Formica ribbeckei Radchenko & Perkovsky, 2021

Radchenko, Alexander G., Perkovsky, Evgeny E. & Vasilenko, Dmitry V., 2021, Formica species (Hymenoptera, Formicidae, Formicinae) in late Eocene Rovno amber, Journal of Hymenoptera Research 82, pp. 237-251 : 237

Formica ribbeckei Radchenko & Perkovsky sp. nov.

Figs 1 , 2

Material examined.

Holotype: worker, SIZK GU-25, Ukraine, Zhitomir Region, Korosten’ District, village Gulyanka, Rovno amber, late Eocene , 33.9-37.8 Ma . Paratypes: worker, SIZK K-3566, Ukraine, Rovno Region, Klesov; worker, SIZK K-3514, Ukraine, Rovno Region, Klesov; worker, collection of M. Khomych, No. KH-F-507, Ukraine, Rovno Region, Vladimirets District , vicinity of village Voronky, Rovno amber, late Eocene, 33.9-37.8 Ma .

Diagnosis.

Worker: maxillary palps 5-segmented, long, reaching occipital foramen, its apical and preapical segments subequal in length; gastral tergites with sparse decumbent pubescence, distance between setae greater than their length; first funicular segment ≤ 1.2 times as long as second one, and ≤ 2.25 times as long as broad.

Description.

Body length 4.7-6.5 mm. Head slightly longer than wide, distinctly narrowing anteriorly (maxHW/minHW 1.31-1.36), with broadly rounded occipital corners and weakly convex occipital margin. Frontal area well defined, sculptured similarly to frons. Eyes large (OI 0.3-0.35), situated distinctly behind (above) midlength of sides of head, genae distinctly longer than maximum diameter of eyes (GI 1.10-1.15). Ocelli distinct, forming equilateral triangle. Frontal groove weakly developed, short and very shallow. Clypeus with distinct longitudinal medial carina, its anterior margin slightly angulated. Antennal scape longer than head, somewhat widened apically, surpassing occipital margin for ca. 1/4 of its length. First funicular segment 1.1-1.2 times as long as second one and ca. twice as long as wide, second and third segments of same length and 1.7 times as long as wide. Maxillary palps 5-segmented but long, reaching occipital foramen, their third segment the longest, apical segment subequal to preapical one; labial palps 4-segmented. Mandibles with 6-8 sharp teeth, apical tooth distinctly larger, but less than twice as long as preapical one.

Mesosoma quite slender, 2.34-2.43 times as long as high, mesonotum slightly raised above pronotum. Dorsal surface of propodeum convex, posterior one almost straight and inclined posteriorly, both surfaces converging at rounded angle. Propodeal spiracles elongate-oval, ca. twice as long as wide. Petiolar scale rather high, its anterior surface convex, posterior one flat and almost straight, sides very slightly convex; its upper margin convex, without notch (seen in anterior or posterior views), in profile quite narrow, with crest. Meso- and metatibiae each with a single, long simple spur.

Body surface with very fine microsculpture, appears quite shiny, especially on genae. Mesosoma, petiole, legs and antennae without standing setae. Several coarse semierect setae present on vertex, frons and clypeus; gastral tergites and sternites with similar setae. Meso- and metatibiae usually without row of coarse bristles, only with 2-4 bristles on distal third of flexor surface. Mesosoma and petiole with very fine, short and dense whitish decumbent pubescence, setae longer than distance between them; head dorsum with sparse pilosity, distance between setae greater than their length. Gastral tergites with sparse decumbent pilosity, distance between setae greater or at most equal to their length. Eyes bare.

Measurements (in mm) and indices are given in the Tables 1 , 2 .

Queens and males. Unknown.

Etymology.

The species is dedicated to Mr Hans-Werner Ribbecke (Mecklenburg-Vorpommern, Germany), who donated us the holotype specimen.

Comparative diagnosis.

Nine undoubted Formica species are known from late Eocene European ambers ( Dlussky 2008). F. ribbeckei Radchenko & Perkovsky, sp. nov. clearly differs from F. horrida Wheeler, 1915, F. kutscheri Dlussky 2008, F. palaeopolonica Dlussky, 2008 and F. zherikhini Dlussky, 2008 by the absence of erect or suberect setae on the tibiae and antennal scape. Among other species with bare tibiae and scape, F. ribbeckei Radchenko & Perkovsky, sp. nov. differs from F. radchenkoi Dlussky, 2008 by the absence of notch on the anterior clypeal margin. It resembles F. phaethusa Wheeler, 1915 by the 5-segmented maxillary palps, but its palps are much longer and reach the occipital foramen; additionally, it differs by the longer antennal scape and by the angulated anterior clypeal margin. New species differs from F. strangulata Wheeler, 1915 by the clypeal shape, the first funicular segment shorter, and the promesonotal and propodeal shape. Both F. flori and F. gustawi Dlussky, 2002 have 6-segmented maxillary palps. In addition, the latter species has dense decumbent pubescence on the gastral segments, which distinguishes it well from F. ribbeckei Radchenko & Perkovsky, sp. nov.

