A case study of Heleomyzidae ( Diptera ) recorded on snow in Poland with a review of their winter activity in Europe

Twenty eight species of winter-active Heleomyzidae were collected during a long-term study in Poland. More than 130 samples of insects, including Heleomyzidae, were collected from the surface of snow in lowland and mountain areas using a semi-quantitative method. Lowland and mountain assemblages of Heleomyzidae recorded on snow were quite different. Heleomyza modesta (Meigen, 1835) and Scoliocentra (Leriola) brachypterna (Loew, 1873) dominated in the mountains, Tephrochlamys rufi ventris (Meigen, 1830) mainly in the lowlands and Heteromyza rotundicornis (Zetterstedt, 1846) was common in both habitats. Heleomyzidae were found on snow during the whole period of snow cover, but the catches peaked from late November to the beginning of February. In late winter and early spring the occurrence of heleomyzids on snow decreased. Most individuals were active on snow at air temperatures between –2 and +2.5°C. A checklist of 78 winter active European Heleomyzidae is presented. Helomyza nivalis Wahlgren, 1918 is herein considered as a new junior synonym of Helomyza caesia Meigen, 1830, syn. n. ZooBank Article LSID: 4C762D80-2771-44AA-8A15-23275096DA86


INTRODUCTION
Heleomyzidae, a small family of Diptera, comprises 151 species in Europe (Lo Giudice & Woźnica, 2013;Woźnica, 2013).It is widely distributed around the world, but is especially numerous in areas with a cool, temperate climate (Woźnica, 2008).Many representatives of this family, especially from the tribe Heleomyzini, are cold-adapted and have a typical boreo-alpine distribution, being abundant in mountains and the north of Europe.Heleomyzid fl ies are one of the most common groups of insects active in winter and are often recorded on the surface of snow in Poland (Soszyńska, 2004; Soszyńska-Maj & Woźnica, 2012) and Scandinavia (Hågvar & Greve, 2003).In the warmer areas of southern Europe they are active at low temperatures, mostly in autumn and winter, at higher altitudes (above 1,000 m a.s.l.) or in caves (Erhard & Spötl, 2010).
Although the snow and winter activity of Heleomyzidae are frequently discussed, no comprehensive research including quantitative or statistical analyses has been published to date.The main aim of this study was to describe the structure of the heleomyzid community recorded on snow in Poland, to compare the heleomyzid assemblage recorded on snow in the Polish lowlands and mountains, determine what infl uence particular weather conditions have on the winter activity of Heleomyzidae in the study area and present a checklist of winter-active Heleomyzidae in Europe.All study sites in central Poland, with the highest point of 284 m a.s.l., were located in large forests, mostly protected as Nature Reserves (e.g.Fig. 2C).Several forest associations, such as mixed deciduous forest with fi r, beech and spruce, mixed coniferous forest, riparian forest (Circeo-Alnetum), acidophilous beech woods (Luzulo pilosae-Fagetum) and dry-ground forest (Tilio-Carpinetum), are characteristic of this part of Poland.The Beskid Sądecki is a mountain range in the western Carpathians with the highest peak 1262 m a.s.l.The study areas were situated in the lower montane forest zone at altitudes of 650-750 m, covered with stands of beech with some fi r and spruce (Dentario glandulosae-Fagetum) (Fig. 2A, B, D).The climates in the regions studied is presented in Table 1 (Liszewski, 2001;Durło, 2003).

