- University of Toronto
The exoskeletal morphology, muscular organization, and innervation patterns of the tymbals of seven sound‐producing species of tiger moths (Arctiidae) were compared with the undifferentiated episterna of two silent species. At least three muscles are involved in sound production: the tymbal muscle, pv2, and the accessory muscles, pv1 and/or pv6. All of the tymbal muscles are innervated by the IIIN2a branch of the metathoracic leg nerve, which contains two axons larger than the others. Backfills of the tymbal branch of the IIIN2a reveal a medial sensory neuropil and a population of five ipsilateral motor neurons whose somata are clustered into three groups along the anterior edge of the metathoracic ganglion. The dendritic arborizations of the motor neurons extend to the ganglionic midline but are separate from one part of the auditory neuropil observed in other noctuoids. The study concludes that the arctiid tymbal reveals only minor modifications (e.g., cuticle thinning) of the episterna of silent moths and represents a primitive form of the tymbal compared to those of the Cicadidae.
The following arctiids were used: (1) sound-producing: Cycnia tenera (n = 12), C. oregonensis (n = 1), Halysidota tessellaris (n = 12), Apantesis virgo (n = 2), A. virguncula (n = 2), Pyrrharctia isabella (n = 3), Euchaetes egle (n = 9); and (2) silent: Spilosoma virginica (n = 8), S. primum (n = 3). Moths were obtained by field collections at the Queen's University Biology Station (Chaffey's Lock, Leeds County, Ontario, Canada), where certain of the studies were conducted. Gross tymbal neuromuscular morphology was studied with pterothoracic (meso + metathoracic) dissections following brief exposures to a saline solution (TES saline (Strausfeld et al., '83)) containing 0.05% Janus Green B. Precise muscle innervations were determined by forefilling (Altman and Tyrer, '80) metathoracic leg nerves with a 10% solution of cobaltous-lysine (Gallyas et al., '78; Edward Arbas, personal communication), followed by precipitation in ammonium sulfide or hydrogen sulfide-saturated saline and silver intensification (Davis, '82). Central nervous system (CNS) origins of episternal motor neurons were determined by backfilling tymbal nerves with Lucifer Yellow (Strausfeld et al., '83) or cobaltous-lysine (10%, 8 h, 10°C). Several fills were done using low cobalt concentrations (<2%) at short times (2–4 h, 10°C) to control for artifactual interpretations due to parallel axonal filling (Altman and Tyrer, '80). All preparations were dehydrated in ethanol, cleared in methyl salicylate, and drawn with a camera lucida-equipped Wild M20 transmission fluorescence microscope. Some fills were embedded in methacrylate (JB-4; Polysciences, Inc.) and sectioned at 10-μm thickness. Muscles were identified by origin, insertion point, and innervation and named using the nomenclature of Nüesch ('53, '57) and Treat ('59).