Simulium (Wilhelmia) lineatum (Meigen, 1804)

WERNER, DOREEN, 2003, The Simuliidae (Diptera) of the River Oder and its tributaries, with special reference to the re-appearance of Simulium (Schoenbaueria) nigrum (Meigen) in larger rivers in Central Europe, Journal of Natural History 37 (12), pp. 1509-1528 : 1513-1526

Simulium (Wilhelmia) lineatum (Meigen, 1804)

Notes:

1. The subgenus Schoenbaueria Enderlein of the genus Simulium Latreille (tribe Simuliini ) is relatively poor in species. At the present time 20 species are assigned to it (R.W. Crosskey, pers.comm.). The descriptions of individual species are often inadequate or not accompanied by any illustrations, and there are therefore difficulties in identifying material on morphological grounds; furthermore, chromosomal data remain very scanty. A comprehensive revision of the subgenus Schoenbaueria is needed in order to rectify this situation. Material studied for the present work is identified as Simulium (Schoenbaueria) nigrum (Meigen) . Although described from Germany, Simulium (Schoenbaueria) nigrum was not included by Zwick (1993: 37), presumably on the grounds that it had never been found in modern times.

2. According to Crosskey (1988), the two nominal species S. reptans and S. galeratum Edwards , which differ in the larval stage by the pattern of the head capsule, are synonyms and in spite of the variation in their features are not now regarded as distinct species. In my material of S. reptans , there is considerable variation in the development of the characteristic morphological features of this species, in particular in the development and colour of the postgenal cleft.

This species list shows that the number of blackfly species occurring in the Oder and its tributaries is relatively small, but certain species are able to develop into vast populations and also to colonise all sections of the River Oder. Rühm (1998) has considered the impoverishment of the species spectrum in flowing waters to be a consequence of the interventions and alterations to the water system brought about by human activities, which have had the effect of enhancing populations of the ‘potamal’ species. Rare species or those of restricted occurrence are suppressed or masked by the abundant species and in the course of time they may disappear from the species inventory.

The widespread or abundant species in the study area are P. hirtipes and S. monticola in the source region, S. reptans and S. morsitans in the upper course, S. reptans and S. (Sch.) nigrum in the lower course. In this region S. reptans and S. (Sch.) nigrum develop massive populations.

The species S. rostratum , S. erythrocephalum , S. ornatum s.lat. and S. equinum colonise different sections of the river and occupy the same niches as the other species. Simulium lineatum , S. aureum -group and S. vernum -group occur only rarely. The patchy occurrence of S. noelleri is dictated by its life-cycle, which is adapted to habitats that have been created or modified by human activities, such as impounded river reaches, lake and pond outlets.

All the species recorded from the tributary rivers in the study area also breed in the River Oder itself. The only exceptions are the species of the aureum -group, which are restricted to the Warta, a right tributary of the Oder. Species that breed in the tributaries and sometimes drift into the Oder do not add to the species spectrum of the Oder since there are already populations developing further upstream.

The distribution of the blackfly fauna through the entire study area is shown in table 1. This table also includes all the individual sites that were sampled within the Oder river system.

The flora of the source streams of the Oder is, with the exception of Glyceria species and rushes ( Juncus species ), limited to aquatic and semi-aquatic mosses. The pre-imaginal stages of S. monticola , S. vernum complex and S. hirtipes attach themselves to moss-covered stones, bare stones, and both living and dead organic material. The stream bottoms range from stones to gravel, but the gravelly texture of the bottom steadily increases towards the town of Odry, with banks of stones distributed here and there. Without exception, aggregations of larvae and pupae of S. hirtipes can be found in large numbers on these accumulations of stones. The herbaceous plant communities along the river margins can spread on to these areas of stones or sand banks, and offer ideal points of attachment in the moving current for S. vernum complex, S. morsitans , S. rostratum and S. noelleri . But even a few kilometres beyond the confluence of these source streams, S. reptans is already the dominant species in the breeding habitats. The number of individuals is up to 50 times greater than that of the other species. Adult emergence begins at the end of the emergence period of S. hirtipes and takes place from mid-May to mid-June.

