MODE OF SWARMING IN RELATION TO REPRODUCTIVE ISOLATION IN MAYFLIES

Mayfly males swarm, that is they fly in a fixed pattern by a specific object, the swarm marker. Females orientate to the same markers. Lepto phlebia marginata mayflies were observed to orientate to two kinds of objects in a single locality in central Finland: to trees and to horiz ontal pale objects on the ground; when dispersed or moved to the other type of marker, they returned to their former orientation. Tree swarm ing is by far the most common mode of swarming, but some horizontally orientating populations were found. Sympatric populations are genetica lly and morphologically distinct, whereas other populations appear to have some gene flow between the swarming types. The tree-swarming mode appears to be primitive and the horizontal mode derived; wind rather than predation is the factor favoring swarming close to the ground. Sw arming constitutes an effective mechanism of premating isolation in ma yflies.