The generation of ultrasonic signals by a New World arctiid moth

Abstract

The sounds generated by the metathoracic tymbal organs of the arctiid mothMeles laodamiaDruce are described and related to their structure. Each tymbal is a swollen air-filled katepisternal sclerite with a vertical band of 15 to 20 striae on its anterior outer face. When compressed vertically by the basalar musculature it buckles inwards along the line of the striated band, which may be regarded as a linear array of 'microtymbals ’. Each stria produces a click which excites the whole structure into damped resonance and yields a briefpulseof sound with a half-decay time of about 80 μs. Muscle contraction and relaxation produce acycleofmodulationboth of pulse repetition rate, up to 1200/s, and of frequency over about an octave as a result of stress changes in the band. The audible fundamental component is relatively weak, and most energy is generated between 30 and 90 kc/s.Burstsof tymbal activity comprise from one to 20 modulation cycles. The acoustic performance of the tymbal organ is compared to that of a simple mechanical model. Previous accounts of the structures are summarized and the possible significance of the sounds discussed.