Koenigsbergia Popov, 2003 is a monotypic assassin bug genus described from the female holotype recovered from Baltic Amber (Eocene, ~33.9–55.8 MYA; Perkovsky et al. 2007). Koenigsbergia shares some features with genera in the ambush bug subfamily Phymatinae that led to its original placement within this subfamily. These features include the strongly developed bucculae that enclose the base of the labium, pronotum with distinct carinae, large membrane of the hemelytron, short antennae and legs, and two-segmented tarsi (Popov 2003). More specifically, this author placed Koenigsbergia within the monotypic tribe Themonocorini. Phymatini, Carcinocorini, and Macrocephalini ambush bugs feature synapomorphic subchelate or chelate forelegs, while Themonocorini have retained the plesiomorphic walking legs similar to those also seen in Koenigsbergia. Other features shared between Koenigsbergia and Themonocoris are the short and stout antennal scape and the two longitudinal pronotal carinae (Popov 2003). He also noted several differences between the two genera, e.g., the length of the anteocular head region, size of the eyes and their prominence in dorsal view, relative length of the antennal segments, and type of vestiture, among others.

However, we noticed that Koenigsbergia in fact shares striking morphological similarities with the assassin bug genera Phimophorus Bergroth, 1886 and Mendanocoris Miller, 1956, which are placed within Phimophorinae Handlirsch, 1897. Among these are two features that were treated by Usinger and Wygodzinsky (1964) as autapomorphic for Phimophorinae, i.e., the subtriangular plates transformed from prosternal processes and a drastically shortened fourth antennomere. Until recently, Phimophorinae (with the two tribes Phimophorini and Mendanocorini) comprised only Phimophorus spissicornis Bergroth, 1886 and two species of Mendanocoris – Mendanocoris browni Miller, 1956 and Mendanocoris milleri Usinger & Wygodzinsky, 1964 (incorrectly named as Mendanacoris). The phylogenetic placement of Phimophorus and Mendanocoris has been enigmatic and controversially discussed as Stenopodainae (Handlirsch 1925), close to the reduviine Aradomorpha Champion, 1899 (Wygodzinsky 1948), or as a distinct subfamily (Phimophorinae) that is part of the phymatine clade (Phymatinae and four related subfamilies) of Reduviidae (Davis 1961). None of the above-mentioned assessments were based on formal phylogenetic analyses, neither morphological nor molecular. The placement suggested by Davis (1961) was accepted in the divergence dating analysis by Hwang and Weirauch (2012), where Koenigsbergia was used as a calibration constraint within the phymatine clade. However, a comprehensive molecular (2268 loci) and morphological (112 characters) phylogenetic analysis of Reduviidae concluded that Phimophorinae are deeply nested within the trichobothrial clade (formerly referred to as “Higher Reduviidae”), the sister group to the phymatine clade (ongoing work by Masonick and colleagues). As part of that study, the concept of Phimophorinae was expanded to include genera formerly treated as Epiroderinae and certain Reduviinae (Aradomorpha Champion, 1899; Marbodus Distant, 1904; Microlestria Stål, 1860; Nalata Stål, 1860; Neostachyogenys Miller, 1953; Sphedanovarus Jeannel, 1919). The biology of Phimophorinae is largely unknown, but Chaverra-Rodriguez et al. (2010) reported that Phimophorus spissicornis were found in the axils of palm fronds. According to the ongoing work by Masonick and colleagues, Phimophorinae are now diagnosed by the following characters: the presence of spatulate setae, trichobothria located distally on the second antennomere, foretibial comb usually located on a subapical spur, 3-3-3 or 2-2-2 tarsal formula, PCu+An1 vein on the forewing long and emanating from the posterior cell apically into the membrane, posterior Cu+An1 closed cell is short, and tergite 8 in females subquadrate, and entirely confined between the connexiva of abdominal segment 7.

The discovery of one male and one nymph of Koenigsbergia provides the opportunity to advance our understanding of the systematics of this unusual fossil assassin bug genus. Our aims are threefold: First, evaluate if the newly acquired fossils represent the described species Koenigsbergia herczeki Popov, 2003 or if the adult male should be described as a new species. Second, document the morphology of Themonocoris, Phimophorus and Mendanocoris and compare them with Koenigsbergia. Third, formally transfer Koenigsbergia to Phimophorinae, should our comparison suggest that the fossil genus indeed belongs to Phimophorinae.

The photographs of amber inclusions were taken in the Laboratory of Insect Anatomy and Morphology of the Institute of Biology, Biotechnology and Environmental Protection, the University of Silesia in Katowice (Katowice, Poland) as follows: the focus-stacked color photographs were prepared with a Leica M205C stereo microscope with a high diffuse dome illumination Leica LED5000 HDI, Leica Flexacam C3 digital camera, and LasX ver. 5.1.025593 software (Leica Microsystems, Vienna, Austria). To be photographed, amber pieces were immersed in glycerin to remove most of the optical deformations due to the non-flatness of the surface of the amber. To obtain high-quality figures, fragments of specimens were imaged at high magnifications. Photographs were combined using the Image Composite Editor (panoramic image stitcher). Figures were prepared using Adobe Photoshop CS6 graphic editor. Measurements were made with LasX ver. 5.1.025593 software.

