Odonata - Entomological Society of Latvia - Latvijas Daba

habitats of open or wooded bogs. According to Pajunen (1962), Leucorrhinia dubia males and
females stay away from water during the prereproductive period and gather at the breeding waters
and their immediate surroundings for mating and oviposition. TWINSPAN analysis identified
Lestes sponsa and Leucorrhinia albifrons as indicator species of wet habitats. These species prefer
to stay at shores of water bodies and above the water. The groups separated by occurrence of Anax
imperator and Aeshna grandis were too small (three and two sampling plots) to allow to make
conclusions.
4.3. THE OCCURENCE AND POPULATION DENSITY OF GOMHIDAE LARVAE IN RIVER GAUJA
Distribution From the four species of Gomphidae family three were found in the research
near the Gauja River – Gomphus vulgatissimus, Onychogomphus forcipatus and Ophiogomphus
cecilia. The most widespread species was Gomphus vulgatissimus. Although, Gomphus flavipes
was not found in the course of research, based on the literature data (Спурис 1956), this species has
been registered near the Gauja River. This may be explained by the fact that the species is mostly
found near large rivers and only found in the lower Gauja (Спурис 1956). It is possible that the
species is less numerous than other Gomphidae species. In literature it is noted that large
populations appear only near large rivers (Dijkstra 2006; Bernard et al. 2009). In Latvia
G. vulgatissimus is a widespread species, while O. forcipatus is moderately spread. Both species
can be found near the Gauja River and it is most likely due to the presence of various micro habitat
substrates along the river. O. forcipatus is less often found in the middle and lower Gauja due to the
gradual decrease of swift and rocky stretches in the direction from the upper to the lower Gauja
(Avotiņa 1995; Kalniņš, Urtāns 2007). Based on the larvae data, O. cecilia is rarely found in the
Gauja River, however, when combining all available (including literature) data on larvae
observations and on imago observations, there are significant differences in the distribution of
O. cecilia. The larvae in the Gauja River were found only in five locations, but adult individuals
were observed almost throughout the river (in 23 squares). It is likely to indicate low density of
larvae, as several authors note that O. cecilia is a physiologically and ecologically flexible species
(Suhling 1994; Müller 2002) and populates also specific habitats like the surfaces of large rocks
(Müller 2004).
Occurrence The most frequently represented in the samples were the G. vulgatissimus
larvae, while the best represented substrates were sand, gravel and mud. The occurrence of the
species depends on various abiotic and biotic factors (Korkeamäki, Suhonen 2002). During the
research the effect of the riverbed substrate was analysed. Gomphidae larvae are common for ritral
rivers with no or scarce vegetation (Müller 2002). The micro habitate substrate components found
in the sample are characteristic to the freshwater habitat types represented in the Gauja River
(Kalniņš, Urtāns 2007). When comparing the substrate distribution in the Gomphidae samples and
in all samples, several differences were found (Figure 9). In Gomphidae samples the proportion of
pebbles and rocks is larger, which accounts for the presence of O. forcipatus, as, individually
analysing G. vulgatissimus and O. forcipatus, a distinct relation of O. forcipatus with pebble and
stone riverbeds in rapid river sections was found (Figure 10). When comparing the habitat
descriptions given by different authors (Table 7), it is evident that most frequently noted is the size
of the river, less popular habitats or substrates populated by larvae. While the literature accounts
(Спурис 1956; Askew 1988; Spuris 1993) are generally concurrent with the results of the research,
they are based on imago observations, which do not allow the evaluation of the importance of
specific habitats and substrates. For example, O. cecilia in the locations in the Gauja River and
other water courses, where larvae have been found (Kalniņš, Inberga-Petrovska 2005), a thin layer
(1-2 cm) of mud above sand was found (including the sites, where the qualitative samples were
collected). Apparently, mud is an important component of the substrate of larvae habitats; however,
with accumulation of mud, the habitat is no longer suited for the species (Müller 2002). The
identification of O. cecilia larvae in two (out of 280) samples indicates the rarity of the species. The
occurrence analysis shows that the species are not found in all seemingly suitable habitats and there
are additional limitations.
71