All living things sense the world they live in, and we are familiar with a set of senses specific to humans. However, the senses humans use are neither the only senses nor the best; rather they simply differ from other species’ sensory systems both in mechanism and range based on differing needs and abilities. Insects have highly developed olfaction or sense of smell, which they use for various activities including predator and food detection, sex, aggregation, spacing, trail forming, and alarm. Often, insects place emphasis on olfaction, which is atypical of most other clades of organisms, many of which rely heavily on vision and audition. Why do insects utilize olfaction to such a great extent? I argue that it is the combination of both the specificity and the lingering nature of olfactory cues that makes olfaction so highly valuable to insects as a clade.
Humans can sense chemical cues through smell conveying messages varying in nature from sexual to warning – we can sense each other’s hormones and emotions through pheromones (de Groot et al. 2012). Insects emit and detect similar signals through olfaction, but are much more sensitive to these cues than humans, and are often able to detect a single molecule of a sex hormone (Zhou et al. 2012). Insect chemoreception begins with signal detection on smell-gathering organs called sensilla that are located on the insect’s antennae, mouthparts, and sometimes genitalia. Chemical signals are then transduced into electrical signals that travel through the insect’s nervous system to the brain, at which point insect has perceived the signal (Gullan & Cranston 2010).
Because chemical signals consist of matter (molecules) rather than energy, they persist in a particular location over a longer period of time than waves of energy in the form of light or sound. For the many insects that are small, travel long distances during the day foraging for food, and live solitarily or widely separated spatially relative to their size, communication is hindered by behavioral and morphological limitations. An insect more easily traces a physical substance because it can almost literally pick up a trail of breadcrumbs in the form of chemicals left hanging in the air if it is following a trail for food, its nest, or a potential mate.
If an insect is searching for a mate, what will matter to that insect? First and foremost, knowing that the signal it detects is actually a member of its own species! In such a “buggy-bug world” as ours, with millions of insect species that are difficult to distinguish even for humans, how can a poor-sighted insect pick one of its own species out of a crowd from miles away? Chemical signals permeate the air in droves (Hansson & Stensmyr 2011), and tracing a path of breadcrumbs through a forest made of breadcrumbs would be impossible if the breadcrumbs weren’t highly distinguishable – and they are, to insects. Chemical signals emitted by females are often species-specific with male counterparts possessing receptors specific to the female’s signals (Vieira et al. 2012). Otherwise, insects would all be trying to mate with each other regardless of special boundaries and get nowhere – and even with highly specialized systems this can sometimes occur.
Social insects are sometimes able to bypass these hurdles. For example, honeybees have evolved highly complex system to visually communicate navigation to food sources through “dances.” Also, the bees live in close quarters with each other and can therefore hardly fail to notice that the other animals in their hive are their own species. However, only certain types of honeybees are fertile, therefore the bees still use pheromones to attract and detect mates.
Speaking of honeybees, alarm signals are very important to these and other insects. The nest of social insects like honeybees provides protection and harbors food and juveniles, making it a vulnerability if it is threatened. Under duress, a bee releases a pheromone that attracts others of its own species over a limited distance. However, if the bee is in close proximity to the hive, others immediately come to its rescue and attack the intruder in order to protect the hive and its resources (Breed et al. 2004). Unlike other forms of communication, chemosignaling can be performed relatively passively and persistently (in contrast to vision in which the insect must be looking at something to see it or audition in which the insect actively works to create sound), which means that the insect can convey messages to many other insects at the same time without using much energy.
While escaping predators, similar advantages to chemical signaling hold true. Visual detection of a predator may help if the insect can fly away, as you can see when a fly detects your fly swatter’s movement and escapes, but for slower, more vulnerable insects this simply isn’t an option. It would be more advantageous if the insect could detect predators from a distance through chemical signals, allowing for time to prepare defenses or flee. If the insect were communicating to the predator, unless it is flashy like some butterflies (Olofsson et al. 2013), it would be unable to communicate its disgusting taste quickly without pheromones like those that the stinkbug employs.
Other social insects like ants, which have no way of visually communicating food location, lead other ants to food and nest through pheromones. Ants actually combine a sun compass, visual cues, and a path integrator (counting its steps) with chemical signals, but research indicates that ants choose to follow chemical signals over others – in fact, ants have four to five times more odor receptors than most other insects (Zhou et al. 2012). In one study in which odors normally associated with a nest were moved away from the nest, ants repeatedly moved toward the odors rather than the nest, meaning that the combination of non-chemical signals was not adequate to overpower the importance of the chemical signals in nest location (Steck et al. 2010). This makes intuitive sense – if an ant can use a passively detected chemical signal to find its way, it can free up other senses like audition and vision for detecting danger, both physical (environmental) and biological (predators).
Clearly, there is a tradeoff whether insects are social or solitary. However, both types of insects use pheromones to counterbalance the disadvantages of these tradeoffs. While a combination of senses is necessary to survival, insects rely more heavily on olfactory cues because of their specificity, substantial and persistent nature, and potential for long-distance emission.
Breed, M. D., E. Guzman-Novoa and G. J. Hunt. 2004. Defensive behavior of honey bees: organization, genetics, and comparisons with other bees. Annual Review of Entomology 49: 271-298.
Gullan, P.J. and P.S. Cranston. The Insects. Ed. 4. Wiley-Blackwell: UK, 2010.
Hanson, Bill S. & Marcus C. Stensmyr. 2011. Evolution of Insect Olfaction. Neuron 72(5): 698-711. DOI 10.1016/j.neuron.2011.11.003.
J. H. B. de Groot, M. A. M. Smeets, A. Kaldewaij, M. J. A. Duijndam, G. R. Semin. 2012. Chemosignals Communicate Human Emotions. Psychological Science, DOI:10.1177/0956797612445317.
Olofsson, M, H. Lovlie, J. Tiblin, S. Jakobsson & C. Wiklund. 2013. Eyespot display in the peacock butterfly triggers antipredator behaviors in naïve adult fowl. Behavioral Ecology 24: 305-310.
Steck et al. 2010. Do desert ants smell the scenery in stereo? 2010. Animal Behaviour, DOI:10.1016/j.anbehav.2010.01.011.
Vieira et al. Unique Features of Odorant-Binding Proteins of the Parasitoid Wasp Nasonia vitripennis Revealed by Genome Annotation and Comparative Analyses. 2012. PLoS ONE 7(8): 43034. DOI:10.1371/journal.pone.0043034.
Zhou et al. Phylogenetic and Transcriptomic Analysis of Chemosensory Receptors in a Pair of Divergent Ant Species Reveals Sex-Specific Signatures of Odor Coding. 2012. PLoS ONE. DOI: 10.1371/journal.pgen.1002930.