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Seasonal and diel dispersal activity characteristics of Sigara lateralis
(Leach, 1817) (Heteroptera: Corixidae) with special emphasis on possible
environmental factors and breeding state
Pál Boda a; Zoltán Csabai b
a Trans Tisza Region Environmental, Nature Protection and Water Inspectorate, Debrecen, Hungary b
Department of General and Applied Ecology, University of Pécs, Pécs, Hungary Online publication date: 17 November 2009
To cite this Article Boda, Pál and Csabai, Zoltán(2009) 'Seasonal and diel dispersal activity characteristics of
Sigara
lateralis
(Leach, 1817) (Heteroptera: Corixidae) with special emphasis on possible environmental factors and breeding state', Aquatic Insects, 31: 4, 301 — 314To link to this Article: DOI: 10.1080/01650420903110519
URL: http://dx.doi.org/10.1080/01650420903110519
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Seasonal and diel dispersal activity characteristics of
Sigara lateralis
(Leach, 1817) (Heteroptera: Corixidae) with special emphasis on
possible environmental factors and breeding state
Pa´l Bodaa* and Zolta´n Csabaib a
Trans Tisza Region Environmental, Nature Protection and Water Inspectorate, Debrecen, Hungary;bDepartment of General and Applied Ecology, University of Pe´cs, Pe´cs, Hungary
(Received 30 October 2008; final version received 13 February 2009)
Both the daily and the seasonal dispersal dynamics of the true water bug,Sigara lateralis (Leach, 1817) were studied in 2000 and 2005 under various environ-mental circumstances by monitoring the number of individuals attracted to highly and horizontally polarising shiny black plastic sheets laid on the ground. The diel and seasonal dispersal activity characteristics ofS. lateralisare described here in detail. The seasonal patterns of mass dispersal of water insects in general were different in the two studied years, the possible reason for which could be that there was a longer period when the environmental factors temporarily influenced the number of flying insects. The diel dispersal patterns ofS. lateraliswere also different in 2000 and 2005. It seems that the diurnal dispersal pattern of S. lateralis described previously can change year by year. Females outnumbered males in most cases, and females with eggs were the most active during mass dispersal.
Keywords:Heteroptera; dispersal flight; flight activity; dispersal pattern; reflected light
Introduction
Freshwater habitats of aquatic insects are continuously changing due to variations of the environmental factors. Thus, the ability to fly and migrate may be important prerequisites of the survival of individuals and populations. For species of permanent habitats, inclination for dispersal is disadvantageous in terms of survival, since they are unlikely to find equally suitable places, whereas it is essential for the survival of species living in temporary habitats (Savage 1989). Temporary habitats are generally not stable for a population even in one life cycle. Dispersal occurs not only under unfavourable circumstances (e.g. dry up, pollution, excess of predators), or in the case of changes in population dynamics (e.g. high number of larvae, high density) (Brown 1954; Pajunen and Jansson 1969), but also when it is required by reproductive activity (Brown 1951). Variations in dispersal rate and the ability of corixid species to distinguish aquatic habitats from non-aquatic ones during flying enable these insects to find suitable pools for breeding and overwintering (Pajunen
*Corresponding author. Email: bodapal@freemail.hu Vol. 31, No. 4, December 2009, 301–314
ISSN 0165-0424 print/ISSN 1744-4152 online
Ó2009 Taylor & Francis DOI: 10.1080/01650420903110519 http://www.informaworld.com
and Jansson 1969). To lay their eggs, aquatic bugs seek for suitable, i.e. shallow, quickly warming up and small-sized water bodies (Brown 1951), in which the number of predators are less; consequently there is a higher chance for survival. In order to overwinter, in autumn some of the species migrate to waters deep enough not to become totally frozen (Pajunen and Jansson 1969; Pajunen and Pajunen 2003).
