An Updated Overview of the Genus Eumerus Meigen, 1822 (Diptera: Syrphidae) in the Iberian Peninsula

The genus Eumerus Meigen, 1822 (Eristalinae: Merodontini) is one of the most diverse hoverfly genera in the west Palaeartic Region with 89 confirmed species and 77 occurring in the European continent. It is also highly diverse in the Iberian Peninsula plus Balearic Islands with 36 species. Recent descriptions of four new species have increased the number of species endemic to the Iberian Peninsula up to five. Nowadays, taxonomy, distribution and biology of the Iberian Eumerus are still far to be fully understood. The aim of this work is to present an updated overview of the Eumerus checklist in the Iberian area, addressing main topics pending of resolution in the

performed a review of the Palaeartic species of Eumerus and supplied an updated diagnosis of the genus summarized as follows: Medium size, stocky flies with more or less thickened metafemora bearing spinae on the ventral side, a face without protuberances, a usually dark metallic-green body, a more or less reddish abdomen (in some species) but in most species with three pairs of white, half-moon shaped maculae on it and a vein M1 bent inwards. Nowadays, modern identification keys added more diagnostic characters to the diagnosis of the genus such as an arista present at the base of the basoflagellomere and always bare, a vein R4+5 moderate to strongly sinuate, a cell R2+3 open at wing margin and a tergum V not visible in dorsal view (in males) [3].
Eumerus belongs to the tribe Merodontini Edwards, 1915 of the subfamily Eristalinae Newman, 1834. This tribe represents a monophyletic clade which comprises six genera in total: Azpeytia Walker, 1865, Cepa Thomson & Vockeroth, 1999 [13,14] [15,16]. Nevertheless, it should be noted that Doczkal and Pape [15] did not include the genus Cepa in their revision of the Merodontini. The genus Eumerus is distinguished from Azpeytia by a shorter scutellum and the presence of short extensions on the apical part of vein M [3], from Cepa by a short basoflagellomere that does not exceed twice the length of the scape and the pedicel together [17], from Lyneborgimyia, Merodon and Platynochaetus by the absence of a triangular expansion on the apicoventral part of the metafemur [15] and from Megatrigon by the presence of pilosity on the lateral sides of tergum I and the absence of a triangular bare area on tergum II anterolaterally, among others characters [16].
Due to the high species diversity found in the genus, Eumerus taxonomy and systematics are complex and difficult. New methodologies such as phylogenetic approaches using molecular markers proved to be a rather useful tool in the framework of species delimitation within this genus. Chroni et al. [18] undertook the first species group delimitation in Eumerus through molecular characters, revealing the following 'molecular' species groups: (1) E. basalis group, (2) E. minotaurus group, (3) E. ornatus group, (4) E. pulchellus group, (5) E. strigatus group, (6) E. sulcitibius group and (7) E. tricolor group. After Chroni et al. [18], some species groups have focused the attention of researchers carrying out the morphological diagnoses [19][20][21][22] while new groups have been defined, such as the E. clavatus group [21]. Despite of the fact that molecular techniques have had a positive contribution in the Eumerus systematics, the phylogenetic relationships between most Eumerus species are still pending of study.

