SEXUAL DIMORPHISM IN RELATION TO SWARMING AND PAIR FORMATION PATTERNSIN LEPTOCERID CADDISFLIES (TRICHOPTERA, LEPTOCERIDAE)

North European Leptoceridae (Trichoptera) perform three types of swarm ing flight patterns: (1) swarming males of Athripsodes and Ceraclea fl y in horizontal zigzag patterns over the water surface, (2) the Mystac ides spp. perform vertical zigzag movements, and (3) the flight of mal es of Triaenodes unanimis McLach. is a mixture of the horizontal and v ertical zigzagging. Also three groups of pair formation behavior can b e distinguished. In the first group, of Athripsodes and Ceraclea, the females fly into the male swarms, where they are grasped and carried t o the riparian vegetation by the flying males with the females hanging upside-down in genitalia coupling. In the second group, a Mystacides female is caught by a male, when approaching a swarm and both use thei r wings to fly in tandem to the shore where they copulate. In the thir d group, of Triaenodes bicolor (Curt.) and Oecetis lacustris (Curt.), the males fly searching for females sitting on aquatic plants and when a female is found the male lands and they copulate immediately while clinging to the plant. The different swarming and mating behaviors mig ht have favored selection for three types of sexual dimorphism: (1) lo nger forewings in males than females in species which fly in copula, ( 2) larger eyes in males of the vertically zigzagging species, and (3) much smaller males in the group where males search for females sitting on aquatic plants. In the second group approaching females are detect ed by males before reaching the swarm and in the third group the femal e almost always mates with the male which is the first to find her. In conclusion, we suggest that females of Athripsodes and Ceraclea have a greater choice among swarming males than do females of Mystacides, T . bicolor, and O. Lacustris.