Human interventions to facilitate the passage of ships have altered the river bed of the Oder between Ostrava and Opole. Large sections of the Oder were deepened and the banks were stabilised with walls. In order to shore up the banks that were not concreted, alders (mainly Alnus incana ) were planted along this section of the river. Along the upper and lower sections, around the water meadows, the dominant woodland trees are Common alder and Grey alder ( Alnus glutinosa , Alnus incana ), Common ash ( Fraxinus excelsior ), elms ( Ulmus species ), and also willows ( Salix species ) and poplars ( Populus species ). The branches and leaves that hang into the water are used by simuliids as attachment points along this section of the river. Simulium reptans is still the dominant species here. In addition, S. equinum , S. lineatum , S. morsitans and S. ornatum complex are also found, together with isolated occurrences of S. erythrocephalum .

The confluence of the Nysa with the Oder at Grodkow brings S. reptans , S. morsitans , S. equinum and S. rostratum into the Oder river, and the confluence with the Strobawa at Brzeg brings in S. reptans , S. morsitans and S. ornatum complex. In the streams, the pre-imaginal stages attach themselves mainly to extensive stands of Ranunculus species and Carex species. When carried into the Oder, these blackfly species can, with the exception of S. ornatum , also be found in the Oder as far as Oława. It is surprising that S. ornatum , a particularly common morphospecies tolerant of a wide range of habitats in the Palaearctic region, does not occur more abundantly in the study area or colonise a more extensive area. It is reported to develop in water at widely different temperatures, can tolerate relatively high levels of pollution, and in other river systems is found up to the potamal zone with its main focus in the lower rhithron (Zwick, 1974; Wichard, 1976; Prügel, 1986; Seitz, 1992; Werner, 1992).

Downstream of Wroclaw the river remains within its natural course. The banks are lined with extensive stands of Phalaris , interspersed with species of Carex and Glyceria . The plant material hanging into the water serves as attachment points for larvae and pupae, and as it extends out into the current provides optimal conditions for the development of S. reptans . Isolated specimens of S. morsitans also occur. The river bed is gravelly to muddy. The few stones that are present on the bed are not used as attachment points. From Scinawa and downstream, isolated larvae of the species of the subgenus Schoenbaueria here identified as S. nigrum occur alongside larvae of S. reptans and S. morsitans . The ratio of reptans : nigrum at the sampling site in Scinawa is about 800:1. The species spectrum of the Barycz, which flows into the Oder before Glogów, consists of four species: S. ornatum complex, S. erythrocephalum , S. reptans and S. equinum . In this section of the river, material of the S. ornatum- group showing several atypical features was repeatedly collected through the period of the investigation. The pattern on the head capsule and the colour of the larvae, as well as the number and arrangement of the pupal gills, varied greatly. These larvae are typical of S. ornatum -group, and are probably close to S. ornatum or S. intermedium Roubaud (J. Bass, pers. comm). At Glogów itself, these four species ( S. ornatum complex, S. erythrocephalum , S. reptans , S. equinum ) together with S. rostratum , S. morsitans and isolated specimens of S. (Sch.) nigrum can all be found in the Oder.

At Nowa Sól, 25 kilometres downstream, S. reptans and S. (Sch.) nigrum are the only species to be found throughout the entire breeding season. It is mainly S. reptans that develops in the rivers Kroisa, Bobr and Lausitzer Neisse, but shortly before their confluence with the Oder these rivers are also the breeding places of S. ornatum complex, S. vernum complex, S. morsitans , S. equinum and S. erythrocephalum . Within the entire study area, S. noelleri was only found in the Bobr, at Zăgań. The natural course of the river has been altered, and the water of the Bobr now runs mainly along an artificial river bed. The further course of the water flow is regulated by a weir. It was not possible to sample at this installation, but vast quantities of pupae and larvae of S. noelleri were found at a point some 50 m behind the weir and onwards, so that it can be assumed that the weir, too, has been colonised.