Eumerus in the Iberian Peninsula
The first monograph dealing with the Iberian hoverflies was carried out by the Spanish entomologist Juan Gil-Collado [23] [25] and E. grallator Smit in Grković et al. [26]. While E. azabense, E. grallator and E. hispanicus inhabit Quercus dehesas or forests, E. bifurcatus and E. gibbosus are present in pine forests (Pinus nigra/P. halepensis) [10]. All these findings increased to five the number of endemic species to the Iberian area. Eumerus truncatus Rondani, 1868 was recently recorded from Spain as well [25]. At present, the total number of Eumerus species from the Iberian Peninsula plus Balearic Islands has increased up to 36 of nine species groups (see Figure 1) (see Table 1). The reported recent increment in the number of species suggests the need of further research on Eumerus taxonomy, distribution and biology in the Iberian Peninsula. In fact, this is particularly urgent for the developmental stages since larva and/or puparium are known only for ten species of those 36 species: E. alpinus, E. etnensis, E. funeralis, E. hungaricus, E. nudus, E. obliquus, E. pulchellus, E. pusillus, E. strigatus and E. tricolor. Furthermore, information bias represents a serious problem in order to establish conservation actions for endangered hoverflies. Since some hoverflies have been catalogued in the Spanish Red Lists [27] no research has been conducted yet to determine the conservation status of any of the Iberian Eumerus. The vast majority of Eumerus species in Europe are greatly associated to a no threatened status, but some species populations, as those of E. clavatus Becker, 1923 or E. ruficornis Meigen, 1822, are thought to be descending [28]. Therefore, an extensive assessment of the conservation status of the Iberian Eumerus is required to better understand their actual threaten situation in this geographic region and design conservation plans if necessary.   [22]. While E. barbarus, E. gibbosus and E. sulcitibius are present in the Iberian Peninsula (see Table 1), E. schmideggeri is restricted to Algeria, Morocco and Tunisia. Eumerus barbarus group was revised by van Steenis et al. [25] provided identifications keys both for males and females of all the species. Nevertheless, they did not test the monophyly of the group using neither morphological characters nor molecular data. Eumerus olivaceus group was proposed by Grković [22] and has three species: E. alpinus, E. nudus and E. olivaceus Loew, 1848. The first two species are present in the Iberian Peninsula (see Table 1) while E. olivaceus is endemic to Sicily. Grković et al. [29] revised the holotype of E. alpinus together with type material of E. olivaceus. In their work, they concluded that E. alpinus is a valid species excluding it from the synonym list of E. olivaceus. However, they did not provide a list of morphological differences nor illustrated the male genitalia of E. olivaceus in comparison with the male genitalia of E. alpinus [30]. In addition, as with the E. barbarus group, the monophyly of the E. olivaceus group was not tested either. We aim to explore the species concepts and the monophyly of these groups assessing molecular data, as sequences of the mitochondrial COI-5' region and other nuclear or mitochondrial genes if necessary, together with morphological characters. The analyses of robust phylogenetic and morphological trees will let to perform much more accurate species delimitation, over all, if we are dealing with possible cryptic species. On the other hand, taxonomic and systematic research on Eumerus is mainly based on male specimens. In males, one of the most diagnostic body parts for species delimitation is the morphology of the genitalia. Nevertheless, female genitalia are not as accessible as that of male. Moreover, Eumerus females exhibit external characters which show a great spectrum of phenotypic variation. These facts make difficult species delimitations based on females only and aggravated if we are dealing with possible cryptic species. Therefore, we aim to carry out an extensive review of the Iberian Eumerus in order to provide reliable and precise identification keys for males but also females.
The second research line entails the construction of a barcode sequence library of the Iberian representatives of Eumerus. We commented in the second section of this work that DNA sequences have proven useful for species delimitation in Eumerus. Unfortunately, the number of molecular data available on public databases as BOLD [31] or GenBank is rather limited as there are only data of a few species. This fact involves that genetic diversity of the Iberian Eumerus is almost unknown which is not the case for other Mediterranean areas like Greece [32]. A reference barcode library is useful for future research involving the identification of larva and/or adult stages of Eumerus. Furthermore, supplying DNA sequences of Iberian Eumerus will allow having a comprehensive knowledge about the genetic diversity of Eumerus in future taxonomic and systematic studies.
The third research line aims to carry out an analysis of the distribution of the genus Eumerus in the Iberian Peninsula. Since the hoverfly faunas of some Iberian regions have been well studied, others are poorly known. In the case of the Iberian Eumerus, while some species as E. amoenus are present in several areas, others like E. azabense, E. bifurcatus or E. clavatus are restricted to one area [4]. The potential new records of Eumerus species in areas where species diversity is underestimated will mitigate the negative effect of information bias about the distribution ranges in the Iberian region. In addition, updating the species distributions would enable the assessment of the conservation status of Eumerus species more accurately. This is particularly important for those species that are endemic to the Iberian Peninsula or whose distributions are restricted to a limited area.
Finally, the last research line is focused on the early stages of the Iberian Eumerus. Eumerus larvae feed on decaying plant tissues or underground storage organs such as bulbs. That is the reason why some species are considered pests like E. funeralis or E. strigatus, causing agricultural damage and economic loss [33]. Despite their important role in ecosystems, the developmental stages are known only for 13 Eumerus species worldwide [33] of which 10 are present in the Iberian territory (see section 3). We aim to perform samplings of Eumerus larvae, for instance, in Amaryllidaceae or Xanthorrhoeaceae plants [34], in order to complete the knowledge about the life cycles and the trophic requirements for the larval stages of the Iberian Eumerus.

Conflicts of Interest:
The authors declare no conflict of interest.