Larval habitats and longitudinal distribution patterns of Cordulegaster heros Theischinger and C. bidentata Sélys in an Austrian forest stream (Anisoptera: Cordulegastridae)

From May 1997 to April 1998 larvae were recorded at the Weidlingbach, a fourth order tributary of the Danube nr Vienna, at 12 sampling stations from source to mouth. From the 14 larval instars reported for the genus, 5 (F to F-4; based on head width) could be identified in both spp.; head widths of $ larvae were significantly larger than in 3. — Both spp. were most abundant within medium sand sediments with a mean grain size (Q J of 2.04 mm in C. bidentata and 2.79 mm in C. heros. Mean water depths and nose current speeds measured at larval microhabitats were 4.4 cm and 2.3 cm s’1 ( (C. bidentata) and 5.6 cm and 2.6 cm s'1 ( (C. heros). During the winter months larvae chose the water depths slightly deeper than during summer. Throughout the observation period, a high proportion of the larvae (C. bidentata; 70-100%; C. hems'. 41-90%) were burrowed in sandy sediments, either totally or displaying the typical ambush posture with only head and anal pyramid visible. In winter, the proportion of burrowing larvae was insignificantly higher than in summer. — C. bidentata larvae were most abundant near the sources, preferring first order stream sections (discharge 0.1-3.2 Is1) with high hardness (up to 34 German degrees) and conductivity (up to 1100 pS cm1) and a high proportion of fine sediments. Although C. hems larvae were also collected at such first order sites, they reached their highest abundance (larval density up to 7.84 specimens per 10 meter shore length) at second order stream sections (discharge 0.3-6.0 I s1) with lower hardness and conductivity and a higher proportion of coarse sediments.