INTERSPECIFIC COMPETITION AND HABITAT SELECTION BY THE RIVERINE DRAGONFLY ONYCHOGOMPHUS-UNCATUS

1. Substratum selection by the burrowing larvae of the dragonfly Onych ogomphus uncatus was examined in an artificial laboratory stream at di fferent larval densities and in the presence of one of three other dra gonfly species. 2. The larvae of O. uncatus, as well as those of the o ther species, clearly preferred gravelly sand substratum rather than g ravel or stone. 3. At low larval density (71.4 m(-2)) in the stream, 8 3% of the O. uncatus were found in gravelly sand. An increase of larva l abundance in the stream to 202.4, specimens m(-2) resulted in greate r density in all substrata, but this increase was proportionally lowes t in gravelly sand. 4. The presence of a second species had various ef fects on the microdistribution of O. uncatus. In the presence of Gomph us simillimus or Cordulegaster boltonii immaculifrons the distribution of O. uncatus changed significantly; their density increased in the n ormally less preferred substrata. This effect is interpreted as asymme tric interspecific interference. The presence of a third species, Onyc hogomphus forcipatus unguiculatus, had no effects.