BAT DEFENSE IN LEKKING GHOST SWIFTS (HEPIALUS-HUMULI), A MOTH WITHOUTULTRASONIC HEARING

The Hepialidae represents an early branch of the Lepidoptera, whose me mbers lack the ultrasonic hearing and other obvious predator defence s ystems present in other extant moths. I observed lekking male ghost sw ifts: Hepialus humuli, being exploited by northern bats, Eptesicus nil ssonii, over a hayfield in southern Sweden. Because the moth's display flight was restricted to a brief (30 min) period at dusk, they avoide d most predators temporally but were exposed to early emerging aerial- hawking bats. Against these, they apparently employed 'acoustic crypsi s: achieved by flying close (<0.5 m) to the vegetation, thereby hiding from the bats among clutter (echoes returning from the background). N evertheless, the predation risk for the displaying moth males was very high (20% per night), mainly because they sometimes left the safety o f the vegetation. The lack of 'advanced' predator defence mechanisms i n H. humuli requires alternative defence strategies, which, however, r estrict the behavioural repertoire and still carry a high predation ri sk.