Melanosiagonserraticornis, Batelka & Prokop, 2021

Batelka, Jan & Prokop, Jakub, 2021, The earliest beetle with mouthparts specialized for feeding on nectar is a parasitoid of mid-Cretaceous Hymenoptera, BMC Ecology and Evolution (207) 21 (1), pp. 1-12 : 3-6

Melanosiagonserraticornis sp. nov.

Etymology. Tis species epithet refers to the characteristic shape of antennomeres III–X. Tis species name is registered under ZooBank LSID urn:lsid:zoobank. org:act:9D3499A6-664B-4073-A550-32F96AE5C1B7.

Material. Holotype: PřFUK056 ( Figs. 1 , 2 ); lowermost Cenomanian; Myanmar, Kachin, Hukawng Valley ; preserved in a polished, transparent yellow piece of amber (14.4 × 10.1 × 0.53 mm). Almost completely preserved female except for four distal pro- and mesotarsomeres and distal half of antennomere XI on the right side (lost during preparation and polishing of the amber). Overall body, especially head capsule and pronotal disc show signs of desiccation of the cuticle and apices of projections on antennomeres VI, VII and XI of the left antenna were broken-off and lost before the body was entrapped in resin.

Description. Female. Body dull, black. Head capsule, orthognathous, compressed antero-posteriorly; eyes small, lenticular, prominent in lateral view; vertex transversely planar, slightly elevated above anterior margin of pronotal disc; antennae serrate, inserted in front of eyes; scapus about 2 × as long as pedicel, subcylindrical and slightly curved; pedicel short, about 2 × as long as wide, slightly narrower than scapus; antennomeres III–X distinctly serrate, flattened dorso-ventrally; projections triangular with more or less rounded apices; surface of antennomeres III–XI covered with sparse semi erect sensilla; antennomeres XI similar to preceding ones, albeit not completely preserved;?labrum dorsally covered by erect setae, mandibles sickle-shaped, ventrally directed, with row of erect setae on their outer lateral edge; labial palpi slightly extending beyond the ligula with terminal palpomere fusiform, apices covered by erect setae; ligula in form of two lobes densely covered by bristles; maxillary palpi 4-segmented; palpomere Inot fully discernible, palpomere II longest and widest at apex; palpomere III and IV equal in length; palpomere IV with acute apex; galea distinctly prolonged with surface microstructures probably microtrichia.

Pronotum convex, long, widest at base, distinctly narrower towards apex, covered with sparse semi erect setae; posterior margin of pronotum trilobate; median lobe with shallowly convex ridge reaching from base to 2/3 along length of pronotal disc; elevated process at base of median lobe absent; ventral part of prothorax dark and poorly visible.

Meso- and metathorax wedge-shaped, only poorly visible ventrally; elytra convex from base to tip, dehiscent, blade-shaped, with sharp apices; surface covered with sparse, backward-leaning semi erect setae; outer borders of elytra nearly straight, inner borders of the last quarter curved outwards.

Hind wings dark brown, folded in resting position with asymmetrically overlapping apices, reaching well beyond apices of elytra; apical fields of vein RP with dense longitudinal secondary “ghost” branches.

Legs very long and slender; tarsal formula 5-5-4; pretarsal claws straight, with comb of five short teeth and one longer apical curved tooth; prothoracic legs quite different from meso- and metathoracic legs; claws on pretarsus almost as long as protarsomere V, claws on meso- and metatarsomeres much shorter than respective ultimate tarsomere; each leg with two tibial spurs; tibial spurs short on protibia, long on meso- and metatibia; protarsomeres without distinct apical setal fringe, meso- and metatarsomeres with distinct apical setal fringe except on last tarsomere; protibia and protarsomeres covered with fine setae; meso- and metatibia covered with dense fine setae and sparse longer spiniform setae; meso- and metatarsomeres covered dorsally with fine and dense short setae, but ventrally they are covered with sparse and longer spiniform setae.

Abdomen short with 5 visible segments (sternites III– VII), much shorter than elytra, laterally compressed; abdominal segments tapering posteriorly, each partially the overlapping succeeding one; sternite VII almost acute at apex.

Measurements. Total body length as preserved approx. 430 µm; length of antennae approx. 150 µm.

Remarks. Melanosiagonserraticornis gen. et sp. nov. is easily distinguishable from other Ripiphoridae by the following combination of characters:

1) Head compressed antero-posteriorly with vertex transversely planar and not elevated. Tis character is present only in some Macrosiagonini : genus Metoecus Dejean, 1833 and Macrosiagon vittata species group sensu Falin [ 29].

2) Tibial spur formula 2-2-2. Complete spur formula is preserved only in basal Ptilophorinae (all genera), in Ivierhipidius [ 30] (a highly derived genus of uncertain placement [ 30], in Ripiphorinae in the genus Ripiphorus Bosc, 1791 and Macrosiagon vittata species group, and in most New Zealand Pelecotominae (whereas in most of the genera of this subfamily and in all Hemirhipidiinae there is a strong tendency for fewer tibial spurs on all legs [ 31, 32]).

3) Antennomeres III–X with distinct triangular projections. Serrate or pectinate antennae are common in females of most Ripiphoridae (with some exceptions at the species level). However, in the Macrosiagon vittata species group (see above paragraphs 1 and 2) antennal projections are thread-like, as in males of the Macrosiagon limbata species group sensu Batelka [ 33].

