Je. Yack et al., Evolution of the metathoracic tympanal ear and its mesothoracic homologue in the Macrolepidoptera (Insecta), ZOOMORPHOL, 119(2), 1999, pp. 93-103
Two independent methods of comparison, serial homology and phylogenetic cha
racter mapping, are employed to investigate the evolutionary origin of the
noctuoid moth (Noctuoidea) ear sensory organ. First, neurobiotin and Janus
green B staining techniques are used to describe a novel mesothoracic chord
otonal organ in the hawkmoth, Manduca sex-ta, which is shown to be serially
homologous to the noctuoid metathoracic tympanal organ. This chordotonal o
rgan comprises a proximal scolopidial region with three bipolar sensory cel
ls, and a long flexible strand (composed of attachment cells) that connects
peripherally to an unspecialized membrane ventral to the axillary cord of
the fore-wing. Homology to the tympanal chordotonal organ in the Noctuoidea
is proposed from anatomical comparisons of the meso- and metathoracic nerv
e branches and their corresponding peripheral attachment sites. Second, the
general structure (noting sensory cell numbers, gross anatomy, and locatio
n of peripheral attachment sites) of both meso- and metathoracic organs is
surveyed in 23 species representing seven superfamilies of the Lepidoptera.
The structure of the wing-hinge chordotonal organ in both thoracic segment
s was found to be remarkably conserved in all superfamilies of the Macrolep
idoptera examined except the Noctuoidea, where fewer than three cells occur
in the metathoracic ear (one cell in representatives of the Notodontidae a
nd two cells in these of other families examined), and at the mesothoracic
wing-hinge (two cells) in the Notodontidae only. By mapping cell numbers on
to current phylogenies of the Macrolepidoptera, we demonstrate that the thr
ee-celled wing-hinge chordotonal organ, believed to be a wing proprioceptor
, represents the plesiomorphic state from which the tympanal organ in the N
octuoidea evolved. This 'trend toward simplicity' in the noctuoid ear contr
asts an apparent 'trend toward complexity' in several other insect hearing
organs where atympanate homologues have been studied. The advantages to hav
ing fewer rather than more cells in the moth ear, which functions primarily
to detect the echolocation calls of bats, is discussed.