Interspecific competition and habitat selection by the riverine dragonfly Onychogomphus uncatus

1. Substratum selection by the burrowing larvae of the dragonfly Onychogomphus uncatus was examined in an artificial laboratory stream at different larval densities and in the presence of one of three other dragonfly species.
2. The larvae of O. uncatus , as well as those of the other species, clearly preferred gravelly sand substratum rather than gravel or stone.
3. At low larval density (71.4 m^–2) in the stream, 83% of the O. uncatus were found in gravelly sand. An increase of larval abundance in the stream to 202.4 specimens m^–2 resulted in greater density in all substrata, but this increase was proportionally lowest in gravelly sand.
4. The presence of a second species had various effects on the microdistribution of O. uncatus . In the presence of Gomphus simillimus or Cordulegaster boltonii immaculifrons the distribution of O. uncatus changed significantly; their density increased in the normally less preferred substrata. This effect is interpreted as asymmetric interspecific interference. The presence of a third species, Onychogomphus forcipatus unguiculatus , had no effects.