This
A total of 131 samples of Heleomyzidae were collected on snow: 51 samples from snow in the lowlands, 64 in the Beskid Sądecki Mountains and 16 from the other study sites.The insects were picked off the snow surface by hand or with tweezers.The do not show any morphological adaptions to snow activity, but they are adapted to low temperatures.Adults of Scoliocentra nigrinervis had the second lowest supercooling point of snow-active insects studied (mean -16.5°C, range -20.5/-13°C) (Sømme & Østbye, 1969).The lowest supercooling point was reported for the chironomid Diamesa mendotae Muttkowski (-21.6°C)(Bouchard et al., 2006).It is noteworthy that some fl ies can survive even long, severe and snowy winters as immature stages: for example, larvae of Heleomyza borealis, a snow-active fl y especially widely distributed in the Arctic, can survive temperatures as low as -60°C and need a lower than -15°C temperature stimulus and thereafter warmer period to pupate (Block, 2002).Despite the known low temperature preferences of the species of this family, snow-recorded Heleomyzidae have been intensively studied only by Hågvar & Greve (2003).During an almost 20-year-long study in southern Norway, they found Heleomyzidae to be the dominant brachyceran family among the Diptera recorded on snow and identifi ed 13 species of these fl ies.Four heleomyzid species were observed on snow in southern Finland (Frey, 1913), one in central Finland (Tahvonen, 1942) and two in mountain areas in Bulgaria (Czerny, 1930).The snow activity of Heleomyzidae was recorded in central Poland (Soszyńska, 2004).Von der Dunk (2006), in his work on Bavarian heleomyzids, noted 10 heleomyzids species on snow.Finally, seven species were recorded from the European part of Russia (Pavlov, 2006).
Winter activity has until now been studied only in Germany; however, the appearance on snow was not studied.Nine species of Heleomyzidae were found by Broen &   air temperature and humidity was measured.A thermo-hygrometer was installed in the shade, about 1 m above the ground.
Only live fl ies were collected from the snow.The time spent collecting was limited (up to 1.5 h) so that the samples could be compared semi-quantitatively.Specimens were also collected using pitfall traps, which caught ground-active invertebrates, under snow cover in the lowland area.Heleomyzidae were caught using this method only in winter 2005/2006 on xerothermic wasteland and in marshy meadows every two weeks.The traps were constructed as described by Aitchison (1974) and in more detail by Soszyńska-Maj & Jaskuła (2013).They contained ethylene glycol (freezing point: -30°C) to preserve the insects with addition of a small amount of detergent to reduce surface tension.They were protected with a roof against snow and rain.These samples, together with qualitative samples only (not subject to a time limit), were excluded from the environmental analysis and only used in the compilation of the list of species and analysis of seasonal dynamics.These traps were operated wherever there was snow cover.
In order to analyse the biocenotic structure of the catches, dominance (D), frequency (F) and Q index (geometric mean of frequency and dominance) were calculated.To explain the relationship between the ambient temperature and number of active Heleomyzidae, Pearson's correlation coeffi cients at a signifi cance level p < 0.05 were calculated.Variables such as the number of individuals and the maximum number of species were included in this calculation.The null hypothesis of no correlation between the activity of heleomyzids on snow and air temperature was tested.All the statistical calculations were done in Statistica 10.0 (StatSoft, 2011).
A checklist of winter active European Heleomyzidae was drawn up.The list was based on data from 71 papers (numbered in References) and the records reported in this study.It included all information about their occurrence on snow, in pitfall traps, yellow traps or just on their activity between November and March/April.

