Summary
On the heads of the femaleCalliphora studied, in the region between the two compound eyes and at the dorsal back margin, there are about 80 macrochaetae and about 500 smaller mechanoreceptive hairs. Their topography is described, with data on size, angle of inclination and direction of curvature (Figs. 1, 2, 3). When a frontal macrochaeta is rotated about its base in different directions, restoring force is maximal with displacement in the direction of curvature; for the bristles on the back of the head, the force is maximal in the opposite direction. As either type of hair is displaced to progressively greater angles in the direction of greatest restoring force, the bristle joint behaves like a nonlinear spring until the shaft meets the edge of the socket (e.g., 8–10° for the ocellar bristles; Fig. 4). When released the shaft returns to the resting position with a non-oscillatory, strongly damped movement.
Each of the bristles and hairs is innervated by one bipolar sensory cell. Nerve impulses are recorded from this cell only when the hair is displaced into the semicircular sector associated with the greatest restoring forces. The threshold angle is about 1°. The angles to which the bristles are moved by air flow (up to 4.5 m/s), airborne sound (50–1000 Hz, 90 dB) and cuticle-conducted vibration (oscillation amplitude in the frequency range of the wingbeat ca. 100 μm) do not exceed this threshold. Stepwise displacement in the direction of curvature elicits phasic excitation that decays approximately exponentially. In the ocellar bristles, for example, the latency to appearance of the first nerve impulse amounts to about 2 ms and the time constant of frequency decay is about 30 ms. With ramp stimuli peak impulse frequency rises in proportion to the logarithm of the rate of displacement, within the range 10–500°/s (Fig. 7). The peak frequency is independent of final angle. The sensitivity of the receptor to movement of the hair shaft within the sector from resting position to socket edge shows a distinct maximum at 1–3° past the threshold angle (Figs. 11, 12). With imposed sinusoidal oscillation the frequency characteristics are approximately linear up to a stimulus frequency of 160 Hz (Fig. 9). But there is considerable adaptation, dependent on stimulus frequency. The macrochaetae are probably pure event detectors, signalling the moment and site at which any suprathreshold contact is made between the body surface and its surroundings. Stimulus-excitation transformation by the ocellar bristles is discussed.
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Supported by a grant from the Deutsche Forschungsgemeinschaft (AL 56/6, Bu 95/10)
I wish to thank Prof. Dr. D. Burkhardt for guidance during this study, and Drs. B. Darnhofer, Marguerite Biederman-Thorson and J. Thorson for their critical comments on the manuscript.
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Theiß, J. Mechanoreceptive bristles on the head of the blowfly: Mechanics and electrophysiology of the macrochaetae. J. Comp. Physiol. 132, 55–68 (1979). https://doi.org/10.1007/BF00617732
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DOI: https://doi.org/10.1007/BF00617732