There is a fly in my food!

I was teaching a lecture on Gustation or “the sense of taste” to my class today. This is the graduate class on sensory physiology which I offer every fall. It is very entertaining because I can tell interesting, fascinating, and sometimes horrible things about animal behavior to a captive audience. Today I happened to remark on the feeding habits of the ordinary house fly (Musca domestica) and it drew some groans and disgusted looks.

Now, the house fly feeds in a really interesting way. It cannot consume solid food, so when it finds something tasty like feces, or your dinner portion of Daube Provençale de Bouef, both being equally preferred, it first secretes saliva to liquefy the food. A fly generates a lot of saliva because it is constantly buzzing around and feeding. After having thoroughly liquefied the food externally, much like mastication in humans, the fly extends a proboscis. This is a monstrous looking appendage with a soft fleshy ending, rather like large hairy lips, that descends from the head. The fly presses the fleshy ending of the proboscis onto the substrate and sucks up the liquefied food. It has to be seen to be believed, because it looks like something out of an alien horror movie. Occasionally it regurgitates what it has eaten and sucks it back up again. I watched it happen up-close once.

I was idling away my time as usual, at a student eatery. One of those graduate student places where cheap food in large volumes is sold at cut-rate prices. They save money by cutting back on cleanliness and hygiene, knowing that it is really, really, hard to kill a graduate student. As I picked up my plastic knife and fork, I noticed a fly settling on my food. By strange coincidence I happened to have a magnifying glass with me, so I whipped it out and observed the fly.

It settled down moving its antennae-like feelers about. Just around the mouth are small projections called the maxillary palps which allow it to taste food. The fly has gustatory (taste) receptors not just around the mouth, but also its legs and the hairy lips-like proboscis. They are chock-full of sensory receptors for tasting all kinds of goodies. The fly is quite extraordinary and efficient in its search for food. I watched its feelers moving about, and could barely make out the proboscis descend as it fed for a few seconds. Then it rubbed its antennae and I watched as it flew to another spot on my plate. It buzzed about for several minutes picking and choosing. Sometimes it didn’t like what it tasted and flew away to another spot. I wondered what it liked and what it didn’t. When it liked something it just hung about, and liquefied my food with its saliva and sucked it up. Then it hopped to the edge of the plate and I watched it closely. It left a tiny drop of yellow fluid before it buzzed away. It had regurgitated its food! I was astounded. Could it be my own food that it had thrown up? Or was it perhaps doggie doo dah feasted upon earlier? I didn’t know. Luckily the fly had left enough food for both of us, and having paid good money for it, I simply ate what was left on my plate. I skirted the suspicious yellow spot. You can never know where that thing had been.

I remember asking myself “why does the fly want to eat my food? Does it like human food?” Well, we don’t eat everything a fly eats, but it pretty much eats what we do eat. Why? Because most organisms require the same kind of nutrients and so taste is a basic sense. It evolved as a class of chemosensory mechanisms that include olfaction or the sense of smell, and gustation or the sense of taste. The basic question that the gustatory brain asks is “Is it yummy?” In the early history of our evolution it probably allowed us to differentiate what is nutritious from what is not.

Human taste receptors are found in the tongue in taste buds. Taste buds are found in a taste pore. They are onion shaped structures that contain several types of receptors in a mixed bag. These receptors put out tiny hairs or microvilli that wave about in the open pore. Our saliva constant flows on the tongue, dissolves the taste molecules, and delivers them to the taste buds. Receptor proteins, found on the microvilli of the receptor, bind to the taste molecule and activate the receptor and send a signal to the brain. Our receptors respond to five different types of taste. These are sweet, salty, sour, umami, and bitter. A single receptor may not be specialized for a given type of taste, as it may respond to more than one type of taste, for example sour and bitter.

Of these, only the first four are really “food” receptors. It is very likely that the bitter taste receptor evolved so that we could avoid poisons. The most common poisons are plant alkaloids and they are usually bitter in taste. So very early on in evolution, this receptor warned us that something bitter was poisonous and allowed us to expel it. In fact the receptor for bitterness is more sensitive than the others, allowing us to quickly expectorate even the tiniest amounts.

This is why, even today, few cultures eat really bitter foods even if they are actually safe and healthy. We are genetically predisposed to avoiding them. But there are some exceptions. Indians have cultivated a taste for bitter gourd, as have the people of the Far East. And the endive (from the chicory family of plants) is quite bitter, perhaps not as much as the bitter gourd. I say “cultivated a taste” because that is really so. You have to cultivate a taste for bitterness otherwise you will gag and expel it. The bitter receptor triggers the gag reflex.

If you think about the kinds of things that are good for us, and what we consider to be tasty, then you will understand why we have two of the other four receptor types. The sweet receptor senses sugars. Sugar is an important source of energy because of glucose, which is found in ordinary table sugar (sucrose is a disaccharide of glucose and fructose). From it we derive energy. The receptor also responds to saccharine, which has no nutritional value. This explains the mad rush to discover sugar substitutes. We want our sweet foods, we want that sweet taste, but we don’t want them to give us so many calories.

