A new monotypic genus of Geometridae, Mirlatia gen. nov., and a new species, M. arcuata sp. nov., are described from Croatia. Based on external and genitalia characters, the new genus is tentatively placed in the subfamily Larentiinae. However, the new genus takes a highly isolated position by having unique characters of the tympanum and showing an unusually long pectination of female antennae. Genetic analysis of a fragmented DNA barcode (mtDNA; cytochrome c oxidase 1) did not result in a clear assignation to any geometrid subfamily or tribe. Adults, male and female genitalia, and habitat photos of the type locality of the new species are illustrated.

Cold-adapted, DNA barcoding, geometer moths, Mediterranean, tympanum

In January 2014, the third author of this paper acquired the Lepidoptera collection of Robert Hentscholek, an amateur entomologist from Linz (Austria). In the course of inspecting the collection, a male specimen of a peculiar geometrid moth collected in 1983 in Podgora, Croatia (then Yugoslavia) was detected. The moth clearly differed from every known European geometrid species and could not even be assigned to a known genus. The collector was contacted for further information about the unusual moth, and it became evident that a male and a female specimen of the same species had been donated to the late Kurt Huber (1941–2002) who had allegedly passed the two specimens on to a geometrid specialist at the Insect Fair in Weiden, the identity of whom could not be clarified. Later in the same year, two Geometridae specialists, Peder Skou and the first author of this paper, were contacted by TM for help in identifying the unknown species; however, both failed to find out more about the identity and potential relationships of the Croatian moth.

In 2015, the second author independently and unexpectedly had located a female specimen of the same species collected in 1982 in Podgora by R. Hentscholek in the Rudolf Pinker (1905–1987) collection during a survey in the Natural History Museum, Vienna. This specimen is likely the female mentioned by the collector to have been obtained by Huber and seemingly had ended up in the Pinker collection. As the second author also failed to identify the species, he contacted the first author hoping to get the species identified and, thus, the two parallel discoveries of the same problematic geometrid had been linked together. However, as no Geometridae specialist who had been contacted for advice could even attribute the species to a genus, the moth remained unidentified and the whereabouts of one of the three specimens remained unknown. In 2022, with the aim to revive the identification efforts and attempt to trace the missing second male, the third author contacted the fourth author for an opinion about the taxon. As a result, a team was formed to work on identifying the enigmatic geometrid moth. Although the authors could not trace the second male specimen, both sexes of the taxon were available for detailed examination.

After ruling out the possibility of an accidental introduction of the species (see Discussion) we regarded it as a resident species new to science. Venation and shape of hindwing suggest the placement of the species in the subfamily Larentiinae. However, many unique characters (e.g. the peculiar morphology of the ansa of the tympanum) make this attribution rather tentative. A set of largely distinctive morphological characters and the highly isolated position of the taxon based on a DNA barcode analysis suggest that the species belongs to a new monotypic genus, which is together with its type species described in this present paper: Mirlatia arcuata gen. et sp. nov.

The phylogenetic position of this new taxon is still unclear and requires further study. Its assignation to Larentiinae is tentative and is mainly based on venation; no larentiine moth is known with a similarly shaped tympanal ansa. The double forewing areole and the long fusion of Sc+R1 and Rs in the hindwing are characteristic for Larentiinae (the latter also present in Alsophilini) (Fig. 6). The distinctly incurved discocellular cross vein is reminiscent of that in Alsophilini (Ennominae), which also possesses a broad-based tympanal ansa. An arcuate hindwing termen occurs in a few species of Larentiinae, e.g. Triphosa dubitata (Linnaeus, 1758).

The COI barcode data suggests a highly distinct position of Mirlatia arcuata within Geometridae, with the closest species at a large p-distance in Sterrhinae (Idaea muricata; in the BOLD barcode-gap analysis). No phylogenetic conclusions can be drawn from p-distances of this amplitude; thus, the available data likely point to an isolated phylogenetic position of M. arcuata, which is in full agreement with its unique morphological characters. Multi-gene molecular analyses would probably shed more light on the potential relationships of this peculiar taxon and allow to its attribution to one of the described subfamilies and tribes (cf. Murillo-Ramos et al. 2021, 2023), or to recognize it as a member of a distinct new tribe or even subfamily. Nevertheless, these molecular studies could much better be carried out on samples with less degraded DNA. Obtaining fresh samples, however, appears to be a difficult task due to the early flight period and the insufficient knowledge of the habitat preference of the species. Recently, the type locality was investigated for the species during 17–20 March 2022 by Stanislav Gomboc deploying 21 light sources over the course of three nights, but despite this intensive sampling, the species was not captured.

Male and female genitalia of M. arcuata sp. nov. do not exhibit any unique characters that would help establish any closer relationships with other geometrid lineages.

As the discovery of a new geometrid in Europe is rather unusual in the third decade of the 21^st century, the possibilities of introduction of the species from other continents had to be scrutinised. A possible introduction of this species from other parts of the world can be ruled out from the following reasons:

1. The three specimens collected in March in two consecutive years points to a (at least temporary) reproduction of the species at the site and also suggests that the species is cold-adapted. Furthermore, the species seems to be established in the area, as it was collected in two consecutive years.
2. As no Holarctic species with even similar morphology could be traced by the authors, the only possible region of origin of the species could be in the least known tropics. It is quite unlikely, however, that a cold-adapted species was transported from a tropical country. Nevertheless, the authors thoroughly screened the known geometrids having southernmost distribution in the southern hemisphere (i.e. Chile, Argentina, southern Africa, and southern Australia including Tasmania), where cold-adapted species occur and could not identify any similar species from these regions.
3. The collecting locality at Podgora is not in a direct vicinity of a port, and, additionally, the traffic at Dalmatian harbours during Yugoslavian times was rather limited. Both large ports of Split and Dubrovnik are some 70–80 km away from the locality, a distance that is rather large for a delicately winged early-spring moth with likely limited flying abilities to travel. Additionally, the probability of a tropical species establishing a population and gradually expanding its range in northern Mediterranean areas, where the winter is often cold, is highly unlikely, especially considering the early-spring flying period of this moth. In the early 1980s, under the communist era of the former Yugoslavia, Split or other Croatian ports were rarely visited by ships from other continents.
4. The collector had not been collecting outside of Europe during the time when the specimens were collected, and he had never collected in the tropics at all. Thus, an error in labelling can be excluded.

It may seem peculiar that such a remarkable species has remained undiscovered for so long. However, Dalmatia has not been a highly popular research area for lepidopterists, especially early in the year, in March. It is worth noting that recently there have been other surprising discoveries from the Balkan Peninsula, for example, Kresnaia beshkovi (Ganev, 1987) which belongs to a separate, hitherto unknown genus, and Idaea millesima Hausmann & Prochazka, 2020 (in Hausmann 2020), which is the first Idaea species with bipectinate antennae.

We are most grateful to Stanislav Gomboc for trying to confirm the present occurrence of this species at the type locality and allowing us to use his photograph of the habitat. We are also indebted to Martin Lödl and Sabine Gaal-Haszler (NHMV) for providing access to the Geometridae collection under their care and the opportunity to use the microphotography system of the collection, and to Robert Hentscholek for collecting efforts and information on the type locality. Images of the tympanal organ of the female specimen deposited in NHMV have been provided through the kind help of László Ronkay.