4) Elytra convex, dehiscent and covering the whole abdomen. Complete but dehiscent elytra are typical for most of the Macrosiagonini , but they are usually flattened dorsally. Convex elytra are typical for species in the Macrosiagon limbata and vittata species groups.

5) Apical fields on hind wings with dense longitudinal secondary “ghost” branches. Acharacter occurring only among Ripiphoridae in Macrosiagon (e.g., [ 27]: Fig. A139, [ 29]: Figs. 1 , 2 ) including the Eocene species M. deuvei Batelka, Collomb et Nel, 2006 [ 18]: Figs. 1 –3.

6) Hind wings folded in resting position with asymmetrically overlapping apices extending beyond apices of elytra. Tis character occurs in Ripiphoridae only in Macrosiagon . Numerous examples are available for Macrosiagon , e.g., [34: Figs. 2 , 3], [29: Figs. 1 , 2 ], [18: Figs. 1 –3], [33: Figs. 2 , 6, 14, 16, 19, 30, 36, 37, 39, 43, 44; [56: Figs. 1 , 4, 6, 7] and others. Tis typical manner of hind wing folding is likely to be due to the narrow width of the wings with reduced venation, ‘compensated’ by secondary “ghost” branches in the apical field, and by a short abdomen and variably dehiscent elytra which usually do not completely cover the hind wings.

7) Meso- and metatibia with spiniform setae. Present also in some Ptilophorinae , but legs in this subfamily are much shorter and stronger, and the apices of tibia are much wider.

8) Long and prominent tibial spurs. Character occurring only in Ptilophorinae and Ripiphorinae , in all other subfamilies there is strong tendency for them to be shorter. In Pelecotominae and Hemirhipidiinae they are even partially hidden in the tibial cavity so they are hardly noticeable under low magnification using a binocular microscope (e.g., [ 31, 32].

9) Serrate pretarsal claws. Pretarsus with series of distinct teeth are present in Ptilophorinae (their number depends on the size of the specimen (Batelka, pers. obs.)), in Ripiphorus , in which the number and size of teeth is a sexual characteristic [ 35], and in Pelecotominae of South America (formerly Micholaeminae), in which all teeth are very strong and robust (Batelka, unpublished). As the teeth in all the above-mentioned groups differ in shape and number we consider this character as homoplastic, depending possibly on the surface conditions and locomotive requirements of adults. In Cenozoic Macrosiagonini the pretarsal claws are smooth and distinctly bidentate at apex.

Traits of Melanosiagon gen. nov. very convincingly place it among Macrosiagonini (see above under characters 1, 4–6); at least characters 4–6 are undoubtedly shared with Macrosiagon . Unfortunately, the metathorax in Melanosiagon gen. nov. is not discernible so it cannot be compared with Cenozoic Macrosiagonini in which the posterior half of the metepimeron is greatly expanded ventrally above the metacoxa, while the anterior half of the metepimeron evenly tapers towards the mesepimeron. Despite some characters being absent in Cenozoic Macrosiagonini (i.e., serrate pretarsal claws and meso- and metatibia with spiniform setae) we consider Melanosiagon gen. nov. to be a member of the Macrosiagonini . Falin [ 29] doubts the monophyly of Macrosiagon in respect to M. vittata species group and genus Metoecus . Te phylogenetic analysis of Ripiphoridae based on molecular markers [ 36] indeed recovered Metoecus nested inside Macrosiagon (members of M. vittata species group were not available for this analysis). More data are needed to reveal which species group(s) of Macrosiagon should eventually be raised to generic status, equal to Melanosiagon gen. nov. and Metoecus . If it is accepted that bidentate pretarsal claws are synapomorphy within Cenozoic Macrosiagonini , it seems unlikely that genera in this lineage are descendants of Melanosiagon gen. nov.

Figures

Fig. 1 Melanosiagon serraticornis gen. et sp. nov. (Ripiphoridae:Ripiphorinae), female, mid-Cretaceous Burmese amber (PřFUK No. 056). A Habitus from dorsolateral view. B Hindwing apices with secondary “ghost” branches.C Antennae with triangular projections on flagellomeres. D Detail of elytron viewed under green fluorescence. E Detail of pronotal disc viewed under fluorescence. F Prothoracic tarsus with five tarsomeres and pretarsal claws. G Detail of three distal mesothoracic tarsomeres and serrate pretarsal claws. H Mesothoracic tarsomere with erect stiff spiniform setae viewed under green fluorescence. I Distal metathoracic tarsomeres and serrate pretarsal claws.el elytron, ml medium lobe of pronotal disc,pe posterior edge of pronotal disc. Scale bars 100 µm (A), 50 µm (B), 10 µm (C, F, G, H, I), not in scale (D, E)

Fig. 2 Melanosiagon serraticornis gen. et sp. nov., micrographs of head with mouthparts and pronotal disc (PřFUK No. 056). A, B Mouthparts with elongated galea.C Detail of pronotal disc. gm galeomere, lbp labial palpus, lg ligula, lmxp left maxillary palpus, md mandible, ml medium lobe of pronotal disc,pe posterior edge of pronotal disc, rmxp right maxillary palpus. Scale bars 10 µm (A, B), 50 µm (C)