RESULTS
A total of 408 adult individuals were sampled, which belonged to 27 species and three subfamilies: Heleomyzinae, Heteromyzinae and Suilliinae.Heleomyzinae were eu- dominant in the material (17 species and 64% of individuals).Detailed data on the species and the structure of the assemblage are listed in Table 2.More males (221 indiv.)than females (187 indiv.)were collected on snow.
Four of the species appeared almost equally dominant in the combined material from lowland and mountains, although they differed in frequency.Heleomyza modesta (Fig. 3C) was the most abundant and most frequent species among all the Heleomyzidae collected on snow.Heteromyza rotundicornis (Fig. 3A), Scoliocentra brachypterna (Fig. 3G) and Tephrochlamys rufi ventris (Fig. 3B) were similarly abundant but less frequent.The Q index, a geometric mean of frequency and dominance, indicated these four species as the most characteristic for the assemblage investigated.
Lowland and mountain assemblages differed in species composition and community structure.Among the 18 species recorded in the lowlands T. rufi ventris was predominant, followed by Eccoptomera obscura (Fig. 3F) and Heteromyza rotundicornis.Fourteen species were recorded in the Beskid Sądecki Mts community.Here, Heleomyza modesta and S. brachypterna predominated, followed by Heteromyza rotundicornis.Two more heleomyzids were caught in other montane areas: Suillia cepelaki and Morpholeria kerteszii.Among the fi ve heleomyzids that were recorded in both lowland and mountain communities, only Heteromyza rotundicornis was almost equally represented in both regions.Two other species were abundant only in the lowlands (T.rufi ventris and E. obscura) and one mainly in the mountains (Gymnomus caesius).

Pitfall traps
No heleomyzids were caught by the Moericke traps placed on snow during winter, neither in the lowlands nor the mountains.However, fi ve species were recorded in pitfall traps in the lowlands (meadows at the edge of a forest) during a period when there was no snow present (Table 3).

Seasonal variation
The study of snow active fl ies was carried out whenever snow was present, from the second half of November to March (in the lowlands) and April (in the mountains).Separate samples for late October, early November and mid-April, were collected following abnormally early or late snowfalls.
In the lowlands, Heleomyzidae began to appear on snow in late November and continued to occur there until the end of December, but later on their number decreased.In the mountains the number of specimens collected increased from the second half of December and reached a peak at the beginning of February (Fig. 4).Table 4 shows the seasonal variation in catches on snow throughout winter.The Heleomyzidae differed in their seasonal activity.One group of species, which occurred in late autumn/early winter and disappeared in mid-December, consisted of Orbellia cuniculorum, Morpholeria kerteszii, Suillia vaginata, S. pallida, O. hiemalis and Tephrochlamys fl avipes.However, the abundance of these species was so low that their occurrence then may be accidental.The other group consisted of winter species, which were active during almost the whole autumn/spring period, i.e. the dominant species and the less abundant Gymnomus caesius.Another group of species consisting of Orbellia myopiformis, Scoliocentra brachypterna, S. nigrinervis, G. spectabilis and Eccoptomera longiseta were mainly collected in mid-winter.A few species started or increased their activity in March and April: Suillia parva (Fig. 3D), Heteromyza atricornis, Heleomyza captiosa and Scoliocentra villosa.This group includes a few individuals of accidentally occurring species.

Infl uence of weather
Heleomyzidae were found alive on the surface of snow at temperatures between -5°C and +8°C (Fig. 5) and at 53-100% air humidity.However, the largest number of individuals and species were collected between -2.5°C and +2°C and at a high humidity (80-100%) (Fig. 6).Three species of Heleomyzidae were recorded when the samples were collected at the lowest temperatures: Scoliocentra nigrinervis (-5°C), Heleomyza modesta (-4°C) and Orbellia myopiformis (-4°C).All of the most important taxa in the assemblage studied were recorded between -5°C and +2°C.Above +2°C, mainly accidental and less abundant species were recorded (Table 5, Fig. 5).
All the Heleomyzidae collected on snow were alive.As air temperatures increased, there was always a greater tendency for the fl ies to fl y away when disturbed.Flying heleomyzids were observed at air temperatures higher than about 2°C.However, part of the population was still sitting in the characteristic "starting position" (Fig. 7C), very often on the higher sites on natural piles or banks of snow.Flying Heleomyzidae were not collected.
According to Gorodkov (1962a) this species is recorded under the name Leria maculipennis Becker, 1897 by Frey (1913), who did not include it in volume VI of Enumeratio Insectorum Fenniae (Frey, 1941).The last published Checklist of the Fly Families Chyromyidae and Heleomyzidae (Diptera) of Finland (Kahanpää, 2014) also does not include this species name, although it should be mentioned that Tahvonen (1942) cites Leria maculipennis from Jyväskylä, central Finland.The distribution of Sc. (Leriola) maculipennis, given in detail by Gorodkov (1962a), indicates that it is an Arctic species probably widely distributed in the Holarctic Region (Gorodkov, 1984).Tahvonen's data are included in the list under the name Scoliocentra nigri nervis.

