Insects have the ability to sense various chemical substances in their environment. When these chemicals are present in gaseous form (at relatively low concentrations), they may be detected as odors (smells) by olfactory receptors. When they are in solid or liquid form (usually at higher concentrations) they are perceived as tastes by gustatory receptors. In general, the sense of taste involves direct contact with a substrate (contact chemoreception) whereas olfaction usually implies detection of compounds in gaseous or airborne form (remote chemoreception).
Gustatory receptors are commonly described as thick-walled hairs, pegs, or pits where the dendrites of several (usually up to five) sensory neurons are exposed to the environment through a single opening (pore) in the cuticle. Each neuron appears to respond to a different range of compounds (e.g. sugar, salt, water, protein, acid, etc.). Taste receptors are most abundant on the mouthparts, but may also be found on the antennae, tarsi, and genitalia (especially near the tip of the female's ovipositor).
Olfactory receptors are usually thin-walled pegs, cones, or plates with numerous pores through which airborne molecules diffuse. Dendrites of sensory neurons branch profusely within these pores and may respond to very low concentrations of detectable compounds (e.g. sex pheromones). Some receptors respond to a wide range of substances while others are highly specific. Olfactory receptors are most abundant on the antennae, but may also be associated with the mouthparts or external genitalia.
Common chemical sense
High concentrations of irritant compounds (e.g. ammonia, chlorine, acids, essential oils, etc.) simulate avoidance reactions and cleaning behavior. Insects can detect these compounds even when all known chemoreceptors have been covered or destroyed. The irritants evidently trigger a generalized response from other types of sensory neurons.