Olfactory sensilla show a large diversification of sensillum types even in the same species. Thus, double-walled and single-walled sensilla with highly different wall pores are usually found on the same antenna, and these may appear in the form of long slender hairs, pore plates or pit pegs. The selective constraints leading to this diversification are evident only in a few cases, e.g. the demand for extreme sensitivity in moth pheromone communication supported the evolution of long sensilla trichodea with high efficiency of capturing odour molecules. The structural diversity continues with the odorant-binding proteins (OBPs) in the sensillum lymph surrounding the sensory dendrites. These proteins may be subdivided into pheromone-binding proteins and two classes of general odorant-binding proteins according to their primary sequence. Different sensilla of the same morphological type may contain different OBPs of the same or of different subclasses. However, OBPs of different subclasses are not co-localized in the same individual sensory hair. The presence of a given OBP is related more to the functional specificity of the receptor cells than to the morphological type of the sensillum, suggesting a role of OBPs in stimulus recognition.