Heleomyzidae -a cold adapted and winter active group
Heleomyzids are known to prefer cold and extreme habitats, such as caves, cold mountains and snow-covered areas.Their most northerly locality is Spitsbergen (Czerny, 1924; Papp, 1981).The preference for a cool climate may have been a characteristic of the Heleomyzidae from early in their evolution, which began in the Cenozoic in northern Europe (Gorodkov, 1972).The current distribution of western Palearctic heleomyzids indicates that the recent European fauna developed in a mountainous area south of the Polar Circle and survived the glaciation in numerous isolated refuges in mountains in Western and Central Europe.It concerns mostly species now classifi ed within the subfamily Heleomyzinae and belonging to the genus Gymnomus (Woźnica, 1996).With all probability the Heleomyzid fauna dispersed from south to north during the interglacial periods (Gorodkov, 1972).This may explain the disjunctive distribution of some snow-active Heleo myzidae, such as Heleomyza modesta, Scoliocentra brachy pterna, Suillia parva and Scoliocentra nigrinervis.The last mentioned species is very common in Scandinavia (Hågvar & Greve, 2003), while in the rest of Europe it occurs only in the Western Carpatians, above 700 m a.s.l.(Soszyńska & Woźnica, 2012).Apparently, Scoliocentra nigrinervis did not colonize lowland regions when the glaciers retreated but remained in the cooler mountainous habitats.
As many as 27 species of heleomyzids were recorded on snow in Poland.Dominant species were Heleomyza modesta, Heteromyza rotundicornis, T. rufi ventris, Scoliocentra brachypterna and to a lesser extent E. obscura.Ten of the 14 Heleomyzidae species recorded on snow in southern Norway (Hågvar & Greve, 2003;Frey, 1913) were also recorded in Poland with the same dominant taxon -Scoliocentra brachypterna.In Germany, Bährmann & Adasch kiewitz (2003) stated that some taxa are typically winter-active: Heleomyza modesta, G. caesius, E. obscura, Suillia oxyphora, S. pallida, S. vaginata, Schroederella iners and Tephrochlamys laeta.In Poland, several species of this cold-adapted family were recorded only in mountains (Table 2).Species of the genera Gymnomus, Heleomyza, Scoliocentra, and Suillia cepelaki are most abundant in natural forests in the mountains.These fl ies naturally occur in wooded areas (Woźnica, 1996).In contrast, species of the genera Eccoptomera, Oldenbergiella and Orbellia occur mainly in less forested areas, and dominate in the lowlands.
Additional results from our winter study are the new faunistic records of Heleomyzidae in Poland.One species of Heleomyzidae is new to the Polish fauna -Suillia cepelaki (Woźnica & Soszyńska-Maj, in press), one was previously known only from a single Polish record -Scoliocentra nigrinervis (Soszyńska-Maj & Woźnica, 2012).The record of Suillia cepelaki in Polish mountains is the most northern known occurrence of this species in Europe, and probably also the northernmost border of its distribution (Woźnica & Soszyńska-Maj, in press).Among the lowland species, records of E. ornata are especially noteworthy.This species was recorded in this study for the fi rst time both on the snow and in the lowlands.Previously, it was recorded in Poland exclusively in the mountains (Woźnica, 2007).These results demonstrate the importance of winter studies when updating lists of some groups of insects, particularly those that are mainly active in cold weather.This was previously demonstrated for winter emerging Chironomidae (Soszyńska-Maj et al., 2016).
On the basis of the new data from Poland and literature on winter-active fl ies and Heleomyzidae recorded on snow, a list of 78 winter-active Heleomyzidae in Europe was compiled (Table 6) (papers are numbered in References and cited in Table 6 by those numbers).One species recorded on snow in Poland had never previously been noted for its winter activity.Every species in the list of 78 winter-active taxa could be potentially recorded on snow.Winter-active Heleomyzidae make up almost 52% of the whole family, making these fl ies the most winter active group of Diptera other than the Trichoceridae (Soszyńska-Maj et al., in prep.).
The absence of Heleomyzidae in pitfall traps under snow leads to the conclusion that these fl ies do not seek protection from the weather in the subnivean space beneath the snow as other insects do (Koshima, 1984(Koshima, , 1985)).This is undoubtedly due to their wings, which are signifi cantly longer than the abdomen and would make boring into deep snow cover impossible.

