Diversity of olfactory structures: A comparative study of antennal sensilla in Trichoptera and Lepidoptera

The antenna is the main sensory organ of insects, housing different types of sensilla dedicated to detect chemicalcues, motion, humidity and temperature. Sensilla are divided into different types based on their wall structureand morphology. Among the olfactory sensilla, there is an enormous variation in the numbers and morphologicaltypes present in different insect taxa. The reasons for this variation remain obscure, though there may be acorrelation between sensillum morphology and the characteristics of the stimulus that the olfactory sensoryneurons inside the sensillum detect. Here, we report the first comparative analysis of the morphology and ultrastructureof sensilla from Rhyacophila nubila (Rhyacophilidae: Trichoptera) and three species of Lepidoptera,Eriocrania semipurpurella (Eriocraniidae), Lampronia capitella (Prodoxidae), and Bicyclus anynana (Nymphalidae),which use different chemical types of pheromones. Our results, together with a thorough literature review,suggest a shift in major types of olfactory sensilla, from a high proportion of sensilla placodea or auricillica inTrichoptera and the most basal moth lineages (including Eriocraniidae), respectively, to sensilla trichodea in themore derived Lepidoptera (including Prodoxidae and the Ditrysia clade), which parallels the change in the typesof sex pheromones used