Umami is Japanese for “delicious”. The taste receptor for umami was discovered more recently than the others and it is receptive to amino acids. Why amino acids? Think meat. Meat is mostly muscle mass with some fat. And muscle is protein. And protein is made up of amino acids. It is good for us. We need it! The umami taste also refers to the savory or meaty taste of broths. It was first discovered because of the strong affinity of this receptor for monosodium glutamate or MSG (sold as Ajinomoto in Asian stores). I have never understood why people are so opposed to MSG. It is a sodium salt of the essential amino acid glutamic acid. Surely, this can’t be bad for us? Otherwise we wouldn’t have a receptor for it. Besides, it tastes so good. I have never suffered from it.

The salty taste receptor probably allows us to seek electrolytes, particularly the sodium cation (Na+) and other alkali metal cations. These electrolytes are important for maintaining homeostasis in cells and tissues. Remember that we came out of the ocean a very long time ago. Our cells are bathed in a solution containing the alkali metal cations. This is saline. We have never really forgotten our origins, nor have we given up that ambience. We still carry it around inside us. I remember seeing a video of monkeys in India once. They were standing in a long line in front of a rocky cave. One by one, they took their turn and licked at the walls of the cave. It was one of the more curious videos I have seen on animal behavior. I would like to lick that wall and find out what it tastes like. I bet it tastes salty.

Perhaps the most mysterious taste is the sour taste. We still do not know why we have this taste, and we know very little about how receptors sense this taste. Sourness is related to pH or the acidity of a solution, and is a measure of the concentration of hydrogen ions (H+) or protons. Why do we need a receptor for it since in every other way gustation is quite parsimonious? I don’t know.

I asked my class today why is it that we humans don’t have a receptor for fatty acids? Humans are carnivorous, and eating meat implies eating fat. Besides, kilo for kilo, fat delivers a huge amount of energy: 9 Kcal/kg. You would have to walk on a treadmill for 45 minutes at 6.5 km/hr every day, for about 18 days, to burn a kilo of fat. That's quite a lot of energy. It is the ultimate store of energy during good times, so that we may survive during the bad. I find it a mystery that we do not have a receptor for fatty acids, yet we do have the taste. Il Sogno, a very nice restaurant in San Antonio, serves a pizza with a topping of lard. Only lard. Yes, long strips of glistening white fat. I believe it is scrupmptious although I have never had it because I am a vegetarian. So, we can eat pure fat and like it. I think it is possible that fat from meat already has a savory or umami taste and so they may go hand in hand.Or perhaps saturated fats are associated with meat protein and the brain associates the former with the latter.

A few decades ago, the US government opened its doors to the import of palm oil from Malaysia. It is one of the three vegetable oils that are high in saturated fatty acids, the kind found in animal fat. So it has a nice meaty taste to it, and it is cheaper than other oils. Besides, saturated fats have longer shelf life. So, the economics of switching to it were compelling. French fries, fried in palm oil taste as though they are fried in animal fat. People in America liked the taste and couldn’t tell the difference. So even though we have no receptors for fatty acids there is something going on here that we need to research. As long as fat tastes meaty it will be consumed. This is the reason why restaurant food tastes so good. The more pricey restaurants cook their food in butter. Try consuming a table spoon of canola oil and you will find it unappetizing. It is a vegetable oil that is low in saturated fats. Go on! Try it! I have done it. But still, we do not know enough, and I am just speculating.

In contrast to humans, the house fly has four types of taste receptors, with some similarities to human taste receptors. The best known receptor is used for tasting sugars and amino acids (the S neuron). Another responds to water (the water receptor). It is known that this receptor triggers a feeding response. This makes sense because the fly generates a lot of saliva and needs a lot of water. We humans don’t have receptors for water which is why we can’t taste it. The third receptor responds to salt and alkali halides (the L1 neuron). The fourth and final receptor responds to sodium salts of fatty acids (Second salt receptor or anion receptor). Aha! At last we find a receptor for fatty acids! There is one, we just don’t have it.

I have not gone into the feeding behavior of flies in detail. One of its more fascinating aspects is the pumping action of food into the gut. There is a pump present in the pharynx where the food is once again tasted, and if it is found to be suitable, it is pumped through the esophagus. This pumping action was recently studied using a real-time X-ray device by some Harvard researchers. They wanted to determine the hydrodynamics of the flow. I think it is absolutely marvelous that someone is taking the trouble to do this kind of research.

Given that human food contains salt, amino acids, water, and fat, it is not surprising that flies want to share our food. We are far apart on the taxa, and yet we have somewhat similar tastes. Like all organisms, here is one more wonder of evolution. Superbly adapted, and pesky, I am in admiration of the fly. You may say that there is no accounting for the taste of some.