Tympanal organ
The tympanum is a specialized hearing system, in which according to Richards (1932), is divided into three types in the Lepidoptera. The first type confined to that group of families known as the Noctuoidea is located on the metathoracic segment; the second type are the first abdominal type (on the first abdominal segment) found in the Geometroidea, Drepanoidea and many of the Pyralidoidea; and third type are the second abdominal type (on the second abdominal segment) confined to the small superfamily the Uranioidea. All of the other groups of moths, as well as the butterflies, lack any resemblance of this organ. In the Noctuoidea the tympanum is located in the posterior part of the metathorax in the region between the epimeron and postnotum, or simply near its junction with the abdomen (Fig. Tympanal Organ: Grimaldi & Engel 2005 ).
Tympanal organs in lepidopterans are similar to those in most insects, being comprised of a smooth, taut membrane (tympanum) pressed against a tracheal air sac. Sound waves cause the tympanum to vibrate, and the waves become amplified by the air sac, which are then transduced into nerve impulses by specialized receptors, in which is called Chordotonal organs (Grimaldi & Engel 2005).
In the opinion of many noctuid specialists, i.e. Eggers (1919), Richards (1932) and Speidel et al. (1996), tympanal organ acting particular useful function in elucidating the basic relationship of the noctuid subfamilies. For instance, Eggers (1919) demonstrated that the tympanal organ is structurally heterogeneous within the Noctuidae. Richards (1932) proposed a noctuid classification based on comparative study of the tympanal organ.
In addition, in the study of Speidel et al. (1996), the newer definition or in a rather broad sense for the term tympanal region have been proposed. It refers to the area around the waist-like cleft separating the thorax and abdomen, commonly known in the Noctuoidea as the tympanal cavity. Thus, the tympanal region includes the tympanal organ proper as well as functionally related structural modifications of the metathorax and first abdominal segment.
- A prespiracular hood in which is present in the Oenosandridae, Lymantriidae, Arctiidae and in a strongly reduced form in the Aganaidae. Surprisengly, the Herminiinae are the only noctuids possessing this configuration.
- A postspiracular hood originating immediately behind the A1 spiracle is typical for remaining Noctuidae (Fig. postspiracular hood: Kitching & Rawlins 1998).
- A perispiracular cup on the A1 pleura is typical for all Notodontidae.
Kitching and Rawlins (1998) studied phylogenetically tympanal organs in more detail. First of all, metathoracic tympanal organ was considered as an apomorphic character in Noctuoidea phylogeny. They mentioned its primary function is considered to be perception of the echolocation signals of bats. However, there is increasing evidence that the tympanum may also be involved in reception of mating signals.
Kitching and Rawlins (1998) also proposed another scene of traditional classification of tympanum, in particular position of sclerotized flap, the counter-tympanal hood, in Noctuoidea. This flap may be anterior (prespiracular) to or posterior (postspiracular) to the first abdominal spiracle, and functions to localize incoming sounds.
They related that precise interpretation of counter-tympanal hood position relative to the spiracle is problematic (e.g. Aganainae, many Herminiinae) as the spiracle may be below the hood, in intermediate configurations, or the hood reduced and its position unclear (Richards 1933). They appended these configurations are probably not homologous, rendering dubious phylogenetic inferences based on treating hood position as either prespiracular or postspiracular (Speidel et al. 1996).
Kitching & Rawlins (1998) at last recounted there are two basic forms of the tympanum amongst Noctuoidea, in Doidae, Notodontidae and some Arctiidae (Syntominae), the metepimeron is strongly concave dorsally, causing the tympanic membrane to be oriented ventrally, while the nodular sclerite and counter-tympanal hood are absent. In Oenosandra and the quadrifid families, the metepimeron is only moderately concave, if at all, so that the tympanum faces posteriorly. A counter-tympanal hood and nodular sclerite are present. The notodontid tympanum is innervated by a singly acoustical sensory cell, while the noctuid tympanum (Fig. Oblique tympanum) is innervated by two such cells.
Back to SUPERFAMILY NOCTUOIDEA