Across the Baltic: a new record for an enigmatic black scavenger fly, Zuskamira inexpectata (Pont, 1987) (Sepsidae) in Finland

Abstract Specimens of the enigmatic, monotypic European genus Zuskamira Pont, 1987 (Sepsidae) were initially collected only from the lower central Swedish provinces of Darlana, Uppland and Västmanland. However, the same species was subsequently found much more south in Lower-Saxony and Schleswig-Holstein although Germany is overall well sampled for sepsid flies. Here we report a further (longitudinal) range expansion based on new localities in Southern Finland. New localities for Finland and Sweden are here added and we discuss briefly the habitat requirements of the species.


Introduction
Sepsidae (Acalyptratae: Cyclorrhapha), or ant-like scavenger flies, form a small to medium-sized, cosmopolitan, family-ranked clade of saprophagous flies with ca. 370 described species (Ozerov 2005). One of the more surprising finds of the last 30 years was the discovery of two new monotypic genera in Europe (Zuskamira, Susanomira) of which Zuskamira was until recently only known from a few localities in Sweden (Pont 1987). More recently, the species was also found in Germany (Stuke 2005) which was surprising because this country is overall fairly well sampled for Sepsidae (Pont and Meier 2002). Zuskamira was described by Pont (1987) and currently contains one species only, viz. Z. inexpectata Pont, 1987. The species is morphologically distinct from other sepsids, based on the following male characters: (1) 4 sternite heavily spinose, (2) syn-sternite 7+8 absent, (3) epandrium enlarged, and (4) hypandrium firmly attached to syntergite 7+8 at one point (Pont 1987). Subsequent phylogenetic research (Zhao et al. 2013, Su et al. 2008, Laamanen et al. 2005, Pont and Meier 2002 showed that this morphological distinctness is associated with a similarly distinct phylogenetic position on the phylogenetic tree of Sepsidae: Zuskamira inexpectata is not nested within any other sepsid genus, and behaves like a "wildcard" taxon whose actual placement on the sepsid phylogeny remains unclear because different topologies are supported by the same data depending on how the data are analyzed (Zhao et al. 2013).
Zuskamira inexpectata is overall a rare and elusive species, and has previously only been recorded in small numbers on horse dung in the lower central parts of Sweden (Darlana, Uppland and Västmanland) and more recently in Northern Germany (Lower Saxony and Schleswig-Holstein). Herein, we report its presence in Southern Finland and add several additional Swedish localities to the known distribution. Overall, this suggests that the species distribution is larger than previously recognized or the species is undergoing a range expansion.

Materials and methods
Specimens were collected during two field collection trips in 2011 and 2014. Zuskamira inexpectata is very closely associated with horse dung, so horse farms and riding centers were targeted. Specimens were caught via sweep-netting; by slowly approaching a pile of dung and quickly placing the net over the dung pat, inducing the flies to walk upwards towards the end of the net (Fig. 2). Sweeping surrounding vegetation yielded very few specimens; it is likely that the species tend to hide very low in the vegetation.
Of the caught specimens, some were immediately stored in 70% alcohol and others ground up in RNA-later for further transcriptomic work. Specimens were identified based on the key given in Pont and Meier (2002) and two alcohol specimens (1 ♂ 1 ♀) from the Finnish locality Uusimaa (see Other Materials tab C in the Taxon Treatment section) were imaged with the Visionary Digital Lab+ photomicrography system (at "CF4-P3" magnification). Their habitus images are provided in the results section. These specimens th are deposited in the Lee Kong Chian Natural History Museum (formerly Raffles Museum of Biodiversity Research, in Singapore). Genbank records for the species can be found here: http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=192462  Photo showing habitat of collection locality in Southern Finland. Red inset shows the particular dungpile where numerous Zuskamira inexpectata were collected; note that it is at the edge of the pasture and close-by to shrubbery.

Ecology
Saprophagous species, obligate breeder on horse dung.

