Olfaction is a chemical sense, not a spectral sense

Franco et al. (1) argued that molecular vibrations contribute to odor detection by Drosophila. They claimed that deuterated odorants have a unique C-D stretch vibration that produces a “deuterium odor character” independent of the “structure and chemical properties of the odorant molecules.” The discrimination of normal and deuterated odorants by flies is a clear and convincing finding, but such isotope effects do not prove that the animals are sensing infrared molecular vibrations using inelastic electron tunneling spectroscopy (IETS) (2).