There are numerous publications about dispersal of aquatic insects (e.g. Richardson 1907; Poisson, Richard and Richard 1957; Fernando 1958; Richard 1958; Young 1966; Macan 1976). Data on the strong relationships between dispersal ability and maturity of wings, wing muscles, and anatomical respect of ovarian development have also been published (Brinkhurst 1959; Young 1978). Leston and Gardner (1953) studied those weather conditions which facilitate dispersal, but they focused on the results of field observations only. These papers described only the phenomenon, or just indirectly studied the dispersal, without capturing flying individuals and analysing quantitative data. Nevertheless, some authors analysed data which originated from capture of flying individuals. According to Benedek and Ja´szai (1972), the number of migratory individuals does not depend on the proportion of individuals able to fly within a population. Based on further investigations, they found that the rate of dispersal is geographically heterogeneous, because it mostly depends upon the environmental conditions and habitat characteristics of the given area. Weigelhofer, Weissmair and Waringer (1992) found that mass dispersal occurs from June to September on the banks of the Danube in Lower Austria. Among their measured variables only air temperature showed significant correlation with dispersal, and below 15.38C flying corixids were not observed (except Callicorixa praeusta). Popham (1964), Pajunen and Jansson (1969), Weigelhofer et al. (1992) and Csabai and Boda (2005a) have described that the dispersal of aquatic beetles and bugs is negatively influenced by wind above a threshold value of wind speed. Studying the dispersal of Corixidae, Popham (1964) revealed that individuals of these species require a minimum air temperature of 12– 158C for flight. He also observed that bugs migrate between habitats within a narrow time interval, mainly at dusk. Other researchers investigated the dispersal in connection with ventral polarisation vision of positively polarotactic water insects, and found that flying during dawn and late evening is one of the three optimal daily time periods to detect suitable water habitats by reflexion polarisation (Berna´th, Ga´l and Horva´th 2004; Csabai, Boda, Berna´th, Kriska and Horva´th 2006; Kriska, Csabai, Boda, Malik and Horva´th 2006a; Berna´th, Kriska, Suhai and Horva´th 2008; Horva´th and Kriska 2008). Studying the flying activity of aquatic insect species, they also described different dispersal patterns.
Using a mark and recapture method, Pajunen and Jansson (1969) investigated small-range dispersal among small pools in Finland. They described that differences can be observed in dispersal among species, which can vary seasonally. They revealed that immature individuals showed a higher propensity to fly, and dispersal mostly occurred at periods when the population was immature. In the case of corixids, an obligatory flight period was observed between March and April, and the autumn dispersal showed positive correlation with the instability of habitat and drying out (Glatz 1976).
In the light of the above-mentioned numerous publications dealing with the dispersal of aquatic and semi-aquatic bugs and its determinants, the most suitable periods for dispersal are the dawn and late evening hours, being usually free of wind and rain, possessing higher relative air humidity, lower air temperature, and maximal
polarotactic detectability. Regarding seasonality, dispersal occurred in the Northern hemisphere from March to November (Popham 1964; Savage 1989; Weigelhofer et al. 1992). However, dispersal intensity and patterns are various, which might be the results of differences in the phenology and ecology of species, geographical differences, or diverse sampling methods. Presumably, dispersal flight is affected mainly by the air temperature and wind. Despite these suggestions, diurnal dispersal patterns of species and seasonal variation of dispersion have not been thoroughly studied. There have been numerous hypotheses about those environmental factors which influence intensity of migration and their effects, but it has not yet been studied how dispersal changes during different years under different environmental conditions. While the anatomy of ovarian development is well known, the relationship between dispersal and breeding characteristics and oviposition has not been investigated. Some data have been published about differences in dispersal between sexes (Pajunen and Jansson 1969; Benedek and Ja´szai 1972; Kecs}o and Boda 2008), but some of them only marginally discussed this subject.
Sigara lateralisis known as a migratory species flying always in mass (Fernando 1958; Popham 1964; Young 1965; Pajunen and Jansson 1969; Benedek and Ja´szai 1972; Glatz 1976, Macan 1976; Weigelhofer et al. 1992). This mainly came from its lifestyle characteristics, since it is a frequent inhabitant of astatic, temporary small pools which rapidly dry up. Generally, this species represents the highest proportion among insects captured by light-trapping (Popham 1964; Weigelhofer et al. 1992). In Hungary it is known as a frequent species (Benedek and Ja´szai 1972), and during our earlier studies it was found in the highest number of individuals (Boda, Csabai, Gido´, Mo´ra and De´vai 2003).
The aim of the present study was to investigate diel and seasonal patterns and variation of dispersal of Sigara lateralis in two different years with different environmental conditions. We wanted to reveal the possible differences in dispersal patterns of the sexes to determine environmental factors influencing dispersal, and how these factors affect the flight activity. Examining the developmental state of the ovaries, we studied the correlation between reproductive cycle and dispersal pattern.
