Anatomic and acoustic sexual dimorphism in the sound emission system of Phoenicoprocta capistrata (Lepidoptera: Arctiidae)

Abstract

Both sexes of Phoenicoprocta capistrata have functional tymbals. The scanning electron microscopy revealed differences in the morphology of these organs in males and females. Male tymbals have a well-developed striated band, constituted by 21 +/- 2 regularly arranged striae whereas female tymbals lack a striated band. This type of sexual dimorphism is rare in Arctiidae. The recording of the sound produced by moths held by the wings revealed that while males produced trains of pulses organized in modulation cycles, females produced clicks at low repetition rate following very irregular patterns. Statistically, there are differences between sexes in terms of the duration of pulses, which were 355 +/- 24 micros in the case of males and 289 +/- 29 micros for females. The spectral characteristics of the pulses also show sexual dimorphism. Male pulses are more tuned (Q(10) = 5.2 +/- 0.5) than female pulses (Q (10) = 2.7 +/- 0.5) and have a higher best frequency (42 +/- 1 kHz vs. 29 +/- 2 kHz). To our knowledge, this is the first report on an arctiid moth showing sexual dimorphism in tymbal's anatomy that leads to a best frequency dimorphism. Males produce sound at mating attempts. The sounds recorded during mating are modulation cycles with the same spectral characteristics as those recorded when males are held by the wings. The morphological and acoustic features of female tymbals could indicate a process of degeneration and adaptation to conditions under which the emission of complex patterns is not necessary.