Microstructure of the adhesive pad in the millipede Orthomorphella pekuensis (Polydesmida: Paradoxosomatidae)

Secondary sexual structures in millipedes have evolved to improve the efficiency with which spermatophores are transferred from males to females. Here, we demonstrate the microstructural characteristics of the sexual dimorphism in the leg adhesive pads of the millipede Orthomorphella pekuensis, using field emission scanning electron microscopy. The adult male of this millipede has developed typical adhesive pads on the legs as a secondary sexual character: these help the male hold on to the female during mating. The adhesive pads are located on the ventral surface of the tarsus in the form of numerous filamentous bristles with horizontal striation. There are approximately 300 bristles, each with an average length of approximately 60 µm, on each tarsus. Each bristle has a round socket on the surface of the tarsus and an acuminate tip. The striation appears only on the distal half of each bristle, typically.