Scuttle flies (Diptera: Phoridae) of subterranean habitats in Slovakia: a review and a case study

During biospeleological surveys in Slovakia in the past, phorid flies were usually overlooked or ignored. Only one species, Triphleba antricola (Schmitz, 1918), has been reported from some caves in Slovakia. The first more comprehensive study of phorid flies of subterranean ecosystems in Slovakia is herein presented. During the investigation of seven caves and one gallery located in the Malé Karpaty Mts. in 2021 a total of nine species of Phoridae were documented. They represent the three genera Borophaga (1 sp.), Triphleba (2 spp.) and Megaselia (6 spp.). Triphleba antricola, distributed along all depths and environmental gradients between entrances and the deepest parts of the caves and the gallery, represents the most common and abundant troglophilous species. Other species probably represent only temporal visitors of the shallow spaces in caves. The position, dimensions and condition of the cave/mine entrances are probably the limits of the species events of scuttle flies in caves. Megaselia breviterga (Lundbeck, 1920) and Megaselia vernalis (Wood, 1909) are recorded in Slovakia for the first time.


Introduction
Scuttle flies (Diptera: Phoridae) belong to the lower Cyclorrhapha, in the superfamily Platypezoidea, with 35 genera and over 650 species in Europe (Weber 2013;Pape et al. 2015).The most recent count for Slovakia is 208 species (Mocek 2009).Phorid flies are minute to medium-sized (0.5-6 mm) flies, often with a somewhat hunched appearance and more or less stout, with a characteristic wing venation and with strong, sometimes dentate or feathered setae on the head, palps and legs (Disney 1983).Larvae are found in various terrestrial and some aquatic habitats, feeding on organic detritus.They may also be predators, parasitoids or parasites of a wide array of organisms, such as earthworms, snails, spiders and their eggs, centipedes, millipedes, insect larvae and pupae (Disney 1983;Oosterbroek 2006).Most adults feed on nectar, honeydew and juices exuding from fresh carrion and dung; some feed on the body juices of living beetle larvae and pupae, others prey on small insects.The adults are conspicuous on account of their fast and somewhat abrupt manner of locomotion, hence the name scuttle flies (Disney 1983).
Species are known from caves around the world (e.g.Disney 2009a; Taylor and Disney 2010;Disney et al. 2011, etc.).Indeed, some are found in European subterranean ecosystems, e.g. from Austria (Strouhal 1939), the Balkan Peninsula (Langourov 2001), Belgium (Leruth 1934(Leruth , 1936(Leruth , 1939)), Hungary (Disney et al. 2014), France (Disney and Withers 2022), Bulgaria (Langourov 2004), Germany (Prescher andZaenker 2005) or Spain (Disney 2009b).Despite numerous studies focusing on dipterans in caves and other subterranean habitats (Košel 2012;Kováč et al. 2014), there has thus far been no study focusing on the family Phoridae in caves or other subterranean habitats in Slovakia.Triphleba antricola (Schmitz, 1918) is the only phorid fly mentioned from caves in Slovakia.Mocek (1997) recognized this species in the dipteran collection from the Domica Cave in the Slovak Karst.Other data on this species from that region were provided by Kováč et al. (2005) from Krásnohorská Cave and Papáč et al. (2006) from Drienka, Majkova and Milada caves, all in the Slovak Karst.Papáč et al. (2020) mentioned the species from the cave Duča in the Slovak Paradise National Park.Melega et al. (2022) found this species as the only scuttle fly species in three caves in the Volovské vrchy Mts.Within the geomorphologic unit Malé Karpaty Mts., or the Little Carpathians (Die Kleinen Karpathen in Austria), are two caves from which this species has been documented.The cave Jaskyňa nad kameňolomom is in the Slovak part of the mountains (Papáč 2018) and Günterhöhle (Schubarthöhle) bei Hundsheim Cave is in the Austrian part, where the species was recorded with another phorid species, Megaselia rufipes Meigen, 1804 (Vornatscher 1950;Strouhal and Vornatscher 1975).
There are some data from the caves situated in the Carpathian regions close to Slovak territory.Dudich (1932) documented three scuttle fly species from the Baradla Cave, which is now recognized as the cross border (Hungary-Slovakia) Baradla-Domica Cave System: Megaselia pulicaria (Fallén, 1823), M. rufipes and T. antricola.Megaselia rufipes was observed in the Baradla Cave for the first time a long time ago (Schmidl 1856).
The main goal of this contribution is to analyse -species diversity of the phorid species assemblage in the caves and a mine in the Malé Karpaty Mts., SW Slovakia; -depth distribution of phorids in the underground corridors.

