Campaniform sensilla

Campaniform sensilla are a class of mechanoreceptors found in insects, which respond to local stress and strain within the animal's cuticle. Campaniform sensilla function as proprioceptors that detect mechanical load as resistance to muscle contraction, similar to mammalian Golgi tendon organs. Sensory feedback from campaniform sensilla is integrated in the control of posture and locomotion. 
Structure.
Each campaniform sensillum consists of a flexible dome, which is embedded in a spongy socket within the cuticle and innervated by the dendrites of a single bipolar sensory neuron (see schematic cross-section). Campaniform sensilla are often oval-shaped with long axes of about 5-10 µm (see SEM).
Campaniform sensilla are distributed across the body surface of many insects. The fruit fly "Drosophila melanogaster", for example, has over 680 sensilla. Campaniform sensilla are located in regions where stress is likely to be high, including on the legs, antennae, and wings. Sensilla may occur alone, but sensilla with similar orientations are often grouped together. On the legs, groups of campaniform sensilla are located close to the joints on all segments except for the coxa (see leg schematic), with most sensilla located on the proximal trochanter. In Diptera such as "Drosophila", the highest density of campaniform sensilla is found at the base of the modified hind-wings, the halteres. The number and location of sensilla varies little across individuals of the same species, and homologous groups of sensilla can be found across species.
Function.
When cuticular deformations compress a campaniform sensillum, the socket edges (collar) indent the cuticular cap. This squeezes the dendritic tip of the sensory neuron and opens its mechanotransduction channels (from the TRP family), which leads to the generation of action potentials that are transmitted to the ventral nerve cord. 
The activity of campaniform sensilla was first recorded by John William Sutton Pringle in the late 1930s. Pringle also determined that the oval shape of many sensilla makes them directionally selective – they respond best to compression along their short axis. Thus, even neighboring sensilla may have very different sensitivities to strain depending on their orientation in the cuticle. For example, stick insects possess two groups of campaniform sensilla on the dorsal side of the trochanter whose short axes are oriented perpendicularly to one another (see inset in leg schematic). As a result, one group (G3) responds when the leg is bent upwards, whereas the other group (G4) responds when the leg is bent downwards. Round campaniform sensilla can be sensitive in all directions or show directional sensitivity if the cap is asymmetrically coupled with the surrounding collar. 
The activity of campaniform sensilla is typically described as phasic-tonic, meaning that they signal the magnitude and the rate of cuticular deformation. But based on their responses to white noise stimuli, campaniform sensilla may also be described more generally as signaling two features that approximate the derivative of each other. 
The activity of campaniform sensilla can affect the magnitude and timing of muscle contractions. In walking control, sensory feedback from leg campaniform sensilla is thought to reinforce muscle activity during the stance phase and to contribute to inter-leg coordination, much like sensory feedback from mammalian Golgi tendon organs. Feedback from leg campaniform sensilla is also important for controlling kicking and jumping. In flight control, sensory feedback from haltere and wing campaniform sensilla is thought to mediate compensatory reflexes to maintain equilibrium.