Materials and methods
It has been proven that the highly and horizontally polarised light reflected from artificial surfaces can attract most of the polarotactic aquatic insects and insects associated with water (Schwind 1991, 1995; Schwind and Horva´th 1993; Horva´th and Zeil 1996; Horva´th et al. 1998, 2007, 2008; Berna´th et al. 2001; Horva´th and Varju´ 2004; Wildermuth and Horva´th 2005; Kriska et al. 2006a, b, 2007, 2008a, b; Malik, Hegedu¨s, Kriska and Horva´th 2008). This reaction is called positive polarotaxis (Kriska et al. 2007, 2008a). In the case of aquatic bugs, Popham (1964) first published that Corixidae find their habitats with the help of water surface reflection. The reflection–polarisation patterns of water surfaces were intensely studied by Schwind and Horva´th (1993), Horva´th (1995), Horva´th and Varju´ (1997), Ga´l, Horva´th and Meyer-Rochow (2001), Mizera, Berna´th, Kriska and Horva´th (2001) and Berna´th et al. (2002). The polarisation sensitivity and positive polarotaxis of water insects are of vital importance, because if they do not find water within one hour flight, they inevitably perish. Despite this well known phenomenon, the dispersion of aquatic insects has been only sporadically studied by a method based on the polarotaxis of these insects (Csabai et al. 2003, 2006; Csabai and Boda 2005a, b). Previously, light-trapping methods were used for dispersal
studies (Benedek and Ja´szai 1972; Weigelhofer et al. 1992). Pajunen and Jansson (1969) analysed the small-range dispersal of water bugs from data of mark and recapture samplings. However, these methods do not provide data about the individuals flying at daylight, or about diurnal variations of dispersion. Thus, we needed a new method to collect migratory individuals (Boda et al. 2003; Csabai et al. 2004). Based on this method tested in many respects, migratory aquatic bugs were trapped on a horizontal shiny black plastic sheet of 9 m63 m (Csabai and Boda 2005b). During the sampling period several such plastic sheets were used, their order was changed randomly, and an aluminium foil of the same size was applied as a control. These test surfaces were placed 40 m apart from each other, and 30 m from the water margin. Using insect aspirators, water bugs landed on the test surfaces were collected continuously by manual sampling. Individuals from the test surfaces were put into separate bottles hourly, and the date of collection was labelled. Collected animals were preserved in 70% ethanol.
Our study area was in north-eastern Hungary, in the territory of Hortoba´gy National Park, at the shore of Hagyma´s-basin marsh (47833029’’N, 20855029’’E;
10610 km UTM grid code: DT 96). The area of the Hagyma´s-basin was approximately 0.3 km2; the water depth ranged between 25 and 60 cm. The marsh was characterised by various and extremely patchy vegetation, therefore its aquatic insect community was rich and diverse.
Samples were taken from early March for 5 months in 2000 until drying up. In 2005 migratory water insects were captured from early April to late October weekly during the entire vegetation period. The water level did not decrease in 2005, when it was high from summer because of the rainy weather. Thus, the two studied years had quite different environmental circumstances. Sampling began every Wednesday, independent of weather forecast, at 8 h (¼local summer time¼UTCþ2) in early spring and in the first month ended at sunset, but from April it was carried out until the next morning (8 h) in both years. During the samplings the environmental variables (e.g. wind speed, air temperature, air pressure, relative air humidity, light intensity), possibly affecting the dispersal, were continuously registered with a cup anemometer and a Kestrel 4000 pocket weather tracker.
Collected samples were examined in the laboratory. Using a stereomicroscope, individuals were identified. Females were dissected to observe their ovarian status, which were classified into the following five categories: immature ovary (I); mature ovary without swollen ovarioles (II); swollen ovarioles without eggs (III); swollen ovarioles and eggs (IV); abdomen full of eggs (V). Numbers of eggs were counted in case of category V. This classification enabled us to reveal the possible correlation between sexual cycle and dispersal ability.
Data were analysed statistically, and evaluated graphically: cumulative numbers of individuals were plotted against sampling dates. Following normalisation with a logarithmic transformation, the differences in numbers of males and females were studied with paired samplet-test. Correlation between environmental variables and individual numbers were analysed by Spearman’s rank correlation because of the non-normal distribution of environmental data (Zar 1984). Statistical tests were done by using SPSS for Windows software package.
