Morphology and ultrastructure of the tarsal adhesive organs of the Madagascar hissing cockroach Gromphadorhina portentosa

The present transmission and scanning electron microscopic study of the ultramorphology of the pliable attachment pads (arolium, euplantulae) of the Madagascar hissing cockroach Gromphadorhina portentosa reveals structural evidence for their function in producing, storing, and secreting an adhesion-mediating secretion and releasing it to the exterior. The exocrine epidermal tissue of both the arolium and the euplantula is significantly enlarged by numerous invaginations stretching into the hemolymph cavity. Its cells show large nuclei, numerous mitochondria, Golgi complexes, and a prominent rough-surfaced endoplasmic reticulum integrated within an electron-dense cytoplasm that contains numerous vesicles of diverse electron density and size. Invaginations of the cell membrane provide evidence for strong membrane turnover. The glandular epithelium of both the arolium and the euplantula releases the adhesion-mediating secretion into a subcuticular void from which it has to permeate the thick cuticle of the adhesive pads. The subcuticular void is compartmentalized by cuticle bands through which the adhesion-mediating secretion permeates via small canals. The secretion subsequently enters a larger storage reservoir before being received by a prominent sponge-like cuticle. The structural differences between the arolium and the euplantula consist of the number and length of the interdigitations spanning the hemolymph cavity, of the subdivision of the subcuticular reservoir by cuticle bands, and of the thickness of the sponge-like cuticle. The structural results are discussed with respect to the production of a chemically complex (emulsion-like) adhesive, its controlled release to the exterior, and the micromechanical properties of the cuticle of the pliable pad.