The tuft organs of the human body louse, pediculus humanus corporis—cryofixation study of a thermo-/ hygrosensitive sensillum

The tuft organs of the human body louse, Pediculus humanus corporis, display a complex cuticular apparatus without wall pores, consisting of the cuticular cone, cuticular pocket, and the sensillar peg with 5–6 tuft processes. The cuticular wall of the sensillar peg exhibits several layers of differing electron density tightly surrounding the dendrites. The sensillar peg is innervated by the unbranched dcndrites of two receptor cells which do not reach into the tuft processes. A third receptor cell forms a lamellar outer dendritic segment which wraps around the two other dendrites and terminates before reaching the base of the sensory peg. This type of receptor cell is named stratoterminal while for those which end within the cuticular apparatus the term conoterminal is proposed. Five auxiliary cells are associated with each tuft organ. The pore organs display a porous cuticular plate innervated by the branching dendrites of four receptor cells. A fifth receptor cell terminates more proximal and resembles the stratoterminal cell of the tuft organ. From the fine structure it is evident that the lull organs represent thermo-hygrosensitive sensilla whereas the pore organs may have an olfactory function.