ABSTRACTConcentrations of hair sensilla have been noted by a number of authors in the joints of the insect body and limbs. Special groups of such sensilla in the form of “hair plates” are described from three positions on the legs of Periplaneta americana. The hairs of the hair plates are mechanical sense organs with a slow rate of adaptation. In life the hairs are excited by a fold of the intersegmental membrane, the excitation varying with the position of the joint. It is suggested that these and similar structures in other parts of the bodies of insects act as “position” sense organs.