One Mendanocoris milleri specimen was examined: female (private collection of R. Hergovits): Malaysia, Pahang distr., Cameron Highlands, Tanah Rata env. (04°28′25″N 101°22′43″E), 20.3.–7.4.2011, R. Hergovits leg. Color photographs were prepared with a Leica M205C stereo microscope (same settings as for amber inclusion). Scanning electron microscopy (SEM) micrographs were prepared using Phenom XL scanning electron microscope (Phenom-World B.V., Eindhoven, The Netherlands) at 15 kV accelerating voltage with a Back Scatter Detector (BSD). The specimen was cleaned with a micro brush and left uncoated. To obtain high-quality figures, fragments of specimens (for both light microscopy and SEM) were imaged at high magnifications. Photographs were edited and assembled as above. — Three Phimophorus spissicornis specimens were examined: one male (AMNH_ENT 00023173), two females (AMNH_ENT 00023174; AMNH_ENT 00023168) and one nymph (UCR_ENT 00127816). Both Phimophorus male (AMNH_ENT 00023173) and female (AMNH_ENT 00023174) are from the same collection event: Peru: Loreto: Requena Jeraro Herrera 31Aug1987; second female (AMNH_ENT 0002316): Columbia, Antioquia, San Carlos, Vereda Jardín, Finca “El Silencio”, 6-Feb-1990; nymph (UCR_ENT 00127816): French Guiana: 11Jan2015. — Two specimens of Themonocoris endroedyi van Doesburg and Jacobs, 2011: one female (UCR_ENT 00001979) and one nymph (UCR_ENT 0010483) from the same collection event (South Africa: Vryheid Hill Nat. Res. 30Jan-2Feb2007). Phimophorus spissicornis and T. endroedyi specimens were assigned 8-digit UCR_ENT specimen identifier (USI) labels for databasing (Arthropod Easy Capture database) and 4-digit lab-internal tracking RCW code labels. Voucher specimens of extant taxa are deposited in the Entomology Research Museum at the University of California, Riverside (UCR). Macro images of dorsal and lateral habitus were captured using a Leica Z16 macro imaging system, 1.0x and 2.0x objectives, and LAS v4.3 software. Scanning electron microscopy (SEM) was used to visualize details for P. spissicornis (male) and T. endroedyi (female and nymph) with a Hitachi TM4000Plus II system of uncoated specimens.

Abbreviations used on figures: abd, abdomen; af, antennifer; apl, anterior pronotal lobe; ats, apical tibial setae; bflg, basiflagellomere; bth, bothrium of pedicellar trichobothrium; bucc, buccula; cl, claw; clv, clavus; clvc, claval commissure; cly, clypeus; cor, corium; dflg, distiflagellomere; dlt, dorsal laterotergite; dpsc, dorsal apical processes on scape; fap, femoral apical process; flg, flagellomere; fsp, fossula spongiosa; gcx8, gonocoxa 8; ge, gena; L2–L4, labial segments 2–4; lbr, labrum; mdp, mandibular plate; mesf, mesofemur; mesp, mesopleuron; mest, mesotibia; metp, metapleuron; mett, metatibia; mst, mediosternite; occ, ocellus; par, parempodium; ped, pedicel; pp, pronotal process; ppl, posterior pronotal lobe; prc, pronotal carina; prof, profemur; prop, propleuron; prot, protibia; ptc, protibial comb; prt, proctiger; s1–8, abdominal sternites 1–8; sc, scape; scl, scutellum; sclr, lateral ridge of scutellum; sf, sensory field; smc, submedian carina; spps, subtriangular process of prosternal process; spr, spiracle; tar1, 2, tarsomere 1 and 2; tbs, tuberculate seta; tp, tarsal projection; tpp, pore of tarsal projection; vlt, ventral laterotergite; vpsc, ventral apical processes on scape.

Our comparative observations suggest that similarities between Koenigsbergia and Themonocoris are superficial and do not warrant the placement of the fossil taxon within Themonocorini. Some of these similarities are also commonly seen in other subfamilies. Examples include the insertion of the antennae at the apex of the head (also found, e.g., in Salyavatinae and Cetherinae), a flattened appearance in lateral view (also found, e.g., in Phimophorinae), and a highly textured integument (also seen in Phimophorinae and certain Emesinae). However, because of the numerous and striking similarities between Koenigsbergia, Phimophorus and Mendanocoris, we place Koenigsbergia within Phimophorinae. Ongoing work by Masonick and colleagues assembled a morphological and molecular dataset across Reduvioidea that includes Phimophorus spissicornis, but this dataset does not include fossil taxa, including Koenigsbergia or Mendanocoris.

Some of the characters that unite Koenigsbergia, Phimophorus and Mendanocoris are also found in certain other reduviids, but there is phylogenetic evidence in the ongoing work by Masonick and colleagues that they are convergently derived. As an example, bucculae also occur in certain Ectrichodiinae, but that group is placed distantly in phylogenetic hypotheses. Due to the differences we observed between Koenigsbergia, Phimophorus and Mendanocoris outlined above, we recognize the three as separate genera. Interestingly, Usinger and Wygodzinsky (1964) described the 4^th antennomere in Mendanocoris as “not free, reduced to a prominence at apex of third article”. In contrast, we found that the apex of the flagellomere includes a ventral area of dense sensory structures, but did not find evidence for that area to be a separate antennomere (Fig. 6A, H). We suggest that the unusual antenna of Mendanocoris should be examined using histological techniques.

The morphological differences between the female holotype of K. herczeki and the newly discovered male appear distinctive, and we are confident that the male represents a distinct species. However, we decided to not determine the new fossil nymph to either K. herczeki or K. explicativan. sp. because the features we used in the diagnosis may vary between nymphs and adults (e.g., relative length of antennifers).