Hearing in moths has evolved to enable them to detect and evade echolocating bats. Thus most moths are silent. Among Arctiidae click production with tymbal organs on the metathoracic episternites is fairly common, but in most species this is also part of the interaction with bats. However, a few moth species produce sound and use their hearing for intraspecific communication. Generally, the noctuid species described so far produce sound by some kind of stridulatory mechanism, often involving the wings and legs. Here we describe quite another mechanism of sound production in the male noctuid moth Pseudoips fagana (Fabricius) of the subfamily Chloephorinae, involving a ventral "tymbal'' organ centrally located on the ventral part of the basal abdominal segment. P. fagana, the green silver line, is common in northern Europe. The sound production has been detected while the moths my around the tree-tops in the dusk. The clicks sound like electric sparks to the human ear and may be heard at several meters distance. We recorded clicks from males in stationary flight in the lab. The moths would only click while flying in place. We elicited the clicks by very intense ultrasonic sound pulses. The moths produce short series of clicks each lasting around 0.3 to 0.4 ms with maximum sound energy around 30 kHz. The sound pressure level was intense, 119 do SPL at 2 cm. The sound producing organ is buried deep in a groove, but may be observed if the moth is placed ventral side up and the abdomen is bent dorsally. The hearing of P. fagana was measured by recording extracellularly from the auditory nerve. Both males and females were most sensitive around 30 kHz, with a threshold of about 35 do SPL, thus matched to the spectrum of the sounds. There are no behavioural observations on these moths, but we believe it most likely that the sounds are part of the sexual display. Research supported by the Danish National Research Foundation.
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