Formica ribbeckei Radchenko & Perkovsky, sp. nov. most resembles F. flori , particularly in the sparse decumbent pilosity on the gastral tergites. The main differences between it and F. flori are:

We examined approximately 100 Formica specimens of five species from Rovno amber in varying degrees of preservation: F. flori (38), F. gustawi (15), F. palaeopolonica (1), F. radchenkoi (2, including the holotype) and F. ribbeckei Radchenko & Perkovsky, sp. nov. (5). These represent half of the known amber Formica species.

The most abundant species in all ambers is F. flori , followed by F. gustawi . The latter clearly differs from F. flori by the dense decumbent pubescence on the gastral tergites. Thirty three specimens of F. gustawi have been recorded from all ambers ( Dlussky 2002b, 2008; Dlussky and Rasnitsyn 2009; Perkovsky 2016), but its separation with F. flori was not agreed upon by all researchers until 2002 when F. gustawi was described. Therefore, when previously examined Formica material will be re-evaluated, the abundance of F. gustawi may increase. The situation with F. ribbeckei Radchenko & Perkovsky, sp. nov. may be similar, as this species is very similar to F. flori , and we cannot rule out that F. ribbeckei Radchenko & Perkovsky, sp. nov. can be found in other late Eocene European ambers after re-investigation of corresponding material.

Nine fossil species originally assigned to genus Formica were first described from Baltic amber in the first half of 19th century ( Presl 1822; Holl 1829; Giebel 1856). However, the type specimens of these species have been lost, and based on their inadequate descriptions, it is impossible to attribute these species to Formica or any other ant genera. As a result, Mayr (1868) suggested that F. cordata Holl, 1829 belongs to Pheidole Westwood, 1839, and F. lucida Giebel, 1856 is not an ant, but a parasitoid Braconidae wasp, and Handlirsch (1907) proposed to consider most of them incertae sedis in Formicidae .

Bolton (2021) agreed with Handlirsch (1907) and excluded F. nigra Presl, 1822, F. gibbosa Presl, 1822 and F. parvula Presl, 1822 from Formica . It is unclear why F. luteola Presl, 1822, F. trigona Presl, 1822 and F. macrognatha Presl, 1822 were not excluded. These species do not belong to the genus Formica for the following reasons.

The body length of F. luteola is ca. 3.2 mm (too small for Formica ), the body is light yellow ( “Flavescenti-alba”) and the head is rounded and somewhat cordate ("Caput rotundatum, fere cordatum"). These features are not known to be present in either extant or extinct Formica species. F. trigona is also too small for Formica (ca. 2.5 mm) with a triangular head ("Caput trigonium") and long antennae ("antennae longitudine capitis et thoracis"). These features raise doubts that this species should even be included to the family Formicidae . F. macrognatha is also quite small (ca. 3.7 mm), with short legs ("Pedes breves"), and its head is large and triangular ( “Caput… magnum, trigonum"), precluding its placement in Formica .

Our data also confirm the exclusion of the following species from Formica : F. nigra and F. parvula are too small, with body lengths of 2.1 and 1.5 mm, respectively. F. gibbosa is too large: ca. 10.5 mm, its head is ovoid ( “ovoideum”) and the mesosoma is extremely elevated ("Thorax ovoideus, eximie elevates"). And, based on the original description, F. quadrata Holl, 1829 cannot be unambiguously assigned to any ant genus, and we agree with its exclusion from Formica .

Figures

Figure 1. Formica ribbeckei Radchenko & Perkovsky, sp. nov., holotype, worker (ANTWEB: casent 0917590) A body, lateral view B body dorso-lateral view C head, maxillary and labial palps, ventral view D head, dorsal view. Scale bars: 1.0 mm (A); 0.5 mm (B-D).

Figure 2. General view of the syniclusion of Formica ribbeckei Radchenko & Perkovsky, sp. nov., holotype, worker (A) and Plagiolepis klinsmanni, worker (B). Scale bar: 1.0 mm (ANTWEB: casent 0917590).