Material and methods
Phorid flies were sampled together with other terrestrial arthropods using pitfall traps exposed in the cave sediment from May/June to the end of October 2021 (5 months).
From 1 to 5 study plots were established per a cave, taking into account the dimensions of underground spaces, the environmental gradient from the entrance to the farthest cave from the entrance and with regard to the heterogeneity of the substrate and diversity of cave microhabitats.In each study plot, five traps were exposed with sterilized woodchips from deciduous trees as an organic substrate (bait).The traps differed in design (diameter of the opening) and fixation solution: two yogurt jars (250 ml, ∅ = 7 cm) filled with a 4% water solution of formaldehyde, two polyethylene bottles (100 ml, ∅ = 3 cm) with 75% denatured alcohol and the same type of bottle was filled with propyleneglycol mixed with clear water (1:1).The material was collected by A. Parimuchová and A. Mock.Collected flies were sorted to the family level, preserved in 75% alcohol and determined by group-specialists.Phorid flies belong to the dominant dipteran groups in the collected material.The specimens were identified at the species level by B. Grundmann (BG) using keys (Schmitz 1943;Disney 1983Disney , 1984Disney , 1989Disney , 1994)).BG also stored the collection of scuttle flies.

Localities
All investigated underground habitats are in the Malé Karpaty Mts., SW Slovakia.They belong to three different karst regions.All of them were formed in Mesozoic limestones.

Results
Nine species of scuttle flies were found in the caves and the gallery in the Malé Karpaty Mts.One to seven species were found per cave.Only one species, Triphleba antricola, was abundant in all of the examined localities and along all the depth and length of the subterranean corridors (Table 1).
Note: Species recorded from only one cave, not abundant.Distribution is limited to the entrance hall, situated at the bottom of a forested valley.
Note: Species recorded from all caves, very abundant (from 2 to 149 individuals per trap).
Note: Species recorded in three caves, not abundant.It is a rare species, formerly also known as T. forfex Schmitz, 1943.In Europe it has been reported from six countries: Austria, Germany, Switzerland, Hungary, Poland and Slovakia (Weber 2013).Nothing is known about the autecology of this species.
Note: Species recorded from three caves, only in one trap in each.First record for Slovakia.
This species is widely distributed in Europe (Weber 2013).It has been reported from caves in the Bakony Mountains in Hungary (Disney et al. 2014).The presence in Slovakia was to be expected.
Note: Species recorded from only one cave, from one trap.
It has been reported from caves in the Bakony Mountains in Hungary (Disney et al. 2014).Megaselia communiformis and M. posticata are closely related species.The identification is easy for both sexes.The females of M. posticata are very striking with their reddish-yellow tip of the abdominal venter.Both species are widely distributed and abundant (Weber 2013).
Note: Group of species (agg.)recorded in five caves, but not abundant.Although the Megaselia pulicaria group has been revised by Disney (1999), it is still not possible to identify some of the species with certainty.The most abundant species in particular, M. angusta Wood, 1909, is part of a complex that has not been resolved satisfactorily.M. angusta is the species formerly known as pulicaria, which now is the valid name of the former M. sinuata Schmitz, 1926, a species that can be recognized easily by its bifurcated spines at the tip of the hind tibiae.
Note: Species recorded from only one cave, from one trap.
This species was also recorded from caves in the Austrian part of Malé Karpaty (Vornatscher 1950;Strouhal and Vornatscher 1975).
There are three further records of this species from caves: one from Germany (Prescher and Zaenker 2005) and two from Spain (Pérez Fernández 2006;Disney 2009b).

