Chemoreceptors
These are receptors included in perception of chemical stimuli. You will see that there are three kinds of Chemoreceptors among metazoans:
i) Those concerned along with general chemical sense perceived by undifferentiated nerve endings or unspecified receptors. These are distributed all over the surface of the body and are common in between lower animals such as cnidarians, worms and molluscs. The most primitive type of chemoreceptor and results in escape reaction of the animal.
ii) Chemotactile sense perceived through contact Chemoreceptors involving contact with molecules in dilute solution; they are generally present in large numbers in special regions of the body. In mammals these are located on the tongue and we call the sensation taste or gustation. But in Octopus they are concentrated around the rim of the suckers, and in insects they are present on mouthparts and in flies, they are found in abundance as well on tarsal segment of foot. So one may say that the fly tastes with their fed.
iii) Olfaction involves distant chemical sense and is the most sensitive type of chemical sense. Olfactory receptors are capable of perceiving molecules of very low concentration in air and we usually call it smell. They are distributed in high density on the antennae of insects. The combined electrical recordings of the antennal nerve generated by the odors on the olfactory receptors of the antennae are termed electroantennogram. Specific insects can recognize their mates even at a distance of a couple of kilometers, from the odour (pheromone) molecules they emit.