Phenology
Records of Heleomyzidae on snow in the lowlands were highest in late November and December.In the mountains, however, they appeared at the end of December and peaked at the beginning of February.These differences are mainly determined by the dominant species.All the dominant species in the lowlands were recorded on snow throughout the autumn-spring period (T.rufi ventris, E. obscura, Heteromyza rotundicornis), but were more abundant in late autumn and early winter.However, the activity on snow of the two most abundant species in the mountains (Heleomyza modesta and Scoliocentra brachypterna) peaked in mid-winter, i.e.January and February.Hågvar & Greve (2003) also report Scoliocentra brachypterna and T. rufi ventris occurring on snow in Norway at this time.In Germany, Bährmann & Adaschkiewitz (2003) reports collecting 27 species of Heleomyzidae in winter and provides details of the winter phenology of 10 species.The phenology they cite for Heleomyza modesta corresponds with our data.

Response to weather conditions
The group of species recorded at the lowest temperatures included those that occur only in the mountains (Scoliocentra nigrinervis, -5°C and Heleomyza modesta, -4°C) and those that were present both in mountain and lowland assemblages (O.myiopiformis, E. ornata, T. rufi ventris).A single species, Heteromyza rotundicornis, common in both assemblages, had the widest temperature range.Species that were recorded in the lowlands predominated at the highest winter temperatures.
Highest number of individuals and species of Heleomyzidae were collected between -2°C and 2°C and at from 91 to 100% humidity.The positive effect of rising air temperature and humidity were statistically confi rmed.Very often, heleomyzids were collected when the weather was cloudy or even foggy, which indicates stable weather without rapid temperature changes.At air temperatures lower than -3.5°C, heleomyzids were only recorded on the snow occasionally, while above 2°C they were commonly seen fl ying.The lowest temperature at which they are recorded is reported by Svensson (1966), who found Scoliocentra nigrinervis at -8°C, and this species can survive down to -16°C in a supercoooled state (Sømme & Østbye, 1969).Our study in Poland confi rmed that this species was active at the lowest temperature recorded (-5°C).The majority of other fi eld observations of these insects on snow were made at temperatures between -2 and + 2°C (Frey, 1913;Tahvonen, 1942;Hågvar & Greve, 2003).However, Sco-liocentra brachypterna, Heleomyza borealis, H. serrata and Heteromyza commixta were also recorded at higher temperatures (up to 5 or even 10°C) on snow in southern Finland (Hågvar & Greve, 2003).