The region around the rivers Kroisa, Bobr and Lausitzer Neisse together with the Oder is one of the principal areas where S. reptans occurs in pest proportions. This species has a Palaearctic distribution and, according to Rubtsov (1956), mainly colonises major rivers and their tributaries in both lowland and hilly regions. This part of the study area is a typical breeding habitat for this species. Present observations do not agree with those of Rühm (1982), who characterised lowland populations of S. reptans as being of low density and of geographically restricted occurrence. As Crosskey (1990) and Rühm (1967) have pointed out, its occurrence as a pest is restricted to particular rivers or river systems. The Oder is a good example of this. Downstream along the river and on both banks there extends a broad area where it attains pest status. In recent years there have been increasing numbers of reports of it attacking both cattle and people. It has become particularly aggressive since about the spring of 1996. On warm and sunny days, it has been impossible for farmers to drive their cattle on to the pastures. Amongst herds that were kept on the Oder meadows all the year round, collapses and deaths among the cattle have been reported (various farmers: Betke, pers. comm.). Collections were made from the lower course of the Oder, between Frankfurt-an-der-Oder and Schwedt, from 1992 to 1995. Simulium reptans was the only simuliid represented in these samples and was recorded from the Oder at an abundance of 20–30 individuals per m2. At the time, samples were not taken from the breeding sites which it is now known that S. (Sch.) nigrum would prefer. No adults were collected attacking cattle or humans, nor were any of the samples collected by local people suitable for analysis.

Isolated records of S. erythrocephalum , S. equinum and S. ornatum complex were found in the samples from 1997 onwards. These three species develop mainly in former arms of the river which still contain running water from the Oder and in narrow side channels of the Oder as well as in the smaller tributaries. Larvae drift into the Oder from these areas and continue their development there. In recent years, since 1997, the species S. (Sch.) nigrum has been found to be breeding in massive numbers in this lower course of the river. The flies seem to be very closely associated with this running water. The pre-imaginal stages of S. (Sch.) nigrum were found in large numbers in the Oder downstream from its confluence with the Lausitzer Neisse onwards, and increased greatly in abundance downstream as far as the mouth of the river. Figure 2 shows the proportions of the species S. reptans and S. (Sch.) nigrum at four localities on the lower course of the Oder, at Lebus, Reitwein, Güstebieser Loose and Szczecin.

At Lebus (52°24.956 N, 14°01.330 E) near Frankfurt-an-der-Oder, 73% of the specimens collected were still S. reptans , whilst 26.6% were S. (Sch.) nigrum . At Reitwein (52°29.932 N, 14°37.823 E), the proportions have changed to 60% S. (Sch.) nigrum and 38.8% S. reptans . If the Oder is followed still further downstream to Güstebieser Loose (52°45.714 N, 14°19.092 E), 78.8% of the blackflies were S. (Sch.) nigrum and at Szczecin as high as 92.8%. Between Reitwein and the mouth of the Oder, cattle are attacked almost exclusively by S. (Sch.) nigrum , with isolated attacks by S. reptans , S. ornatum complex and S. equinum . The same is true with blackfly attacks on goats, sheep, pigs, dogs, and especially on humans. Particularly interesting is the fact that the blackflies follow the wind direction when searching along the Oder for breeding sites, whereas they fly against the wind when searching for hosts. Flies can easily cover distances of 20 to 30 km. Although adults were collected up to 30 km from the breeding sites whilst attacking grazing cattle for their blood meals, the species was never found to be colonising rivers in the immediate vicinity of the Oder even though these too appeared to provide optimal conditions for the development of the pre-imaginal stages. In the forests of Karelia, adults of S. reptans are found no more than 7 km from their breeding sites (Ussova 1964), and may possibly drift passively with the wind or with air currents.

Along with S. reptans , S. (Sch.) nigrum is a potential major pest species in this region and is also now one of the most aggressive pest species in Central Europe. Investigations were carried out in the lower course of the Oder to establish whether local adaptations are reflected by the presence of genetic subtypes of the species S. reptans and S. (Sch.) nigrum (Lentzsch and Werner, in press). Currently no evidence of ecological preferences by the various subtypes along this lower course of the Oder has been found, either among the pre-imaginal stages or in the host-searching by the adult blackflies. Shifting the grazing regimes of farm animals in time or space does not influence blackfly attacks.

