Conventional chemical ecology approach for identification of semiochemicals from natural sources is based on isolation of active ingredients after fraction and bioassay. Instead, we identify potential semiochemicals using reverse chemical ecology approach. In this way, we discovered an odorant receptor (CquiOR136) for DEET, which was not know how and why it work. Additionally, we identified a link between this synthetic repellent and methyl jasmonate, thus suggesting that DEET might work by mimicking defensive methyl jasmoante from plant. Moreover, we serendipitously found that a receptor, CquiOR36, responded to an old sample of nonanal in ethanol. Careful investigation led us to identify the active ligand as acetaldehyde (derived from ethanol) from “contaminants” in the old nonanal sample. We explored that acetaldehyde in a wide range of dose both attracts gravid mosquito and stimulate them to lay eggs in oviposition trays.