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 chemical
cues, motion, humidity and temperature. Sensilla are divided into different types based on their wall structure
and morphology. Among the olfactory sensilla, there is an enormous variation in the numbers and morphological
types present in different insect taxa. The reasons for this variation remain obscure, though there may be a
correlation between sensillum morphology and the characteristics of the stimulus that the olfactory sensory
neurons inside the sensillum detect. Here, we report the first comparative analysis of the morphology and ultrastructure
of 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 in
Trichoptera and the most basal moth lineages (including Eriocraniidae), respectively, to sensilla trichodea in the
more derived Lepidoptera (including Prodoxidae and the Ditrysia clade), which parallels the change in the types
of sex pheromones used.