, adult male, PIN 3130/224; Late Cretaceous, Santonian; Yantardakh, Taimyr Peninsula, Russia. A, inclusion in amber embedded in epoxy resin. B, habitus. C, head. D, antenna (arrowheads indicate borders between flagellomeres fm 1–14: black – distinctly separated, grey – weakly separated or partially fused). E, head and thorax in lateral aspect.

Figure 2. Mesoacentron kaluginae, adult male, PIN 3130/224; Late Cretaceous, Santonian; Yantardakh, Taimyr Peninsula, Russia.A, B, wing, with anal lobe and squama magnified (B). C, D, tibial armature of mid (C) and hindleg (D). E, hypopygium, ventral aspect. F, anal point. G, H, hypopygial volsellae: superior (green), inferior (blue), digitus (yellow), pseudovolsella (red) and pars ventralis (purple) photographed (G) and drawn (H).

Figure 3. Pseudochironomini? incertae sedis, adult male, PIN 3130/223; Late Cretaceous, Santonian; Yantardakh, Taimyr Peninsula, Russia. A, inclusion in amber embedded in epoxy resin. B, habitus. C, D, wing, with RM vein area magnified (D). E, F, tibial armature of mid (E) and hindleg (F). G, H, hypopygium in dorsolateral (G) and ventrolateral aspect (H).

Figure 5. Palaeocentron krzeminskii, adult male, MP/4020 (ISEZ PAN); mid-Cretaceous, probably Albian–Cenomanian; Hukawng Valley, Kachin State, Myanmar. A, habitus. B, head and antenna (arrowheads indicate borders between flagellomeres fm 1–14).

Figure 6. Palaeocentron krzeminskii, adult male, MP/4020 (ISEZ PAN); mid-Cretaceous, probably Albian–Cenomanian; Hukawng Valley, Kachin State, Myanmar. A–E, tibial apices of fore (A, B) mid- (C) and hindleg (D, E), with bristle fan magnified (E). F, G, thorn-like bristles on hindleg tibia: arrangement (F) and shape (G).

Figure 7. Palaeocentron krzeminskii, adult male, MP/4020 (ISEZ PAN); mid-Cretaceous, probably Albian–Cenomanian; Hukawng Valley, Kachin State, Myanmar.A, B, hypopygium in dorsolateral (A) and ventrolateral aspect (B). C, D, anal point photographed (C) and drawn (D). E, superior volsellae. F, G, inferior volsellae drawn (F) and photographed (G).