Results
Since in 2000 the pools dried up in July (in the 28th week), dispersal could be studied only until the end of summer (Figure 1). In 2000S. lateralisindividuals appeared on
the plastic sheets at the beginning of April (15th week), but they migrated in high numbers from the end of April (17th week); considerable dispersal took place from the end of May to mid-June (21st to 23rd weeks). The seasonality of dispersal patterns shows a typical bell-shaped curve.
Despite the extreme weather conditions in 2005, dispersal of aquatic bugs was observed for the entire vegetation period (Figure 1). Migration ofS. lateralisbegan at the end of May (21st week), increased from the 24th week, and its maximum was detected in the 28th week when more than 1000 bugs were collected. Except for days with insufficient weather conditions for migration – 18th, 19th, 20th, 23rd and 33rd weeks – mass dispersal occurred from mid-June to August (24th to 32nd weeks). Following this, S. lateralis was also found among the collected insects, but its numbers were always less than 23.
According to the data from the two sampling years,Sigara lateralis migratesen massein the summer months at dusk, in the late evening hours (Figure 2). Analysing the diurnal migration of S. lateralis in 2000, a period with a high numbers of individuals was observed during early evening and evening: migration began at 18 h (¼UTCþ2), was strengthened and continuously increased from 19 h. Within an hour the number of migratory insects increased eight-fold, and reached the maximum between 20 and 21 h. During this intense flight the capture rate of insects highly exceeded the capture rate averaged for the whole day. Inclination to disperse decreased continuously until 23 h. Analysing the results of the 18 sampling days in 2000, it was revealed that 90% of the collected individuals migrated from 18 to 23 h. At around midnight the number of insects decreased further, dispersal totally
Figure 1. Seasonal flight activity patterns of Sigara lateralis(Leach, 1918). Vertical axis: number of individuals captured. Horizontal axis: time (number of weeks beginning from January). The bright and the dark columns refer to the data of 2000 and 2005, respectively.
stopped, and migration of bugs was not observed at dawn. Flight began at 4 h, but in an inconspicuous numbers of insects.
In 2005 two dispersal peaks were observed forS. lateralis: evening flight began at 19 h, increased from 20 h, and reached its maximum at 21 h (Figure 2). The number of collected insects decreased continuously up to 22 h. Evaluating the data of 29 sampling days in 2005, 77% of the water bugs captured in this year dispersed from 19 to 22 h. After 23 h the number of flying individuals diminished considerably, but dispersal did not end. At dawn the observed flying activity was weaker than in the evening, but it was stronger between 4 and 6 h (number of individuals N4129) than during daytime hours (N548). 10.4% of the individuals collected in 2005 migrated between 4 and 6 h.
Among the measured meteorological variables, only the wind speed (Spearman’s rank correlation: r¼70.410, p ¼0.000), light intensity (r¼70.234, p¼0.001) and air temperature (r¼70.146,p ¼0.006) showed significant negative correlation with the number of flying individuals. Relative air humidity and air pressure did not show significant correlation.
In 2000 the rate of sexes among the collected insects was R¼male:female¼
46:54. In this year females exceeded males on each sampling day except during the 20th, 23rd and 27th weeks; the difference was marginally significant (paired samplet -test: t¼2.235, df¼10, p ¼0.049). Rates of males and females varied between
R¼29:71 and 59:41. Regarding only the days with high numbers of individuals collected (N4100), Rvaried between 46:54 and 48:52.
In 2005Rwas 40:60. Females were trapped on the test surfaces in higher numbers (except the 22nd, 32nd and 36th weeks, when the numbersNof collected insects were very small); the difference was also significant (paired sample t-test: t¼2.269, df¼18,p ¼0.036).Rvaried daily from 25:75 to 75:25. Because of some days with
Figure 2. Diel flight activity patterns ofSigara lateralis(Leach, 1918). Vertical axis: number of individuals captured. Horizontal axis: time (hours of day). The bright and the dark columns refer to the data of 2000 and 2005, respectively.
small numbers of collected insects (N5150), these results cannot be relevant. On those days when high numbers of bugs were trapped (N4150),Rvaried from 41:59 to 37:63. Diel and seasonal dispersal patterns of the sexes did not show any differences.