Simulium (Sch.) nigrum was not found to have colonised the tributary rivers in the immediate vicinity of the lower course of the Oder, such as the Warta, Obra, Wetna and Notec. The species spectrum of these rivers during the study period consisted of S. erythrocephalum , S. ornatum complex, S. morsitans and S. equinum . In this connection it is worth mentioning that the entire river systems of the Warta and Notec are breeding sites for massive populations of S. erythrocephalum . Fluctuations in the water level of these rivers are very slight and have only a minimal influence on the development of the egg batches and the pre-imaginal stages. According to Rühm (1972), S. erythrocephalum is unable to react with sufficient flexibility to great fluctuations in the water level, and first instar larvae dry out when their contact with water is broken. In contrast to the Oder, it is S. erythrocephalum which is the principal pest species in the areas around the Warta, Obra, Wetna and Notec. Females of S. ornatum complex and of the subgenus Wilhelmia were found as pests. These blackflies also aggressively attack humans and grazing animals, but in comparison with S. reptans and S. (Sch.) nigrum they are neither as aggressive nor as persistent in their attacking behaviour.

In the Warta at Poznań and Konin, isolated pupae of the S. aureum -group were found in June 2000. A species identification was not possible, as neither pharate nor experimentally hatched adult males were available, and larvae and females lack discriminating characters.

It is interesting that S. reptans too does not breed in the extensive lowland area around the Warta and the Notec. As it is a species of turbulent water courses, these rivers do not provide optimal breeding conditions, and so it avoids this region completely and is restricted in its mass occurrence to the Oder.

Simulium ornatum , S. erythrocephalum and S. reptans were found in small numbers in the Rurzyca, which joins the Oder before Ognica. Hardly any individuals of the first two species were carried into the Oder. Egg batches of S. ornatum and S. erythrocephalum were never observed in the lower course of the river Oder.

Rubtsov (1964) considered species of this subgenus to be typical inhabitants of large rivers such as the Volga and the Dnieper. Species of the Simulium subgenus Schoenbaueria like nigrum or pusillum are characteristic members of the potamal, that is to say their occurrence is restricted without exception to large rivers and streams. These blackfly species are never found in small streams or around springs. However, a few species are now assigned to the subgenus Schoenbaueria that are found in small streams and drying brooks, so that Rubtsov’s characterisation of the habitat cannot be included in the definition of the subgenus.

In earlier times, Schoenbaueria species were probably found in other large rivers such as the Rhine, the Danube, the Elbe, the Spree and the Havel. Meigen (1804) described the first species as Atractocera nigra from at least two males, one of which (in Paris Museum) is the lectotype (Zwick and Crosskey 1981), they were collected by Meigen himself in May and July around Stolberg in Germany. The material worked on by Enderlein (e.g. 1921) also came from Germany (and other countries), but since that time S. (Sch.) nigrum has not been found again in Germany (Zwick, 1995). Perhaps the species is very sensitive to the impact of human activities. It has not re-colonised any of the large rivers such as the Danube, Rhine, Elbe, Spree, Havel or even the Warta in the last ten years. Up to early or mid March, water levels in the Oder remain normal. But strong fluctuations in the water level are caused by the spring snow melt in the mountains, periods of rain, followed by high evapo-transpiration as summer temperatures increase. Fluctuations of more than 2 m in the water level can be expected within the course of a few months. Neither larvae nor pupae of S. (Sch.) nigrum and S. reptans occur in their breeding sites before the high water levels initiated by the snow melt at the beginning of March. Only isolated larvae of S. equinum , S. ornatum complex and S. erythrocephalum are found. When the water level is high (figure 3 compared with normal water level in summer figure 4), the larvae of S. (Sch.) nigrum and S. reptans can be found in astronomical numbers on the new shoots of Phalaris stands and on Carex spp. and Glyceria spp .. At this time the larval stages are to be found at depths of 1.2 to 2 m. The pre-imaginal stages are subjected to large variations in water pressure and current speed. Both species take about 3 weeks to pass through the larval stages. During this period the water-flow is extremely rapid, up to 1–1.5 m /second in the current, and the width of the river also fluctuates, at normal water levels occupying some 180–220 m in its lower course. Depending on the water level, larvae and pupae can be found at depths of 20–180 cm. Plant surfaces are generally used for pupation by S. reptans , whereas S. (Sch.) nigrum also pupates on the stones of the breakwaters which line the river bed on the west bank of the Oder. These breakwaters consist mainly of densely-layered aggregations of stones. Breakwaters destroyed during World War 2 were replaced by concrete blocks, and these are occupied by pupae but only along their upper edges, very close to the water surface. Pupation strategies are not species-specific. Both species pupate at random, usually with the opening of the cocoon facing downstream in the direction of flow. Pupation on the stalks of old Phalaris stands from the previous year takes place with the opening of the cocoon usually directed towards the water surface, and there are often several layers of pupae with the two species mixed together. Up to 1000 pupae could be counted on many stalks measuring 1–1.2 m in length and with a diameter of 0.5 cm. With the texture of the river bed and the growth of its plant communities, the lower section of the Oder appears to provide optimal conditions for the development of S. (Sch.) nigrum . In contrast with the other species, it is strictly limited to the main channel of the Oder.