Ecology
For some specialized arthropods, snow cover is an important refuge from extreme weather, e.g.wingless females of Geometridae (Soszyńska-Maj & Buszko, 2011) and rich communities of Chironomidae (Soszyńska-Maj et al., 2016) and Triphleba trinervis (Phoridae) (Soszyńska & Durska, 2002).The genera Boreus (Mecoptera) (Hågvar, 2001) and Chionea (Limoniidae) (Hågvar, 1971(Hågvar, , 1976) also use snow surface as a refuge, as do many Collembola (review by Hågvar, 2010).The snow surface is also a stage for sexual behaviour and copulation for some taxa, such as, species of wingless and fully-winged Chironomidae ( Svensson (1966) reports Scoliocentra nigrinervis moving on snow at a rate of 8-10 cm per minute.All Heleomyzidae collected in Poland were standing on the snow motionless, unless they were disturbed.Hågvar & Greve (2003) also note that Heleomyzidae recorded on snow do not use the snow surface for walking, migration, copulation or sexual behaviour (only one such record), or for feeding or basking.
All available data on the biology of Heleomyzidae indicates their larvae are either saprophagous or mycetophagous, and this infl uences the phenology and habitat preferences of the adult.The adults of mycetophagous species of the genus Suillia occur in forests and in cool and shady places during autumn, which is confi rmed by our results (Table 5).Autumnal and winter species whose larvae are necrophagous feed on carrion and prefer grassy and steppe areas (viz.Neoleria spp., Oldenbergiella spp., Orbellia spp.and Schroederella spp.).Larvae of the genera Heleomyza and Scoliocentra, some species of Hetero myza and Tephrochlamys rufi ventris are probably saprophagous.Their adults appear in early spring and as the temperature rises they fi nd shelter in caves and diapause there in summer.Locally, they stay permanently in caves as troglophiles.Heleomyza serrata, H. borealis, Scoliocentra brachypterna and S. villosa are mostly regarded as troglophiles and Heleomyza modesta and Eccoptomera pallescens as trogloxenes and are often recorded in caves in Norway, Austria and Poland (Østbye et al., 1987;Kjaerandsen, 1993; Christian & Spötl, 2010; Østbye & Lauritzen, 2013; Weber & Weber, 2013).Populations of some heleomyzids also occur in burrows and soil tunnels made by small rodents or rabbits, in birds' nests, on guano in caves or even in old-fashioned toilets where they reproduce.Their saprophagous larvae develop in excrement, e.g. the larvae of Eccoptomera spp. in droppings of winter active rodents (Czerny, 1924;Papp, 1981;Woźnica, 2008;Carles-Tolrá, 2011).The habitats that winter active Heleomyzidae prefer make them possible prey for shrews, which are active beneath the snow.Shrews feed on winter-active species rather than hibernating invertebrates (Ackefors, 1964;Randolph, 1973;Pernetta, 1977;Aitchison, 1984;Itämies & Lindgren, 1989).Heleomyzidae may also be a source of food for bats in caves, as the winter diet of bats consists mainly of winter-active Diptera (Kaňuch et al., 2005;Williams et al., 2011).
During this study in Poland, slightly more males than females were collected on snow (male-to-female sex ratio is 1.2), which indicates that both sexes overwinter and copulate either in winter or not until spring.It should also be mentioned that some freshly emerged specimens with a balloon on the front of their heads were seen on the snow -Scoliocentra nigrinervis (end of December) and Scoliocentra brachy pterna (beginning of March).The latest studies on Diptera collected on carcasses in winter (Carles-Tolrá, 2011; Carles-Tolrá & Prado e Castro, 2011) show that there are several species of Heleomyzidae that lay eggs in winter, and can be classifi ed as cadaver colonizers [viz.O. brumalis Czerny (Fig. 3E)] with imagines common in late autumn and winter (Carles-Tolrá, 2011; Carles-Tolrá et al., 2011; Roháček, 1997).
Hågvar & Greve (2003) point to the fact that during winter, the snow cover accumulates excrements and carrion like a big freezer, which is released when the snow melts.They hypothesized that cold-tolerant Heleomyzidae could be the fi rst to colonize these resources at snow melt.In this way they could outcompete more heat demanding saprophagous insects which exploit the same food source, for example beetles and other Diptera.In support of this hypothesis, Hågvar & Greve (2003) show that females of Scoliocentra nigrinervis contain mature eggs throughout winter, which they lay as soon as a favourable substrate becomes available.The occurrence on snow might refl ect their continuous search for possible substrates, even at low temperatures.The overwintering of males furthermore indicates copulation when the substrate is available.We support this hypothesis.Also, some species may be able to fi nd substrates for egg-laying in burrows or in caves, and stay active throughout the whole winter.However, we have not dissected females of Heleomyzidae to determine the number of egg they contain or the status of their ovaries.
Much is still left to learn about the ecology of this interesting, cold-tolerant family of fl ies.It would be interesting to determine whether carrion or dung placed in typical Heleomyzidae habitats during late winter is fi rst colonized by these species and whether they are winners in utilizing resources exposed when snow melts.proved the manuscript.Special thanks go to R. Danielsson (curator emeritus of the Entomological Collections at Lund University, Sweden) for making available the holotype of Helomyza nivalis Wahlgren, 1918 ; Hågvar & Aakra, 2006; Soszyńska-Maj, 2008; Jaskuła & Soszyńska-Maj, 2011; Soszyńska-Maj & Buszko, 2011; Soszyńska-Maj & Jaskuła, 2013).Winteremerging aquatic insects are regularly observed on snow and are a signifi cant subgroup within this community, with stonefl ies (Plecoptera) and caddisfl ies (Trichoptera) being the most abundant members besides Chironomidae (Bouchard et al., 2009; Hågvar, 2010; Soszyńska-Maj, pers.observ.).
investigation was carried out from 1997 to 2013, with different intensities of sampling in different years from November to the fi rst half of April, depending on the weather and presence of snow cover.This was part of a more general, long-term research project on snow-active invertebrates in Poland.Depending on favourable atmospheric conditions samples were collected regularly, at 10 or 14 day intervals.The lowland assemblage was studied in central Poland at regular intervals in 1999-2003 and 2005-2007.The assemblage in the Beskid Sądecki Mountains (mountain community) was sampled regularly over three winter seasons 2010-2013.Other samples were collected intermittently in different years and at different localities.Only samples containing at least one heleomyzid were included in the present analysis.The material was collected at 24 sites in central Poland (Figs 1, 2C) and at 15 sites in the Beskid Sądecki Mountains (Figs 1, 2A, B, D, E).Additional data come from isolated samples collected in the Tatra, Gorce, Bieszczady and Bialskie Mountains and the Polish uplands.