Table 1
The summary of the recorded species per locality + 1-2 ex.; ++ 3-10 ex.; +++ dozen to hundreds of ex.; * calculated for plot with positive occurrence of the species; + sole locality for the species; D the dominance, C the constancy, ČK Čachtický karst, PK Plavecký karst, BK Borinský karst Species/cave Note: Unidentifiable females.

Discussion
In total, eight species of scuttle flies and one species aggregate were recorded in subterranean ecosystems in the Malé Karpaty Mts.Previously, only T. antricola was known from Malé Karpaty caves (Strouhal and Vornatscher 1975;Papáč 2018).Therefore, most of the recorded species are new to subterranean ecosystems in Slovakia, and two species (namely: M. breviterga and M. vernalis) represent the first records for Slovakia.
It seems (see Table 1; entrance hall) that the morphology of the entrance parts of a cave has a significant influence on its colonization by Phoridae.The presence of a horizontal entrance hall, but poorly ventilated and shaded, is apparently positively preferred by the majority of detected species (the absence of such an entrance will affect the biodiversity of the studied flies in the entire cave).The sufficient food resources near the entrance to the cave is probably another important factor for the settlement of the cave by representatives of Phoridae.
Triphleba.antricola is the most typical troglophilous representative of the family Phoridae in European caves.It is a troglophilous species, and although rarely, it is nonetheless found outside of caves (Langourov 2001;Mocek et al. 2005).This species is saprophagous (Disney 1994).The rest of the species enter caves sporadically, mainly to hibernate or aestivate.The majority of them are characterized by a broad saprophagy.M. rufipes is also characterized by coprophagy (Disney 1979(Disney , 1994)).
All taxa recorded in this study occurred in the entrance hall of the caves.Only three species (namely M. breviterga, an unidentified species of the M. pulicaria-agg.and T. antricola) were also recorded slightly further inside, and only T. antricola was found deeper in the corridors.While these flies (except T. antricola) may not be troglobitic, they may be using the cave entrance and twilight zones as a refuge from temperature extremes and harsh, drying conditions outside.Deeper in the caves temperatures fluctuate little; they approximate the average annual temperature of the region on the surface, and humidity levels are typically elevated (Disney et al. 2011).Almost nothing is known about the ecology and life cycle of the recorded Megaselia species.Females of M. breviterga are reported from dead mice in Finland (Disney 2012a), and a record of this species from dead snails in North America (Robinson 1971) refers to the sibling species Megaselia spelunciphila Disney, 1999, as do all Nearctic records of M. breviterga (Disney 2012b).
Although M. vernalis is said to be a real cave species (Zaenker and Prescher 2012), it has been found outside of caves at several localities.It is widely distributed in Europe (Weber 2013).Little is known about the ecology of this species.It has been bred from mushrooms (Eisfelder 1956).However, care must be taken with the records of M. vernalis in the older literature.It has often been confused with other species.
Despite the fact that Phoridae in the Malé Karpaty caves make up only a small part of dipteran biodiversity (about 70 species of all dipterans detected so far, see Mock et al. (2023), quantitatively they are not to be overlooked, and their representation is often dominant -this especially concerns the species T. antricola, very numerous and present in all the studied localities.What role they have as decomposers, food source, vector microorganisms and phoretic animals still deserves attention in the future.
The investigation of scuttle flies (Diptera: Phoridae) in subterranean habitats in Slovakia is still far from being finished.This argument can be supported by Prescher and Zaenker (2005;with 11 spp.) or Disney et al. (2014;with 10 spp.), who examined the scuttle fly fauna in in the subterranean habitats in neighbouring countries.