Effects of bubbling behaviour in insects

June 2, 2021   5:30pm IST

   You might have blown bubbles with bubble gum probably for fun as it looked cool. Do you know that some flies and some other insects are observed to indulge in bubbling their semi-digested food from their crop? (crop is the part of the alimentary canal where food is stored before digestion.) Bubbling behaviour is extensively observed and studied in some flies, so, we will only be looking at flies. Even though this behaviour is termed as bubbling, they actually regurgitate a droplet and not a bubble which is hollow and contains air unlike a droplet. Why do they do that? To look cool?

It seems that flies bubble to look cool, not in front of other flies but in front of thermal imaging. They bubble their crop contents and then stay idle for some time to let the water in it evaporate. You would know evaporation causes cooling and it does happen in the droplet too. When the droplet is reingested, it cools down the fly's body and it appears blue in thermal imaging which represents lower temperatures.[1] By pooling the reingested liquid in certain regions of the oesophagus, they cool the vital organs. Cooling is most effective in the head followed by the thorax and the abdomen. The above process hints towards cooling the energy-expensive brain and flight muscles that acquire heat due to flight.

Knowing this fact, we would expect bubbling frequency to increase with ambient temperature. It is not as simple as we hope. Bubbling frequency also depends on the activity of the fly. When the temperature is perfect for them, their activity levels would be high. Such a state requires their muscles to be warmed up. In such a condition, cooling would be undesirable, and bubbling frequency would be very low. Above their preferred temperature, bubbling frequency increases as expected. However, above a specific threshold temperature, the bubbling frequency decreases again as the flies become inactive at higher temperatures.

Bubbling has another utility! These insects utilize evaporation the same way we do while separating salt from seawater. Whenwater evaporates, the solutes don't and thereby they increase the concentration of the solution. Evaporation concentrates the droplet and reingesting it reduces the water load in their crop.[2] Reducing the crop load also allows the flies to forage for more resources. Thus, bubbling flies are observed to have higher concentration of crop solutes than non-bubbling flies.[3]

It is even more fascinating to learn how effectively they use this strategy. The flies use muscles and sphincters (muscles which contract in the resting state and relax upon activation) in the crop and some mouthparts to manoeuvre the crop contents and form the droplet. It is observed that they dynamically manipulate the droplet using the above organs and by behaviours known as ‘proboscis shaking’ and ‘proboscis pumping’. Researchers have shown that the above dynamics help in effective evaporation as compared to a static drop. Researchers have also found low amounts of volatile compounds in the droplet which contains water, amino acids, hydrocarbons and carboxylic acids as the major constituents. The volatile compounds would ease the process of evaporation and hasten it.

We have pondered enough on the question “why the flies bubble?”. There remains another unanswered question that we have to talk about. Why is this particular mechanism used by flies? For that, we have to know what other mechanisms are possible. Sweating, panting and licking are the commonly observed mechanisms in the animal kingdom, where evaporation is used. The presence of an exoskeleton prohibits sweating and their respiratory system lacks the capacity to modulate heat dissipation. Both of the above-mentioned factors prohibit sweating and panting. Licking can’t be done by flies in the sense of using the tongue as they don’t have one. However, they can effectively use surface tension to spread the liquid over their body which is the requirement for the cooling effect. There isn’t any evidence for why flies do not prefer this. The possible reason might be the ineffective heat dissipation from the body to limbs, due to poor circulation, as compared to direct cooling.

References:

---

1. Gomes, G., Köberle, R., von Zuben, C. J., & Andrade, D. V. (2018). Droplet bubbling evaporatively cools a blowfly. Scientific Reports, 8(1).   
2. HENDRICHS, J., COOLEY, S. S., & PROKOPY, R. J. (1992). Post-feeding bubbling behaviour in fluid-feeding Diptera: concentration of crop contents by oral evaporation of excess water. Physiological Entomology, 17(2), 153–161.   
3. Vergnes, A., Pellissier, V., Lemperiere, G., Rollard, C., & Clergeau, P. (2014). Urban densification causes the decline of ground-dwelling arthropods. Biodiversity and Conservation, 23(8), 1859–1877.