Fig. 4 .
Fig. 4. Phenology of Heleomyzidae recorded on snow in Poland, (CPUE -catch per unit effort).Ten-day periods and months are as follows: e.g.2/XII stands for mid-December.The slight differences in the colour of each of the columns indicate the catches recorded in the lowlands and mountains.

Fig. 5 .
Fig. 5.Total numbers of individuals and species of Heleomyzidae recorded on snow at different air temperatures; curved dotted line indicates the temperature when Heleomyzidae were recorded fl ying.

Fig. 6 .
Fig. 6.The relationship between numbers of Heleomyzidae recorded on the snow and humidity.

Fig. 8 .
Fig. 8. Linear regression analysis showing the correlation between the number of individuals and species of Heleomyzidae recorded on snow and temperatures up to 2°C (A) and humidity (B), at p < 0.05.

Table 3 .
Heleomyzidae collected in central Poland using pitfall traps when there was no snow; X -xerothermic wasteland, WM -marshy meadow.

Table 2 .
List of Heleomyzidae recorded on snow in Poland, with data on the number of individuals and biocenotic indices: D -dominance, F -frequency, Q -ecological signifi cance; L -lowlands, BSM -Beskid Sądecki Mountains, M -other Polish mountains.

Table 4 .
Heleomyzidae recorded on snow in Poland during this study.Ten-day periods and months are given; e.g., 2/XII stands for second decade of December.

Table 5 .
Heleomyzidae recorded on snow in Poland: numbers of specimens and temperatures (°C) at which they were recorded.