Lamellated outer dendritic segments of a chemoreceptor within wall-pore sensilla in the labial palp-pit organ of the butterfly, Pieris rapae L. (Insecta, Lepidoptera)

Abstract

The structure of the sensilla in the apical pit of the third segment of the labial palps in Pieris rapae was investigated in cryofixed and chemically fixed specimens. There is a field of about 80 club-shaped sensilla, 94% of which house a single sensory cell; 6% contain two sensory cells. All sensory cells are of the same type and are characterized by the structure of the dendritic outer segment. This consists of a proximal cylindrical and a distal lamellated section. The lamellae contain a lattice of longitudinally arranged microtubules. Filamentous strands connect the microtubules with the surface membrane of the lamellae. The surface area of the lamellated section amounts to about 40 μm2. Pores and pore tubules are present in the cuticular wall of the peg. Electrophysiological recordings show that the sensory cells are olfactory receptors, which react to a variety of complex plant odors and to the odor of conspecifics. It is shown that (a) the usual modality-specific characteristics of insect olfactory sensilla apply here also; (b) lamellation is not only a characteristic of thermoreceptors, but also of olfactory chemoreceptors; (c) there are pore tubules that are separated from the dendritic membranes by an extended dendritic sheath; and (d) in the labial palppit sensilla only the lamellated dendritic tip region may be involved in sensory transduction.