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, wings, and halteres. Sensilla may occur alone, but sensilla with similar orientations are often grouped together. Campaniform sensilla on legs 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. The number and location of sensilla on the legs varies little across individuals of the same species, and homologous groups of sensilla can be found across species. Campaniform sensilla on wings and halteres On the wings, most campaniform sensilla are found near the wing base on veins towards the leading edge (see wing schematic). Campaniform sensilla typically occur on both sides of the wing, but the exact number and placement varies widely across species, likely mirroring differences in flight behavior. In Diptera such as Drosophila, the highest density of campaniform sensilla is found at the base of the modified hind-wings, the halteres (see haltere schematic). Function Response properties 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 insect analogue to the vertebrate spinal 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,