Considering the ovarian status in 2000 (Figure 3), most of the females belonged to the ovarian categories II and V. The proportion of category V decreased
Figure 3. Seasonal changes of the ovarian status in 2000 and 2005. Barely dotted columns refer to the data of immature ovary (I); checkered columns to the data of matured ovary without swollen ovarioles (II); horizontally striped columns to the data of swollen ovarioles without eggs (III); diagonally striped columns to the data of swollen ovarioles and eggs (IV); black columns to the data of abdomen full of eggs (V).
continuously with time, but it was never less than 25%. The average number of eggs was 6.3, the maximum was 24. Females belonged to category II with 15–37% of the migratory females, the highest proportion of them was observed during the 24th week, and their number did not decrease considerably (35%) until the marsh dried up. After their appearance, the rates of females in categories III and IV increased continuously, but never exceeded 20%. Only eight individuals were collected from category I, their rate being less than 0.05%.
In 2005, distribution among the ovarian categories showed a more diverse pattern (Figure 3). The proportion of the most frequent category V varied between 33 and 100%, except in the last migratory period, when it had the highest proportion. The average number of eggs was 4.3, the maximum was 22. After the 22nd–23rd, weeks the proportion of category IV was steadily 15%. The most variable category was III, in which case a clear tendency was not detected because of the high fluctuations of the proportion. Females belonging to this category are characterised by their proportion of 8–31%. Females of category II appeared in 14% in the 22nd–23rd weeks, but after one month this category presented 33% of the females. Following this, the proportion of category II decreased below 5%, and disappeared. On the last days suitable for migration, 46% of the females belonged again to this category. Category I did not exceed 2% during the mass dispersal, but after that period 15–33% of the females belonged to this category. In the 38th–39th weeks females were not collected.
Discussion
Dispersal of aquatic bugs is driven by a number of physical, environmental, ecological and physiological factors (Savage 1989). Studying all of them is beyond the capability of a team. In the present study we analysed some aspects of the dispersal ofSigara lateralis.
Since our sampling was frequent and long-term, it enabled us to study the seasonal dispersal pattern. Seasonality showed typical bell-shaped curves in both years, but in 2005 both activity peaks began three weeks later. The reason for this may be the fact that the annual mean air temperature was much higher in 2000, when the spring began earlier, thus the weather conditions were suitable for dispersal earlier. At the same time, this dry weather without rain led to the drying up of the marsh in the 28th week. However, it is interesting to mention that although the environmental factors did not inhibit flight, the drying up did not result in a distinctive new dispersal wave.
Considering flight, in 2000 the most active period was between the end of May and the end of June (21st–26th weeks) until drying up, while in 2005 it occurred from mid-June to mid-July (24th–32nd weeks). On the warmer days of September in 2005 water bugs flew in small numbers, a stray individual was collected in October, but considerable dispersal activity of S. lateralis was not observed in the spring. According to Savage (1989), spring and autumn are the most suitable seasons for flight, especially for species of temporary habitats, such asS. lateralis for instance. Popham (1964) described August and September as the most suitable months for water insect migration, while Benedek and Ja´szai (1972) observed mass migration from June to August. Differences might have resulted from geographical differences. Since our sampling sites were in the same region as that of Benedek and Ja´szai (1972), it is evident that our results more or less correlate with theirs. However, the
dispersal pattern is geographically heterogeneous, because it mostly depends upon the habitat conditions characteristic of the given area. Moreover, our results show that the yearly changes in the same habitat may result in variations of the seasonal dispersal pattern. From our results mass-dispersal of the seasonal pattern can be determined only for a longer period, within which temporary environmental factors (meteorological conditions) influence the number of flying insects.