The adult emergence is both characteristic and significant. In contrast to populations of S. erythrocephalum , for example, in which a mass emergence is triggered when the water temperature rises rapidly above 10°C (Rühm, 1970b), S. (Sch.) nigrum and S. reptans are influenced by a second important factor in addition to the water temperature of the Oder system. The water level of the river drops substantially and rapidly at the end of April and beginning of May (figure 4), usually within a few days. A high proportion, more than 90%, of the pupae that are left dry on plants and stones are then able, at least if the pharate adult stage has been reached, to complete their development with the aid of the atmospheric oxygen. The remainder fail to complete their development and succumb to desiccation, fungal attacks or physical destruction.

Simulium (Sch.) nigrum females emerge first, and are followed by both sexes of S. reptans and males of S. (Sch.) nigrum in an overlapping sequence. This behaviour may be based on an intraspecific genetic variability (Lentzsch and Werner, in press), but the extent to which this variability may also influence geographical distribution and ecological interactions is not yet known. The very aggressive females of both species emerge between early and mid May, according to temperature, and continue until early June. During this period the areas around the breakwaters are regular carpets of simuliid pupae. The onset of serious blackfly harassment in the areas around the river can be expected as early as 3–4 days after the drop in water level. Attacks on warm-blooded mammals continue for some 5–6 weeks, approximately until the end of June. Both species, S. reptans and S. (Sch.) nigrum , produce one generation of particularly aggressive females in this breeding habitat. Catches of females attacking a person, collected with only 10 strokes of a plate-sized net, produced up to 5000 blackfly females at times, with the ratio of the two species varying between sampling locations. With the assistance of a farmer, it was possible to count the attacks by aggressive females on a tethered calf. Some 1500–1800 females arrived within 15 minutes. Up to 6000 attacks per hour were recorded. Farmers and smallholders used normally to drive their herds to pasture at the beginning of May. In the last few years it has no longer been possible to do this until the middle of June in the infested areas around the Neisse and in the lower course of the Oder, as most of the tethered animals in the polders immediately suffer circulatory collapse and die. The areas around the bites are marked by swellings that may be as large as 2 cm in diameter, and these wounds generally become inflamed and then suppurate. Bites on man are equally unpleasant. The bite is painful, the area around it swells and becomes inflamed, and it is easily infected if it is scratached or comes into contact with any dirt.

In contrast with other work dealing with blackfly attacks on grazing animals in the lowlands, in which it was pointed out that plague years occur aperiodically (Rühm, 1970b; Gräfner et al., 1976), the mass occurrence of these two species takes place every year because the most essential prerequisite, the possibility for an extremely high individual or adult density to develop in the Oder, is always present.

In addition to providing a species inventory of the blackflies of the Oder and its tributaries, the geographic and seasonal distribution of the sampling survey also provides information on the phenology of the species and on their ecological interactions with each other.

The phenology of the two indicator species in the Oder, S. reptans and S. (Sch.) nigrum , was very uniform throughout the five year period of this investigation, 1997–2001. Table 2 shows the seasonal distribution of the pre-imaginal and imaginal stages of S. (Sch.) nigrum and S. reptans during the study period, at the sampling site near Reitwein on the Oder (52°29.932 N, 14°37.823 E).

