Campaniform sensilla are a class of mechanoreceptors found in

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...

s, which respond to local stress and strain within the animal's cuticle. Campaniform sensilla function as

proprioceptors Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neurons ...

that detect mechanical load as resistance to muscle contraction, similar to mammalian

Golgi tendon organ The Golgi tendon organ (GTO) (also called Golgi organ, tendon organ, neurotendinous organ or neurotendinous spindle) is a proprioceptor – a type of sensory receptor that senses changes in muscle tension. It lies at the interface between a musc ...

s. 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 ''Halteres'' (; singular ''halter'' or ''haltere'') (from grc, ἁλτῆρες, weights held in the hands to give an impetus in leaping) are a pair of small club-shaped organs on the body of two orders of flying insects that provide infor ...

. Sensilla may occur alone, but sensilla with similar orientations are often grouped together. Distribution of campaniform sensilla on leg

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

Distribution of campaniform sensilla on wing and haltere

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 ventral nerve cord is a major structure of the invertebrate central nervous system. It is the functional equivalent of the vertebrate spinal cord. The ventral nerve cord coordinates neural signaling from the brain to the body and vice versa, i ...

, the insect analogue to the vertebrate spinal cord. The activity of campaniform sensilla was first recorded by

John William Sutton Pringle Sir John William Sutton Pringle (22 July 1912 – 2 November 1982) was a British zoologist. His research interests were in insect physiology, especially proprioception, flight muscle, and cicada song. Life and career Pringle was born in 1912, an ...

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 their legs' 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 may be slowly-adapting (tonic), signaling the magnitude of cuticular deformation, and/or rapidly adapting (phasic), signaling the rate of cuticular deformation. 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. In addition, activity adapts to constant loads and shows hysteresis (history dependence) in response to cyclic loading. Campaniform sensilla project directly to motor neurons and to various interneurons, which integrate their signals with signals from other proprioceptors. In this way, campaniform sensilla activity can affect the magnitude and timing of muscle contractions.

Function of leg campaniform sensilla

Campaniform sensilla on the legs are activated during standing and walking. Their sensory feedback is thought to reinforce muscle activity during the stance phase and to contribute to inter-leg coordination, much like sensory feedback from mammalian

s. Feedback from leg campaniform sensilla is also important for the control of kicking and jumping.

Function of wing and haltere campaniform sensilla

Campaniform sensilla on the wings and halteres are activated as these structures oscillate back and forth during flight, with the phase of activation depending on the placement of the sensilla. The campaniform sensilla on the wing encode the wing's aerodynamic and inertial forces, whereas sensilla on the base of the haltere are thought to encode Coriolis forces induced by body rotation during flight, allowing the structure to function as a gyroscope. Feedback from wing and haltere campaniform sensilla is thought to mediate compensatory reflexes to maintain equilibrium during flight.

Computational models

To better understand the function of campaniform sensilla, computational models that mimic their response properties are being developed for use in simulations and robotics.

References

{{DEFAULTSORT:Campaniform Sensilla Insect anatomy Sensory receptors es:Sensilia fr:Sensille ru:Сенсиллы членистоногих