The receptors which respond to gravitational influences are often termed equilibrium receptors, though; the equilibrium of the body of any animal is usually the result of a combination of visual, proprioceptive, tactile, and gravitational stimuli sensed by appropriate sensory receptors. Various equilibrium receptors found in the animals are the following:
(i) Statocysts:
These are very old types of a specialized mechanoreceptor found mainly in crustaceans, molluscs and vertebrates. These respond to either to static forces of gravity or to acceleration forces.
Statocysts are generally rounded or cylindrical closed sacs with sensory cells which have sensory hairs.
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These are filled with a fluid. In crustaceans the sensory hairs of sensory cells are chitinous structures innervated at their bases by the primary sense cells.
In most animals such as in Pecten and LeptomySis, the statocysts contain a harden concretion, the statolith which consists either of sand grains picked up from the environment or calcareous concretions produced by the animal.
The statolith moves about in the fluid medium of the statocyst and exerts mechanical distortions on the sensory hairs.
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From the hairs the impulses are transmitted to the nervous system to provide information concerning the position of the animal with respect to gravity.
Hyman, 1940, 1951, reported that such receptors are also present in the coelenterates and flatworms.
(ii) Tactile and chordotonal sensillae of insects:
These are sufficiently specialized structures or receptors of insectsto act as simple acoustic organs.
Sensilla trichodea are tactile sensory organs that are located in different parts of the body in hollow extensions of the exocuticle.
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The sensilla (=sensory hairs) are found in abundance at joints and regions between body segments.
They consist of a sensory cell in a socket. The shaft of the hair is rigid, and any force applied to it is transmitted to and amplified by the membrane of the sensory neuron, resulting in the production of a nerve impulse (Pringle, 1938).
Some of these receptors are innervated by only one neuron others may be innervated by several neurons, indicating a variety of sensory functions including that of chemoreception.
Sensilla chaetae (sensory spines) are also found all over the cuticle of the insect and are mechanoreceptors that function in proprioception as well as other mechanoreceptor activities.
Sensilla campaniformia (sensory pores) are mechanoreceptors that function in gravity reception and proprioception.
Like sensilla, there are also found some specialized cells over the insect body, these are known as scolophours sense cells.
A group of scolopidia forms a chordotonal organ. These organs may stretch across a fluid-filled space as in the subgenual organs in the leg’s or ..Johnston” organ in the second antennal segment.
The former serves as a tactile organ or pickup for low-frequency vibration (200 to 6000 cycles per second) from the ground or substratum where the animal lives ; the latter are probably “statical organ” (Autrum. 1959 ; Dethigr, 1963). Chordotonal organs may also stretch across an air sac or tracheal space and form auditory organs of many different patterns and varying complexities.
The chordotonal organs consist one to 400 sensilla (sensory hairs), each with one nerve cell and 2-3 characteristic cells.
The distal cell generally called cap cell is attached to an organ or part of the body wall, whereas a ligament is usually attached to some other part of the body.
The sensory cell is thus attached to two different parts of the body, and any movements of the body cause impulses to arise in the neuron.
In some insects as many as 1500 or more scolopidia in association with a thin membrane, the tympanum form specialized acoustic organs called tympanal organs. The membrane improves the pickup of distal vibrations.