Evolution of the metathoracic tympanal ear and its mesothoracic homologue in the Macrolepidoptera (Insecta) |
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Authors: | J E Yack Geoff G E Scudder James H Fullard |
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Institution: | (1) Department of Biology, Carleton University 1125 Colonel By Drive, Ottawa, Ontario, Canada, K1S 5B6, e-mail: jyack@ccs.carleton.ca, Tel.: +1-613-5203887, Fax: +1-613-5204497, CA;(2) Department of Zoology, University of British Columbia Vancouver, British Columbia, Canada, CA;(3) Department of Zoology, Erindale College, University of Toronto Mississauga, Ontario, Canada, CA |
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Abstract: | Two independent methods of comparison, serial homology and phylogenetic character 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 chordotonal organ in the hawkmoth, Manduca sexta, which is shown to be serially homologous to the noctuoid metathoracic tympanal organ. This chordotonal organ comprises a
proximal scolopidial region with three bipolar sensory cells, 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 nerve branches and
their corresponding peripheral attachment sites. Second, the general structure (noting sensory cell numbers, gross anatomy,
and location 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 segments was found
to be remarkably conserved in all superfamilies of the Macrolepidoptera examined except the Noctuoidea, where fewer than three
cells occur in the metathoracic ear (one cell in representatives of the Notodontidae and two cells in those of other families
examined), and at the mesothoracic wing-hinge (two cells) in the Notodontidae only. By mapping cell numbers onto current phylogenies
of the Macrolepidoptera, we demonstrate that the three-celled wing-hinge chordotonal organ, believed to be a wing proprioceptor,
represents the plesiomorphic state from which the tympanal organ in the Noctuoidea evolved. This ’trend toward simplicity’
in the noctuoid ear contrasts an apparent ’trend toward complexity’ in several other insect hearing organs where atympanate
homologues have been studied. The advantages to having fewer rather than more cells in the moth ear, which functions primarily
to detect the echolocation calls of bats, is discussed.
Accepted: 18 June 1999 |
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