Taxon discussion
An excellent description of Z. inexpectata was provided by Pont (1987) and further discussed in relation to other sepsid genera in Pont and Meier 2002. The specimens from Finland and Sweden fit the descriptions well, but we are here nevertheless providing a high-resolution image of the male lateral habitus (Fig. 3a) and ventral abdomen (Fig. 3b) based on specimens acquired from Finland in 2014 because such images can serendipitously capture morphological differences that may become important in future research Ang et al. 2013a. This image is also placed on the digital reference collection for the Sepsidae, Sepsidnet Ang et al. 2013b.
Zuskamira inexpectata appears to have fairly specific habitat requirements while many other sepsid species are generalists, are found in many habitats, and can be bred on bovine dung (Pont and Meier 2002). Exceptions include Orygma luctuosum Meigen, 1830 which only breeds on beached kelp-wrack and some Themira species that only breed on waterfowl dung (Pont and Meier 2002). Zuskamira inexpectata appears to be an obligate specialist for horse dung. This is shared with Ortalischema albitarse Zetterstedt, 1847 while Susanomira caucasica is now also known from cow dung (pers. comm., A. Pont). Another unusual life history feature of Z. inexpectata is that the puparia require a winter for diapause before the adult stage emerges (Pont and Meier 2002). The narrow choice of substrate and habitat may explain why Z. inexpectata appears overall fairly rare and why only two specimens were caught in Finland in late August 2011. This is also reflected in the low numbers recorded by Stuke (2005) in Germany.
It is possible that Z. inexpectata is actually currently expanding its distribution, given that European Sepsidae has been extensively sampled (Pont and Meier 2002), and no Z. inexpectata have been found outside of Sweden until 2005. Climate has often been an effective barrier for dispersal (e.g., McGlynn 1999, Kobelt and Nentwig 2008, Smith et al. 2007), but it is not the case here: the Finnish locality is within the latitudinal range of the Swedish distribution and has a similar climate, while the German localities do not have drastically different (and in fact, milder) climates than the localities in Sweden and Finland.
As mentioned earlier, Z. inexpectata is dependent on horse dung for breeding -one may speculate that the species is being spread to new areas as a synanthrophic commensal alongside equestrian activities, given that humans commonly transport arthropods to new areas (McGlynn 1999, Smith et al. 2007 Nentwig 2008). It is thus conceivable that Z. inexpectata may eventually be found in other countries in Western Europe and Scandinavia.
In 2011, a field collection trip by Ang in Southern Finland yielded two specimens of Z. inexpectata from (F1) a horse farm in Lohja (Uusimaa, sub-Helsinki region. A subsequent field collection trip by Ang to the same locality on 28.vii.2014 yielded many additional specimens. The same trip also yielded Z. inexpectata from (S5) Orsa County (Dalarna) in Sweden. These records currently constitute all the known collecting localities for Z. inexpectata (Fig. 1).
Despite exploring numerous (including previously sampled) pastures, only one pasture in Sweden yielded small numbers of Z. inexpectata specimens during our 2014 collecting trip. The collected specimens were almost always seen on the horse dung itself or in the immediate surrounding vegetation. Sweep-netting of vegetation even a meter away from horse dung almost never yielded this species. The microhabitat was also specific: only fairly fresh piles of horse dung (i.e., still moist on the exterior) in moist -but not flooded-pastures with relatively short grassy vegetation or hay, and close-by to wooded or bushy areas (see Fig. 2) yielded specimens. Furthermore, specimens were collected only during sunny, warmer periods of the day. Overall, the species initially appeared fairly rare in Finland when only two specimens were collected while other sepsids, such as Themira annulipes (Meigen 1826), were very common. However, during the trip in July 2014 the reverse was found as Z. inexpectata was fairly common while T. annulipes was rare.
An updated checklist based on previous records (Pont and Meier 2002, Ozerov 2005, Kahanpää and Winqvist 2014 indicates that Finland is home to 33 species of Sepsidae across 7 genera (see Table 1 for list of species). Zuskamira inexpectata Pont, 1987 Total: 33 spp.