ULTRASTRUCTURE OF A SENSILLUM ASSOCIATED WITH THE PHARYNX OF THE COCKROACH PERIPLANETA-AMERICANA

A sensillum associated with the pharynx of the cockroach Periplaneta a mericana was examined in serial ultrathin sections using electron micr oscopy. This sensillum consisted of a group of 10-20 similar sensillar subunits. Each sensillar subunit possessed one 60- to 70-mu m long de ndritic sheath that made direct contact with the cuticle. The dendriti c sheath enclosed 3-5 sensory cilia arising from 3-5 sensory cells loc ated in a cluster approximately 30 mu m proximal to the base of the sh eath. Between the sensory cell body and the base of the sheath the den drites were wrapped by the sheath-forming cell. Before entering the de ndritic sheath itself, the dendrites crossed through an extracellular space, the ciliary sinus. No cuticular specializations, such as a well -defined sensory hair or pore, were observed. The structure of this se nsillum suggests that it responds poorly to mechanical distortion of i ts surroundings. This characteristic supports the hypothesis that this sensillum measures the osmotic concentration of the ingested food.