Structure of campaniform sensilla on the haltere ofDrosophila prepared by cryofixation

The structure of sensory cilia of campaniform sensilla has been examined in theDrosophila haltere by cryofixation. The sensory cilium is dilated to 1.8 μm thick, and becomes distally a fan-shaped process (1.5 μm wide and 0.15 μm thick) where the tubular body occurs. In the dilated region microtubules and dense filaments of 8–10 nm are connected by fine filaments with one another and with the ciliary membrane. In the fan-shaped process microtubules appear in a regular pattern as major components of the tubular body. They extend distally in two rows with respect to the medial plane of the fan. In the proximal region adjacent microtubules in each row and those between the two rows are linked by meshworks of a dense substance. In the distal region dense bars occur horizontally in the medial plane of the fan, and microtubules are attached to the bar. Microtubules are connected with the ciliary membrane of the fan-shaped process at regular intervals of 15–20 nm, and the ciliary membrane is connected externally with the cuticular sheath by filaments. These cytoskeletons and extracellular fibrils seem to make the receptor membrane more sensitive to mechanical stimuli by balancing external and internal tensions, and facilitate resetting of the receptor at the resting position to allow mechanoreception at a higher frequency.