POSTEMERGENCE MIGRATION OF STONEFLIES (PLECOPTERA) INTO THE NEARBY FOREST

1. The post-emergence lateral migration of both sexes of eight stonefl y species was examined in a dystrophic, fourth-order forest river in e astern Finland. 2. For this purpose, 7351 stonefly adults were collect ed with eighteen trunk funnels positioned in rows of six at distances of approximately 1, 15 and 60 m from the river. A further 1880 adults were also caught from the vegetation of the bank zone by sweep netting and with slit traps. 3. The species could be grouped into two types w ith respect to migration distance: Isoperla difformis, I.grammatica an d Leuctra fusca tended to stay in the bank area, whereas Nemoura flexu osa, N. avicularis, Amphinemura borealis, L. hippopus and N.cinerea te nded to disperse into the forest, so that the majority were found some distance away from the shore. 4. The males of the leuctrids, N.flexuo sa and N.cinerea migrated farther than the females. 5. The sex ratio w as significantly biased in all species except N.flexuosa. The isoperli ds, leuctrids and A.borealis showed a significant predominance of fema les in the trunk funnel catches, but males were significantly dominant in N.avicularis and N.cinerea. The material caught by other methods r eversed the ratio for I.difformis and N.avicularis, Comparison of the sex ratios of the species with other reports revealed marked variation and deviation from unity, much of which could be attributed to bias i ntroduced by the sampling methods. 6. Lateral migration seems to be th e first phase in the colonization cycle, although the latter as such w as not studied here. Adults of Euholognatha species migrate farther th an those of Systellognatha, a difference of which may be due to their ability to feed as adults.