Both species are strictly univoltine, that is to say there is only one generation per year, in the spring months. Because of the development of these mass populations, it was possible to take a continuous series of samples which then give an almost uninterrupted picture of the developmental periods for the individual instars and of the development of the whole population. No uniform sampling of the pre-imaginal stages was possible in 1998 before the middle of April, because the water levels of the Oder were unusually high up to this point (up to 4.84 m) and collecting at the breeding sites was not possible even by boat. The same was true of the sampling period in 2000.

If the developmental stages are compared with the water levels of the Oder as recorded by the water gauges in the study area (table 3), it can be seen that the larvae of both species first appear together in March, some 10–20 days before the onset of the high water levels that are caused by snow melt in the mountains. The high levels of water that occur every year between December and January have no influence on the onset of simuliid development. It is probably a combination of temperature and water level that is the decisive factor for initiating development.

No general conclusions can be drawn from the data in table 2 about the seasonal occurrence of the developmental stages of the aquatic forms. However, final instar larvae of S. (Sch.) nigrum can always be found some 10 days before those of S. reptans . Depending on temperature, pupation of S. (Sch.) nigrum begins in early or mid April and S. reptans a little later. The observations of Rubtsov (1962), Ussova (1964), Knoz (1965), Davies (1968) and Zwick (1974), who reported that the pupal period of S. reptans was during May and June in the areas that they studied, could not be fully corroborated here as May and June fall within the main flight period of adult blackflies on the Oder, for example in 1998. A shift in the onset and in the length of development took place in spring 1998. Persistent cold temperatures and extremely high water levels delayed the pupation of both species until mid and late April. Consequently adults emerged later that spring. Results from the year 2000 stand in contrast to this. In spite of prolonged high water levels, the period of warm weather in March and April accelerated blackfly development and isolated females of S. (Sch.) nigrum were collected as early as the end of April.

The habitat requirements of S. (Sch.) nigrum need to be examined more closely, because there are contradictions between my personal observations and the findings of Rubtsov (1956, 1962) and Ussova (1964). According to these authors, the pre-imaginal larvae and pupae attach themselves only to herbaceous plants at a temperature of 15°C or more. In the Oder, however, pupation takes place mainly on stones, and plant material is probably used only when there is no other substrate available. Final instar larvae and new pupae can be found from April onwards, with water temperatures of 6–8°C. The pupal period lasts for two weeks, and the first adults can emerge at the end of April. This is probably a process that is influenced by the temperature regimes in the water. These show slight fluctuations from year to year, but the blackflies always reach their maximum emergence in mid May. Rubtsov (1956) and Ussova (1964) also recorded a relatively long flight period for the adults, from May to the end of July, and considered it possible that this may run into a second generation in late summer and autumn. Within their breeding area on the Oder, these blackflies are univoltine and the flight period lasts for a maximum of 5–6 weeks. There is certainly no second generation here. Shifts in the seasonal appearance of the adults can be attributed to shifts in the temperature regime. Carlsson (1962), for example, records one flight period in July and one generation in the year in North Sweden. If the changes in temperature regimes at these latitudes are taken into consideration, then Carlsson’s data are comparable with those from Central Europe.

Knoz (1965) has published a record of the Simulium subgenus Schoenbaueria from the Czech Republic, from the River Morava near Kromĕrĭz˘. This may be a misidentification, because the habitat is very untypical for Schoenbaueria . The record was from the upper course of the Morava, which at this point is some 8–10 m wide, densely overgrown with riverside vegetation, with a gravelly bottom and moderate current. My personal collections from this region showed a species spectrum on this section of the river of four species: S. erythrocephalum , S. ornatum complex, S. reptans and S. equinum . Species of the subgenus Schoenbaueria were not found in that river system. Further work needs to be done on other species of this subgenus, to define their occurrence and the associated habitat requirements.

