Millions of substances are dissolved in the air, giving rise to the smells we perceive. Nature is full of pleasant aromas, such as most flowers, wood, and citrus fruits, but there are also unpleasant odors, such as those given off by decomposing bodies or excrement. Among the animals with the best sense of smell is the elephant, which has almost five times more olfactory receptors than humans, rats, cows, horses, or sharks. Flies also have to be included on this list. An exquisite olfactory radar. Flies' sense of smell is highly developed, allowing them to detect volatile compounds at great distances and reach the source without wasting time or energy. This "prodigious nose" is located in the antennae, where there are neurons—olfactory receptors—capable of capturing the molecules that form odors. Related News standard No Matter of Size: Life on Earth Prefers Larger and Smaller Individuals José Manuel Nieves A surprising study reveals that the biomass of our planet is concentrated at both ends of the size spectrum. Flies are capable of differentiating hundreds of volatile organic compounds at very low concentrations and can also differentiate chemically very similar odors. Once they have been identified, the receptor neurons generate a specific electrical signal that is sent, along a neuronal axon, to the primary olfactory center of the fly's brain (antennal lobe). In this lobe there are two types of neurons, some that are fundamentally excitatory and others that are inhibitory. From there, the information is sent to the brain area that is responsible for regulating the learning of responses to odors and to the regions responsible for giving innate responses to odors. These connections explain, among other things, why flies meticulously and regularly groom their antennae. It's basically a way of avoiding substances that could hinder olfactory perception. Skatole is to blame. Four substances are responsible for the unpleasant odor found in excrement: indole, cadaverine, putrescine, and skatole, which are produced by putrid fermentation. Indole originates from the metabolic degradation of tryptophan, a process carried out by some bacteria (Bacterium linens and Clostridium sporogens) that possess enzymes capable of hydrolyzing and deaminating tryptophan (tryptophanases). This biochemical process generates pyruvic acid, ammonia, and indole. Cadaverine is a colorless liquid that, when in contact with humid air, gives off vapor and a foul odor. It is a metabolite that originates from the decomposition of another amino acid, lysine. This liquid is responsible for the putrid or dead smell. A smell that also involves putrescine, a substance produced from the breakdown of amino acids. Finally, skatole is produced from the breakdown of hemoglobin, the protein responsible for transporting oxygen in the blood. Chemically, skatole is similar to indole and contributes to the unmistakable smell of intestinal gas. Flies are irresistibly attracted to skatole, as it helps them, like a GPS, locate excrement, the place where they will lay their eggs. Strategic 'maternity' Generally, flies do not build nests, that is, they do not construct ad hoc structures to reproduce, but rather lay their eggs in strategic locations that can be used by their larvae as food. For this reason, some flies lay their eggs in excrement or garbage, since decomposing matter, like feces, contains a large amount of bacteria, proteins, minerals, fats, and undigested carbohydrates that the larvae use for their nutrition. MORE INFORMATION news No Horses, Native Americans, and the Spanish Empire: The untold story now revealed by genetics news No 'Jurassic Park' gets it wrong again: the Tyrannosaurus rex had lips and no visible teeth These flies typically lay up to 2,000 eggs throughout their lives—between 75 and 100 in each clutch—which hatch in a day. This is when they will need to feed, for about ten days, before creating the cocoon. From this, the adult fly will eventually emerge three weeks later. In some ways, it was said that excrement acts as a strategic and improvised "maternity ward." ABOUT THE AUTHOR Pedro Gargantilla is an internist at El Escorial Hospital (Madrid) and the author of several popular books.
