Ultrastructure and mechanical properties of an insect mechanoreceptor: stimulus-transmitting structures and sensory apparatus of the cercal filiform hairs of Gryllus.

1. The following features of the cercal filiform hairs of the cricket Gryllus were investigated: (a) the ultrastructure and geometrical peculiarities of the various auxiliary structures in the region of the hair base, as well as those of (b) the stimulus-receiving outer segment of the dendrite (including the tubular body), and (c) the mechanical properties (directionality and linearity and frequency dependence of mobility) of the hair. 2. When stimulated by vibrations of the medium, the filiform hairs show regular or irregular oscillations depending on stimulus intensity. At higher stimulus intensities (xi > congruent to 100 microns at 100 Hz) the hairs flutter irregularly in various directions, at somewhat lower intensities preferentially in the plane of best mobility in even lesser intensities in the plane of stimulus vector. In the plane ob best mobility the maximal angle of deflection from the resting position is 5.3 +/- 1.4 degrees. 3. The dependence of hair mobility on stimulus frequency was tested in the range 20-1000 Hz. Best mobility was found in the range 100-200 Hz. 4. The directional characteristic of hair mobility has the form of a figure eight. Hairs can be grouped into three classes on the basis of direction (with respect to the long axis of the cercus) of best mobility: parallel (L-hairs), transverse (T-hairs), and diagonal (D-hairs). 5. The plane of best mobility corresponds with the plane symmetry of the hair base. The hair can be deflected furthest from the resting position in the direction of a cuticular peg at the hair base, which projects toward the lumen of the hair and marks the flat side of the tubular body within the terminal dendrite segment. Deflection of the hair shaft in the opposite direction is limited by a fibrous cushion, which exerts a counter-pressure. When the hair is deflected, the cuticular peg causes deformation of the tubular body. 6. The direction of best mobility of the hair is the direction in which the sensory cell is depolarized; the direction of depolarization can thus be determined entirely by morphological criteria.