As a kid I used to love blowing bubbles. A few years ago when I started to pay more attention to insects, I noticed that flies blow bubbles too. At the time I tried to find more information about this behaviour, but unfortunately couldn’t find that much.
Fast forward to a month ago when I was observing flies for this blog. Again I noticed various flies blowing bubbles. This time around I decided to really look into scientific literature to find some answers to questions such as: Why do flies blow bubbles? What is the purpose of this? When do they do this? Do all flies blow bubbles? How do they make these bubbles?
Three different fly species bubbling.
Delving into the science behind fly bubbling or regurgitation as it is called, I found some answers. No definitive answers yet, though. Let’s start with the last questions. Fly bubbling is actually the formation of a droplet on the tip of the proboscis (mouthparts). These droplets range in colour and transparancy. After a short time, droplets will be sucked in again and this behaviour repeats itself for some time. The main idea is that it has something to do with its feeding behaviour and digestion. If a fly bubbles, it occurs some time after a meal. According to some sources, not all flies bubble. Apparently the blood-sucking flies and the hoverflies do not display this behaviour. It’s not yet clear why they don’t. Some other insects like bees, lacewings and wasps also bubble. But, again, no one seems to know why.
Over the years, several theories have been proposed regarding this bubbling behaviour. These range from temperature regulation, to aeration and to digestion. One hypothesis is that bubbling reduces water in the fly’s crop via evaporation, thereby increasing the concentration of sugar. However, a study specifically designed to test this idea, showed the opposite. Flies that sipped a 80% sugar solution bubbled a lot more than flies that were fed with a 20% sugar solution. The researchers also found that levels of osmolality differed between the solutions. A change in osmolality might be an indication for the fly that sugar levels also change. Sugar is converted to trehalose, a type of blood sugar in flies (humans produce glucose). Trehalose in turn is used as an energy source. While bubbling, a fly is practically motionless, which means it doesn’t use much energy. During this time, the fly gets a chance to convert concentrated sugar from the crop into trehalose in the blood. Depending on the concentration of blood sugar, the crop contraction rates (which is an indication of digestion) also increases or decreases. So a higher concentrated sugar meal may mean that the fly needs to process this more/sooner. Still, this doesn’t explain why bubbles are formed.
My non-scientific speculation is that bubbling could be just a side-effect of this digestion phase. I.e., while the crop contracts, fluid may ‘leak’ out onto the proboscis, which we see as droplets. Maybe it is similar to us humans eating a heavy meal and then need to chillax for some time due to acid refluxes? And perhaps the fly species that never bubble have differences in anatomy and/or no need to regulate their blood sugar levels in a similar way. But as with all speculations, they could be wrong and something else might be the explanation.
Some links to sources I have read: