Fine structure of integumental glands in the labial palp of adult Graphoderus occidentalis Horn (Coleoptera: Dytiscidae)

The terminal segment of the labial palp of the diving water beetle Graphoderus occidentalis contains numerous integumental glands. Each secretory unit is composed of three cells: a long, thin distal duct cell; a long, wider proximal duct cell; and a bulbous basal gland cell. A cuticular duct, which opens through a pore at the surface, extends centrally through all three cells, after penetrating the cuticle and underlying epidermal layer. The proximal duct cell is joined to the distal duct cell and the gland cell by desmosomes and septate junctions between their interdigitated membranes. The distal duct cell is joined to adjacent epidermal cells similarly. The distal duct cell has a small nucleus and few cytoplasmic inclusions. The proximal duct cell extends numerous microvilli centrally, from which packed epicuticular filaments extend perpendicularly to the central duct. It has a large nucleus, small mitochondria, smooth tubular endoplasmic reticulum, and scattered ribosomes. The gland cell has very abund...