The first time I saw a fly with a droplet of water protruding from it’s mouth (or the tip of the proboscis) I was quite surprised. Why would a fly be blowing bubbles like this?
It turns out that flies do this to cool down! They move a droplet of fluid in and out and of their buccopharyngeal cavity a number of times before eventually swallowing it (Gomes et al., 2018). This is called bubbling behaviour, which helps them lower their body temperature by evaporative cooling.
Fully gorged flies eliminate excess water by bubbling behaviour. The evaporation of water concentrates the suspended nutrients and thereby reduces the crop volume, allowing the fly to resume fluid feeding, or minimize the water load while foraging for other resources (Hendrichs et al., 1992)..
As the droplet is extruded, evaporation lowers the fluid´s temperature, which, upon its re-ingestion, lowers the blowfly’s body temperature (Gomes et al., 2018).
Flies rarely move whilst bubbling, but if disturbed, they quickly reingest the liquid – although a few dropped their bubbles – and move off! (Stoffolano et al., 2008).
Some Hymenoptera (bees and wasps) also reportedly engage in bubbling behaviour, and occasionally deposit the regurgitate on the substrate, followed by subsequent re‐ingestion.
The droplet fluid consists of a mixture of liquids from the ingested food, enzymes from the salivary glands etc., with evidence pointing to a role in liquid meal dehydration (Gomes et al., 2018).
Coronado-Gonzalez, P. A., Vijaysegaran, S., & Robinson, A. S. (2008). Functional morphology of the mouthparts of the adult Mediterranean fruit fly, Ceratitis capitata. Journal of Insect Science, 8(1).
Gomes, G., Köberle, R., Von Zuben, C. J., & Andrade, D. V. (2018). Droplet bubbling evaporatively cools a blowfly. Scientific reports, 8(1), 1-7.
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.
Stoffolano Jr, J. G., Acaron, A., & Conway, M. (2008). “Bubbling” or droplet regurgitation in both sexes of adult Phormia regina (Diptera: Calliphoridae) fed various concentrations of sugar and protein solutions. Annals of the Entomological Society of America, 101(5), 964-970.
Links and blogs on fly bubbling