Larvaal habitat van de Beekrombout (Gomphus vulgatissimus) in enkele Nederlands Limburgse beken

The locations and timing of emergence of Gomphidae in the river Rur have been thoroughly investigated in recent years. It is assumed that gomphid dragonflies generally emerge close to their larval habitat. Since surveys of actual larval habitats in the Netherlands have been very rare, four transects of the Rur were checked for the presence of larvae of Common Clubtail (Gomphus vulgatissimus) during the years 2006-2009. The aim of this study was to discover what type of substrate the larvae prefer, and if the places where the dragonflies emerge are indeed situated close to the actual larval habitats. The larvae were caught with a hand brailer, which is normally used for fish and amphibian surveys. For each of the larvae caught, the type of substrate in which it was caught and its distance to the river bank were noted. Most of the 615 larvae were found within one metre distance of the river bank, and almost 50% even within 0,5 metre from the bank. The largest distance from the bank at which larvae were caught was 5 metres. Most larvae were found in mixed substrates, dominated by a combination of silt and detritus. Only a few larvae were found in substrates dominated only by silt, detritus, sand or gravel. Because the river Rur is too deep to investigate the whole streambed, it was not certain if the results were a good representation of the actual situation. Therefore, in 2009 and 2010 three smaller streams were investigated, using the same method as in 2006-2009. During this period, 51, 108 and 62 larvae were caught in the rivers Swalm, Vlootbeek and Worm respectively. These results were similar to the situation in the river Rur. In the three streams most larvae (53% to 63%) were caught within 0,5 metre from the bank as well. Same as in the Rur most larvae were found in mixed substrates, which were dominated by a combination of silt and detritus. In substrates that consisted of just one type, almost no larvae were caught. Most likely, the preference for locations near the river banks is a result of the greater diversity in types of substrate. Further up the streambed, the streaming velocity is too high for sedimentation of silt and detritus. Therefore, the diversity of types of substrate is much higher along the banks. The majority of larvae of Gomphus vulgatissimus were found close to the banks, and it is most likely that they emerge near the larval habitat.