Owing to our sampling method, based on polarotactic water detection, the diel dispersal pattern of Sigara lateralis could be described properly. Sigara lateralis
migrates in the highest number during the evening hours. The dispersal pattern of this species has only one peak in the early/late evening from 18 to 22 h. This result accords with the observations of previous researchers (Richardson 1907; Poisson et al. 1957; Fernando 1958; Richard 1958; Popham 1964; Young 1966; Macan 1976), and this period is optically the most suitable for dispersal by positively polarotactic aquatic insects (Berna´th et al. 2004; Csabai et al. 2006). However, it is important to declare that different species have different flight ability and activity. Based on data from studies of several flying aquatic insect species, Csabai et al. (2006) described four typical dispersal patterns (curves with typical and discrete diurnal activity peaks) having activity peak(s) (i) in morning, (ii) in the evening, (iii) both in mid-morning and the evening, and (iv) around noon and in the evening. These activity maxima are quite general and cannot be explained exclusively by the daily fluctuations of the air temperature, air humidity, wind speed and risk of predation, which are all somewhat stochastic.
Based on dispersal characteristics (i.e. dispersal pattern), many aquatic insect species were classified to these four categories (Csabai et al. 2006). However, there can be differences in dispersal patterns among species belonging to the same category: variations in rate and time of the dispersal peak(s), for example. Since many factors can affect dispersal at every moment, the dispersal activity on a given day may be different from the main dispersal pattern. Differences caused by the environmental factors could become permanent. The consolidated differences may influence the diurnal pattern of the whole year, as in the case ofS. lateralispresented in this work. Compared with 2000, in 2005 the dispersal peak of this species occurred one hour later, which was observed over the year, and it was a permanent change of the diurnal dispersal pattern only. Beyond that, in 2000 a minimal number (N510) of individuals migrated during every hour except the evenings. On the other hand, in 2005 dispersal with higher numbers but not mass was observed at dawn. This local peak could be caused by daily differences in the environmental parameters, since it was observed on each day when high numbers of individuals were collected. Consequently, this can be explained as an explicit change in the diurnal dispersal pattern. As a result of this, depending on the meteorological conditions, diurnal dispersal patterns ofS. lateralis can change year by year. In the case ofS. lateralis
the dispersal pattern changed from the type with one peak in the evening to the type with peaks both during mid-morning and evening. Regarding variations because of geographical differences, we suggest that switches can occur between the above-mentioned four dispersal patterns discovered by Csabai et al. (2006) even in a given species.
Weigelhofer et al. (1992) studied how variations in air temperature, wind speed and air humidity influence aquatic insect migration. Among their measured variables only the temperature showed a significant correlation with migration; below 12.48C flying of Corixidae was not observed with the use of light-trapping. In accordance
with this, we also observed a significant correlation with the change of air temperature, andS. lateralis was not collected bellow 12.98C; furthermore, we also did not find correlation between air humidity and dispersal activity. At the same time, wind speed was found to correlate significantly negatively with the number of migratingS. lateralisindividuals. A previous study analysed the effect of wind speed on dispersal of aquatic bugs (Csabai and Boda 2005a). Above 12 km/h wind speed, migration of water beetles and bugs were not observed. Wind speed between 6 and 12 km/h showed a significant negative correlation with the number of flying insects, but below 6 km/h wind speed had no controlling effect on dispersal. However, in this study only the total numbers of migratory insects were analysed, species were not studied separately. The wind speed may variously influence species with different size and flying ability. Our result that we did not observe migration ofS. lateralisabove 11.3 km/h agreed with Weigelhofer et al. (1992) and Csabai and Boda (2005a). Nevertheless, S. lateralis is more sensitive to the effect of wind speed than other aquatic insect species, because above 2 km/h the correlation was negatively significant between wind speed and dispersal intensity (r¼70.247, p¼0.003). Below this value wind speed had no effect on flight (r¼70.094,p ¼0.522). This is understandable, because this species is a small-sized insect.
Some mention of the effect of light intensity on water insect dispersal have also been published, but without quantitative data (Fernando 1958; Popham 1964). It is again understandable that the change in the number of migrating S. lateralis
correlated significantly negatively with light intensity, since this species disperses at dusk.
Taking the above facts into account, one can see that meteorological variables exceeding given values are able to considerably limit the number of flying insects. Strong wind and heavy rain can completely inhibit dispersal. We observed this in the seasonal dispersal pattern on the 16th, 19th and 24th weeks in 2000, and on the 23rd, 27th and 31st weeks in 2005.
