The anti-bat strategy of ultrasound absorption: the wings of nocturnal moths (Bombycoidea: Saturniidae) absorb more ultrasound than the wings of diurnal moths (Chalcosiinae: Zygaenoidea: Zygaenidae)

The selection pressure from echolocating bats has driven the development of a diverse range of anti-bat strategies in insects. In this study, we investigate how some moths may utilize their wings to dampen their telltale echoes in order to become inconspicuous to bats. The absorption factors of the wings of three saturniid (Bombycoidea) and three chalcosiine (Zygaenoidea: Zygaenidae) species were measured over the ultrasonic frequency range 20 – 100 kHz with a small reverberation chamber. The absorption factors of the male and female saturniids peak significantly higher than the absorption factor of the chalcosiines, presumably because the chalcosiines are predominately diurnal and consequently experience less intense selection pressure from bats than the mainly nocturnal saturniids. However, the ultrasound absorbance of the chalcosiine wings is not negligible. Following a phylogenetic analysis on the character state of diurnality/ nocturnality in the Zygaenidae, we propose that diurnality in the Chalcosiinae is pleisiomorphic (retained); hence, the absorbance of their wings is not a vestigial trait from an ancestral, nocturnal form but an adaptation to bat activity that overlaps theirs. On a within-species level, the absorption mechanisms of the saturniids Argemma mittrei and Samia cynthia ricini are sexually dimorphic – a trait attributed to differential bat predation on males and females. In the female S. c. ricini the higher absorption factor corresponds to a detection distance by bats that is at best 20-30% shorter than that of the male. Although sexual dimorphism driven by predators that hunt with visual cues has been demonstrated in the Saturniidae before, this is the first report of sexual dimorphism due to differential selection pressure from echolocating bats.