Sensing touch, movement and sound:
Part 3: Campaniform sensilla
The second type of mechanoreceptor is the campaniform sensillum. This appears as a small dome in an area of this cuticle. The dome is usually oval in shape and from 5-30 microns in length. A specialized bipolar nerve ending joins with the inside of the cuticular dome and is stimulated as the cuticle is stretched and sheared. These domes usually occur close to joints. They are also present on mouthparts, basal segments of antennae, near wing veins and on ovipositors – anywhere that the cuticle is bent and flexed.
[Diagram: BW Cribb. Modified from Chapman, edition 4]
The campaniform sensillum can vary in appearance but the basic form is shown here. Instead of a hair shaft, there is a low dome-like structure in the cuticle: the cap of the sensillum. There is a joint membrane surrounding the dome which links to a collar of cuticle that forms a ring. Beneath the dome-like cap there may be a layer of spongy cuticle and fibrous cuticle.
The accessory cells around the bipolar neuron are similar to those seen for the sensilla with shafts. The sensory neuron produces a distal dendrite that connects with the center of the dome. The dendrite is surrounded by a dendrite sheath (produced by the thecogen cell) and ends in a tubular body, as seen in the mechanoreceptors with shafts.
Details of the structure of this type of sensillum vary and there are nine different types known from adult Calliphora (flies).
These sensilla are stress receptors. They often occur in clusters all orientated in the same direction and work as a unit
Go on to part 4: Scolopidia & chordotonal organs