The olfactory code of insects
The following is an extract from On The Scent by Paolo Pelosi and explores how insects taste and smell.
When we describe the aroma of foods, we frequently use the term taste to indicate sensations that are usually quite complex and include, to a large extent, olfactory elements. In introducing food into our mouth, taste receptors located on the surface of the tongue are immediately stimulated and send signals to the brain which we can properly define as taste. At the same time, however, a multitude of volatile components present in our food are released, also as a result of chewing, and reach the olfactory mucosa, the area at the top of our nose, where receptors for smell molecules are located, through an opening situated on the upper wall of the palate.
It is thanks to these volatile chemicals that we appreciate the richness and variety of different food aromas. We use the terms flavour or aroma to describe the complex sensory experience generated when we taste foods, which includes olfactory and taste components, but also tactile aspects (the crispiness of chips, the crunchiness of biscuits, the melting softness of chocolate), temperature detection, and other sensory elements. Above all, smells (entering our nose through the palate and often mistaken as tastes) are responsible for the great variety and subtle differences that we appreciate in our foods.
But, of course there are proper taste sensations, which we perceive with our tongue and which provide unique contributions to the aroma of foods. At this point we should perhaps clarify the differences between smell and taste both with reference to the molecules eliciting such perceptions and to the relative sensory systems.
From a purely anatomical perspective, at least in humans and other mammals, tastes are perceived with the tongue and smells through the nose. In other words, we call tastes the sensations coming from our tongue and smells those which originate in the nose. In other species, however, this definition cannot be always correctly applied. For example, the bifurcate tongue of reptiles, which is rapidly moved back and forth, is used to sample the air and carry the odours present in the environment to the organs of chemoreception. Its rhythmic movements correspond to the rhythmic sniffing of a dog exploring the environment and navigating with its olfactory map.
If we then look at insects, we cannot find tongues or noses, but tiny sensilla, elementary sensory organs, located on the antennae, mouth parts, legs and sometimes also on the wings and other parts of the body.
However, we can regard the antennae as the main olfactory organ (the equivalent of a nose), while legs and mouths are generally dedicated to taste. But can we still talk about olfaction and taste in insects, even in the absence of noses and tongues? We certainly can, if we refer to the chemical nature of the stimuli rather than to the organs of perception.
In this case, we call odorants those chemicals carried by air to the chemosensilla of insects (and to the noses of humans and terrestrial vertebrates), while taste is elicited by non-volatile compounds, often water soluble and present in the environment.
Therefore, the antennae of insects can be regarded as olfactory organs, as they catch molecules present in the environment as gases, while the legs perceive sugars and other non-volatile compounds, which are brought into physical contact, for example when a butterfly lands on a flower or a mosquito on its host’s skin. It would therefore be more appropriate to talk about contact chemoreception in insects, rather than taste.
So a distinction between olfaction and taste based on the nature of chemical stimuli rather than the anatomical structures that receive them would seem clearer and more appropriate. But it isn’t so. Nature is always more complex than our schemes and definitions, and every time we feel we have arranged all the phenomena in our artificial scheme we come across examples that we are not able to classify.
For fish, and for aquatic organisms generally, all chemicals are carried in water, and the above definition of odours and tastes does not make sense any longer. But fish, at least, possess a nose and a tongue and we can refer to these organs to distinguish again between the two chemical senses.
Paolo Pelosi is currently Visiting Professor at the Chinese Academy of Agricultural Sciences (CAAS) in Beijing, China. His research work has been dedicated to the study of olfaction and includes the use of organic conducting polymers as sensors for an electronic nose, in a study performed in collaboration with Krishna Persaud at the University of Manchester, UK. He has authored around 150 scientific publications and several book chapters. He is the author of On The Scent.