Morphology and histology of vom Rath's organ in brush-footed butterflies (Lepidoptera: Nymphalidae)

Vom Rath's organ, located at the distal end of the third segment of the labial palp, is one of the recognized synapomorphies of Lepidoptera (Insecta). Information about the structural and histological morphology of this organ is sparse. The structure of vom Rath's organ in four species of Nymphalidae, three frugivorous: Fountainea ryphea (Charaxinae: Anaeini), Morpho helenor achillaena (Satyrinae: Morphini) and Hamadryas epinome (Biblidinae: Ageroniini), and the nectarivorous species Aeria olena (Danainae: Ithomiini) is described by means of scanning electron microscopy and histology. The species showed significant differences in the cavity shape, setal morphology and arrangement, opening shape and location, associated with the organization of cell groups, type of axon, and degree of development. These differences do not seem to be related to feeding habit. No cell groups were found in Actinote thalia (Heliconiinae: Acraeini) and Heliconius erato phyllis (Heliconiinae: Heliconiini), and for the first time the absence of vom Rath's organ is documented in the clade Ditrysia. A terminology is proposed to improve understanding of the organ morphology, with an extensive analysis of the previous descriptions.