As was mentioned, not only external, but also internal factors (e.g. reproductive features, population biological factors) may influence migration. To study these factors is quite difficult. Investigating the sex rates of flying insects and their ovarian status is a method by which the relation between reproductive cycle and migration can be revealed. Based on a previous study using light-trapping (Benedek and Ja´szai 1972), S. lateralis males are more active than females. During the summer mass dispersal (June–August) rates of males exceeded 70%, but when migration was weak (May, September, October) females had the same rate. Contrary to this, females outnumbered males (by ca. 60%) on our sampling days with high numbers of individuals; males exceeded females only on three days, when small numbers of insects were collected. Contrary results could have been obtained with the use of different sampling methods, or for different habitat characteristics.
In 2005, when examining the ovarian status of female S. lateralis, significantly higher numbers of individuals with eggs were found, while females in the other ovarian categories did not represent a remarkable proportion. Different distribu-tion among the ovarian categories was revealed in 2000. Those females represented a higher proportion, which had mature ovaries without swollen ovarioles or eggs. During the last two weeks before drying up of the area, higher numbers of individuals belonging to category II were caught. Females belonging to the other categories remained in small numbers. There could be several explanations for this phenomenon: one may be the diapause of females, another could be the
two-generation hypothesis, and a third can be a drastic effect of certain environmental factors. The first hypothesis is unlikely, because the dispersal patterns with typical bell-shaped curves were the same in both years. In case of species with two generations, the first summer generation consisting of non-migratory, reproducing individuals is followed by a second, overwintering generation with high dispersal ability (Savage 1989). The stability of habitats may considerably alter this. In fact, study of the relationship between dispersal ability and habitat characteristics cannot be separated from the examination whether one or two generations of the species appeared in a year, because flying polymorphism can change during the life cycle in a year (Savage 1989). Sigara lateralis has been described with a bivoltine life cycle (Crisp 1962; Savage 1979). However, based on the determined migration patterns we did not observe this. Since larvae are not able to fly, and we have no data about the proportions of larvae and imagos, we cannot contradict it. In 2000, because of the dry-up, there was no observable migration in early spring, and in 2005 migration was not considerable. According to Benedek and Ja´szai (1972), there was no correlation between the number of migrating individuals and the proportion of insects which are able to fly within the population. Pajunen and Jansson (1969) described that flightless adults existed only in deep pools. In temporary habitats flightless adults represent a small proportion. The seasonal dispersal pattern revealed from our study did not clarify that S. lateralis has a uni- or bivoltine life cycle.
In 2000 the factor which influenced migration most seriously was exsiccation of the marsh. Since high numbers of females belonging to category II were caught on days before drying out, we suggest that continuous decrease of the water level was a stress that facilitated the individuals – also those with no mature ovarioles or eggs – to seek other habitats. In 2005, the water level of the marsh was stable. In this year we did not observe increasing numbers of females of category II.
Summarising our results, we suggest that females with eggs are the most active during mass dispersal. After the breeding period, females with swollen ovarioles without eggs begin to fly. This is in contrast with the results of the previous study of Pajunen and Jansson (1969) who reported that dispersal mostly occurs in periods when the population is immature.
To determine the dispersal activity pattern of every aquatic insect species, and to reveal the factors governing and influencing this pattern requires long-term and more complex studies. The dispersal activity characteristics discussed in this paper considered only the speciesS. lateralis. The dispersal can be influenced by a number of factors. Our future goal is to clarify this phenomenon and to reveal the dispersal behaviour of other taxa as completely as possible.
Acknowledgements
This work was supported by the grant OTKA F-046653 received by Z. Csabai from the Hungarian Science Foundation. Special thanks are due for valuable comments on the manuscript to Dr Eszter Csoma, Dr Gyo¨rgy Kriska and Dr Ga´bor Horva´th and also to two anonymous reviewers. Authors’ thanks are also due to all colleagues and students who gave us any help in the field, namely Dr Arnold Mo´ra, Dr Zsolt Gido´, Kla´ra Kecso, Enik} o Kova´cs,}
La´szlo´ Papp, Mo´nika To´th, Andrea Erdosi, Lı´via Kosztka, Erika Barnucz, Csilla Ko¨ve´r,}
Bala´zs Luka´cs, Dr Pe´ter Taka´cs, Tı´mea Szasza´k, Anna Kira´ly, Tibor Varju´, Korne´l Szila´gyi, Rita Fo¨ldesi and A´gota Csirik. We are thankful to the Directory of the National Park of Hortoba´gy for the authority, and the University of Debrecen, Department of Hydrobiology for their cooperation.
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