Development of the egg batches is strongly influenced by fluctuations in the water level. Species whose eggs are not adapted to these occasional dry spells are particularly at risk and die when exposed to the air (Rühm, 1983; Timm, 1988; Welton and Bass, 1980). Oviposition by S. reptans was observed in June, but these egg batches dried out as the water level continued to fall still further. Ussova (1964) has described the oviposition behaviour of females of Simulium (Sch.) pusillum, during which large communal egg masses are laid on the riverside vegetation. However, I was never able to observe ovipositing females of S. (Sch.) nigrum at any point on the lower course of the Oder, in spite of their vast abundance. Bearing in mind the diverse oviposition strategies in Simuliidae , it is possible that females drop their eggs singly on to the water surface in the line of the current, and that these sink to the bottom and remain in the sediment until the following spring (P. H. Adler, pers. comm.). However, searches for eggs among plant roots, in snail shells and caddis tubes, or in the sediment were unsuccessful, and this is made more difficult by the fact that in the summer months water is only let into the Oder along the shipping channels. The movements in the water following on from this together with the natural current of the river give rise to extreme perturbation and transport of the sediment downstream. Eggs would be unable to remain on the river bed for any length of time, which could be 9 months in the case of subgenus Schoenbaueria , without drifting away downstream. It is possible that eggs of S. (Sch.) nigrum survive in dried out breeding sites by means of a period of dormancy. A dormant period of 8–9 months is possible, according to Timm (1987). It would then be possible for oviposition to take place on the stones of the breakwaters or in the same area, in damp cracks and crevices. The next high water level would initiate the further development of the eggs, through the effect of the water or of the temperature, and larval development would begin. According to Rühm (1970a) and Post (1982), first instar larvae remain on the oviposition substrate or in its immediate vicinity. In theory, such a strategy would be optimal for the first instar larvae of S. (Sch.) nigrum , because the first instar simuliid larva lacks the anal sclerite on the circlet of hooks above the tip of the abdomen with which the larva attaches itself securely to the substrate (Barr, 1984). In this case the damp cracks and crevices on the breakwaters would offer some protection from the current. This would also explain the sudden appearance of larvae at particular spots where the current is strongest and at depths up to 2 m.

Similar observations were reported by Ladle et al. (1985) and Welton et al. (1987) for the species S. (Simulium) posticatum Meigen which is also univoltine at the site where it was studied. The larvae of S. posticatum also hatch between February and March and develop within the same time-period as S. reptans and S. (Sch.) nigrum do in the Oder system. The adult blackflies emerge from May to June. The characteristic eggs of S. posticatum have been found in small numbers in the river sediment, but it is impossible to equate these numbers with the enormous densities of the larval populations.

Searches for eggs of S. (Sch.) nigrum among the stones of the breakwaters have been unsuccessful so far. Small stones were taken back to the laboratory and kept there for several days. In June 2001 a single first instar larva was found on a stone, but it could not be reared through to the pupal stage and so no species determination was possible. It would be too much to draw any definite conclusions from this single find. This first instar larva was not included in the discussion of species phenology.

Ladle et al. (1985) have described the oviposition site of S. posticatum at the River Stour in Dorset ( England), which differs from all previously known blackfly oviposition sites. These authors found some of the highly characteristic eggs of S. posticatum in desiccation cracks in the soil surface mainly in the rooting zone of nettles and other marginal plants on the river bank. However, such desiccation cracks in the vertical earth banks of the river are hardly ever to be found on the Oder. Moreover, the vegetation and the river bed at these two breeding sites are very different. The choice of egg-laying site and the quantitative influence on them, as depending on the time of year selected for oviposition, remain unknown for S. (Sch.) nigrum .

The species inventory from the Oder is fully comparable with that known from the other large rivers of Central Europe. Because of the development of vast populations of Simulium reptans and Simulium (Schoenbaueria) nigrum , other species such as Simulium (Wilhelmia) equinum or Simulium ornatum complex are masked by the abundant species and play a very subordinate role in this river system. It would be interesting to undertake a study of the River Weichsel system and to make comparisons between the blackfly populations of these large, eastern, lowland rivers. Study of the known localities for S. (Sch.) nigrum indicates that the breeding sites in the River Oder mark the most western points in the distribution of this species known since the time of Enderlein.