Olfaction is an essential sense in animals especially insects. A large family of odorant receptors (ORs) has evolved in insects, to become involved in behaviours such as finding food, mates and avoiding predators. Insect ORs are ligand-gated ion channels formed as a complex between a ligand-binding OR and an obligate co-receptor, Orco expressed together in each sensory neuron. Genomes of higher insects sequenced to date typically contain one Orco and up to several hundred ligand-binding ORs. We are undertaking genomic and antennal transcriptomic surveys from species at the base of the Hexapoda to understand the origins and patterns of expansion of this multigene family. The presence of multiple Orcos was detected in the antennae of the silverfish, Lepisma saccharina. In comparison what look like ligand-binding ORs, together with a single Orco were detected in the antennae of the giant bush dragonfly, Uropetala carovei. The nature of the OR complex and what these receptors detect remains to be determined. Phylogenetic analysis with these and other OR sequences is building a picture of the evolution of insect ORs initially through duplication of Orco followed by the evolution and expansion of specialised ligand-binding ORs. This evolutionary scenario remains consistent with the idea that the evolution of the OR multigene family was associated with the evolution of flight in insects and the requirements to rapidly navigate based on volatile cues within a three dimensional gaseous medium.