Noses on the wing: the olfactory capacity of hawkmoth wings

In addition to their primary olfactory organs, the antennae, insects can smell using other parts of their bodies. In this study, we examined the olfactory capabilities of the wings of the tobacco hawkmoth, Manduca sexta (Lepidoptera: Sphingidae). Using scanning electron microscopy, we identified an average of 21-32 sensory bristles along the margins of each wing. In addition to raised sockets that indicate mechanosensory function, we observed a subapical pore and numerous wall pores. These pores are signs of chemosensory function, suggesting that the wings can detect chemicals upon contact or in gaseous form. Gene expression analysis revealed the presence of genes encoding chemosensory receptors, such as gustatory receptors (GRs) and ionotropic receptors (IRs), including IR76b, the co-receptor of amine-sensing IRs, and IR8a, the co-receptor of acid-sensing IRs. Genes encoding several odorant receptors (ORs) were also expressed in the wings; however, expression of the gene encoding ORCo, the obligatory OR co-receptor, was not detected. Electrophysiological recordings revealed that only the volatile amines pyrrolidine and piperidine elicited responses from the wings. Protein modeling and molecular docking simulations identified members of the Lepidoptera-specific IR7d clade as encoding candidate receptors for these compounds. These are characteristic alkaloids found in solanaceous plants, which tobacco hawkmoths prefer for laying eggs. Together, these findings demonstrate that the wings of M. sexta function as accessory olfactory organs that may influence oviposition choice.