Insects Taste More Than Food and With More Than Mouths
By Panchalie B. Gunathunga
Insects taste food, sites to lay eggs, and even mates. Some insects also use taste to know where on their body needs grooming. We humans taste using taste buds on our tongues. The insect equivalent of taste buds are taste sensilla, which tend to look like hairs. Taste sensilla have specific protein receptors that are activated by certain compounds. This in turn triggers nerves in the sensilla, which may result in acceptance or rejection behavior. Taste / gustation in insects can be defined as detection of nonvolatile chemicals, either liquids or solids, whereas olfaction is detection of molecules traveling through air.
Why I Reviewed Insect Taste
In 2020, I was accepted into a Ph.D. program at Northern Illinois University, to work with Bethia King, Ph.D. Soon I would be off to the U.S. to begin experiments on house fly taste—or so I thought. Instead, because of COVID-19 and visa issues, for the first year of my Ph.D. work I was stuck in my home country, Sri Lanka.
So, Dr. King and I began work on a review to figure out which body parts different insects use to taste. That review ballooned into two reviews. Our first review paper “Gustation Across the Class Insecta: Body Locations” was published this month in the Annals of the Entomological Society of America. In a subsequent review (not yet published), we are summarizing the types of evidence used to determine if a given sensillum or body part has taste function, as well as what taste qualities insects respond to.
Previous insect taste reviews focused mostly on Drosophila fruit flies, because they have been so well studied. But, by going through hundreds of papers we began collecting information on taste in other insects. We believe that these new reviews will be of use and interest to entomologists, as well as to other people who study taste. Understanding taste in insects is important, especially in controlling pest insects.
Insect Body Parts Used to Taste
Although insects taste with their mouthparts, it seems rarely to be with a “tongue.” Beyond mouthparts, body parts like legs, antennae, wings, and ovipositors (egg-laying organs) are also used. Insects taste with these body parts because determining if a compound is good (appetitive) or bad (aversive) before mouth contact may save time and energy and reduce the risk of consuming toxins.
For many insects, the first point of contact with a food source is the legs. When a house fly’s legs are stimulated by an appetitive food source, the mouth parts extend for consumption, whereas contact with aversive compounds inhibits extension. Before doing our review, we had read about flies and lepidopterans that taste with their feet (tarsi), but what about insects from other orders? We learned that tasting with tarsi has been documented in five additional insect orders: Orthoptera, Blattodea, Hemiptera, Coleoptera, and Hymenoptera. Whether there are any entire insect orders with no tarsal taste function are yet to be discovered.
Prior to our reviews, we thought of insect antennae as being used for touch and smell. However, we learned that insect antennae are used to taste in some, but not all, fly species; many lepidopteran, hymenopteran, beetle, and hemipteran species; a few cockroach species, and a few mayfly species, as well as in a locust, a termite, a thrips, a stonefly, and a firebrat species. Taste function in wings, meanwhile, has been demonstrated in only a few fly species, but this sparsity is at least partly due to technical challenges in testing wings.
Apart from legs, antennae, and wings, some insects also have taste sensilla on their ovipositors. Ovipositor taste sensilla recognize good places to lay eggs. Evidence of taste function in male external genitalia is weak, and there is no evidence of vaginal taste sensilla, yet.
In addition to the external body parts used in taste, in some insect species, taste sensilla are found in internal organs like the pharynx (between the mouth and esophagus in insects). These sensilla are important because not all aversive compounds are present before ingestion. Some are breakdown products of digestion.
Which body parts can taste has been mostly studied in adult insects. More work on larvae is needed. Also, we found no studies of which body parts taste for any species of 15 insect orders.
What Insects Can Taste
Besides learning which body parts various insects taste with, we also learned that sweet, bitter, sour, salty, and umami are not the only important taste qualities. Like dogs and cats, at least some insects taste water. Other compounds that at least some insects taste include fatty acids, metals, carbonation, ribonucleic acid (RNA), adenosine triphosphate (ATP), pungent tastes as in horseradish, bacterial lipopolysaccharides, and certain pheromones.
Insect Taste Experiments: Undergraduates Like Them
With the current emphasis on course-based undergraduate research experiences (CUREs), we are pleased to report that many taste experiments can be conducted by undergraduate students accurately, cheaply, safely, and with minimal training. These experiments can help students understand experimental design and controls, such as how to think about, and rule out, alternative hypotheses, like smell and touch. So, for those of you looking for fun and easy experiments that can be conducted with undergraduates, we encourage taste experiments on understudied insect groups or on body parts that have not been tested before. Who knows what other body parts you may find that insects can taste with?
“Gustation Across the Class Insecta: Body Locations”
Annals of the Entomological Society of America
Panchalie B. Gunathunga is a Ph.D. student in the Department of Biological Sciences at Northern Illinois University in DeKalb, Illinois. Email: email@example.com.
What other experiments can be done to test how and what insects can taste?