Hygroreceptors:
Hygroreceptors (Hygro=moist) detect the water content of air. For example some insects have hygroreceptors that can detect small changes in the ambient relative humidity. This sense enables them to seek environments with a specific humidity or to modify their physiology or behaviour with respect to the ambient humidity to control the opening or closing of spiracles.
Zoologists have identified a variety of hygrosensory structures on the antennae, pulps, under side of body and near the spiracles of insects.
Phonoreceptors:
Phonoreceptors (Phone=voice + receptor) that respond to sound have been demonstrated only in insects, arachnids and centipedes although other invertebrates seem to respond to sound induced vibrations of the substratum. For example, crickets, grasshoppers and cicados possess phonoreceptors called tympanic or tympanal organs. This organ consists of a flexible tympanum that covers an internal sac that allows the tympanum to vibrate when sound waves strike it. Sensory neurons attached to tympanum are stimulated and produce a generator potential.
Proprioreceptor:
Proprioreceptors (Propricus=one’s self + receptors) commonly called “stretch receptors” are internal sense organs that respond to mechanically induced changes caused by stretching, compression, bending or tension. These receptors give an annual information about the movement of its body parts and their position relative to each other.
Proprioreceptors have been most thoroughly studied in arthropods where they are associated with appendage, joints and body extensor muscles. In these animals the sensory neurons involved in proprioreception are associated with and attached to some part of the body that is stretched. These parts may be specialized muscle cells, elastic connective tissue fibres or various membranes that span joints. As these structures change shape, sensory nerve endings of the attached nerves distort accordingly and initiate a generator potential.
Tactilereceptors:
Tactile or touch receptors are generally derived from modification of epithelial cells associated with sensory neurons. Most tactile receptors of animals involve projection from the body surface e.g. various bristles, spines, setae and tubercles. When an animal contracts an object in the environemtn these receptors are mechanically deformed. These deformations activate the receptor which in turn activates underlying sensory neurons initiating a generator potential. Most tactile receptors are also sensitive to mechanically induced vibrations propagated through water or a solid substrate.
Tube dwelling polychaetes bear receptors that allow them to retract quickly into their tubes in response to movements in their surroundings. Web building spiders have tactile receptors that can sense struggling prey in webs through vibrations of the web threads.
Thermoreceptors:
Thermorecepetors (Thermo=heat + receptors) respond to temperature changes. Some invertebrates can directly sense differences in environmental temperatures.
Protozoa paramecium collects in areas where water temperature is moderate and it avoids temperature extremes. Heat sensing mechanism draws Leeches and ticks to warm blooded hosts. Certain insects, some crustaceans and horse shoe crab (limulus) can also sense thermal variations.
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