In the air there are dissolved millions of substances that give rise to the smells we perceive. Nature is full of pleasant aromas, such as most flowers, wood or citrus, but there are also unpleasant odors, those given off by decomposing bodies or excrement.
On the list of 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. In this list we also have to include flies.
An exquisite olfactory radar
The sense of smell of flies is highly developed, which allows 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.
Flies are capable of differentiating hundreds of volatile organic compounds at very low concentrations, and can also differentiate chemically very similar odors.
Once the receptor neurons have been identified, they 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 classes 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 brush their antennae. Basically, it is a way of avoiding substances that could hinder olfactory reception.
It’s the skatole’s fault
In the excrement there are four substances responsible for the bad smell: indole, cadaverine, putrescine and skatole, which are produced from putrid fermentation. Indole originates from the metabolic degradation of tryptophan, a process that is carried out by some bacteria (Bacterium linens and Clostridium sporogens) that have enzymes capable of hydrolyzing and deaminating tryptophan (tryptophanases). In this biochemical process, pyruvic acid, ammonia and indole are generated.
Cadaverine is a colorless liquid that in contact with humid air gives off steam and a fetid odor. It is a metabolite that originates from the breakdown of another amino acid, lysine. This liquid is responsible for the putrid or dead smell. A smell in which putrescine also participates, a substance produced from the fracture of amino acids.
Finally, skatole is produced from the breakdown of hemoglobin, the protein that is responsible for transporting oxygen in the blood. Chemically, skatole is similar to indole and contributes to the distinctive odor of intestinal gases.
Well, the flies feel an uncontrollable attraction towards the skatole, since it helps them, as if it were a GPS, to locate the excrement, the place where they will deposit their eggs.
Strategic ‘maternity hospitals’
In general, flies do not usually build nests, that is, they do not build ad hoc structures to reproduce, but rather lay their eggs in strategic places, which can be used by their larvae as food.
For this reason, the laying of eggs of some flies takes place in the excrement or in the garbage, since the decomposing matter, like the feces, contains a large quantity of bacteria, proteins, minerals, fats and carbohydrates not digested that the larvae use in their food.
Usually, these dipterans lay up to 2000 eggs throughout their lives – between 75 and 100 in each clutch – which hatch in one day. This is when they will need to feed, for about ten days, before creating the cocoon. The adult fly will end up breaking out of this three weeks later. Somehow it was produced to say that the excrements act as strategic and improvised “maternities”.