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Today we have a text-and-photo essay by reader Athayde Tonhasca Júnior.  His captions and descriptions are indented, and you can enlarge his photos by clicking on them.

Bubbling sans blubbing

We don’t know much about Marcus Terentius Varro (116–27 BC), but it’s safe to assume this Roman polymath was no slacker. Varro wrote about history, theology, philosophy, language, literature, rhetoric, music, medicine, geography, architecture, law and agriculture, including apiculture (the Varroa genus of parasitic mites that plague honey bees was named after him). Near the end of his life, Varro wrote res rusticae (Country Matters), the last of his reported 490 publications. Possibly gripped by a sense of irrevocability about what was soon to come, Varro opens res rusticae with a sentence that says …ut dicitur, si est homo bulla, eo magis senex (…as they say, man is a bubble, all the more so is an old man). Varro’s image of life as ephemeral and fragile as a bubble was picked up by the Dutch humanist and philosopher Erasmus (1466-1536), who popularised the saying homo bulla est (man is a bubble). Erasmus in turn hit a chord with baroque Dutch painters who had started the vanitas (futility) movement, a style focussed on depicting the transience of life, the pointlessness of earthly pursuits, the inevitability of death. Vanitas painters were partial to dark environs, extinguished candles, flowers, clocks, hourglasses, skeletons, skulls, and lots of bubbles.

Quis evadet? (Who evades [death]?), by Hendrick Goltzius (1558–1617). The last two lines of the poem on the lower margin read: “Likewise the life of man, already ebbing in the newborn babe, vanishes like a bubble or like fleeting smoke” © The Metropolitan Museum of Art.

The appeal of this less than cheery theme wasn’t to last: not many art patrons wanted to be reminded of their approaching death every time they looked at a painting on the wall. The Dutch artists needed another source of inspiration. They kept the bubbles, but now depicted as aids to children playing games and having fun – bubbles of a joyful variety instead of portents of oblivion (Kareem, 2005).

Blowing bubbles, by Luigi Bechi (1830–1919) © Wikimedia Commons.

To this day, blowing soap bubbles thrills and entertains many a child. But to several bee species, blowing bubbles is no laughing matter.

The great majority of the more than 20,000 known bee species depend on nectar from flowers as their main source of energy. Nectar, secreted by specialised glands (nectaries), contains sugars, mostly glucose, fructose and sucrose. Free amino acids, proteins, minerals and lipids add to the mixture’s nutritional content, although we have only a vague understanding of their workings. Besides bees, wasps, hover flies, mosquitoes, butterflies, moths, hummingbirds and bats are among the most enthusiastic consumers of this ready-to-use source of carbohydrates and nitrogen.

The sugar content of flowers from different plants varies considerably, and one could expect bees preferring the sugar-rich ones. But they face a mechanical obstacle: the sweetest nectars offer the greatest energetic rewards, but are hard to extract because of their viscosity. And the longer bees work on the sticky stuff, the more they are exposed to predators. Based on the balance sheet of energetic rewards and costs, sugar concentrations in the 35-65% range seem to be optimal. That suits plants fine, because 40% is the median value of sugar concentration for bee-visited flowers in many habitats and geographic regions (Pamminger et al., 2019)​.

Most bees take nectar by lapping it, while orchid bees, moths and butterflies suck it up © Pseudopanax, Wikimedia Commons.

Nectars with 35 to 65% sugars may be ideal for harvesting, but not so for preventing spoilage and fermentation during long-term storage. Some bees sort out the problem by removing excess water, thus concentrating the nectar. This process is best understood for the European honey bee (Apis mellifera). A worker returning to the hive passes the nectar to one of her sisters by trophallaxis, which is the transfer of food from one individual to another.

Honey bees engaging in stomodeal trophallaxis (mouth-to-mouth transfer of food). For termites and some ants, trophallaxis is proctodeal (anus-to-mouth) © Kate Anton, The Center for Pollinator Research, PennState.

The receiving bee regurgitates a bubble of nectar between her mandibles, then swallows it again. The nectar bubble is exposed to the warm air inside the hive and loses some water to evaporation. The bee repeats the process several times, making the nectar more and more concentrated. But the work is not done. The nectar is transferred to the honeycomb, which the bees fan with their wings to further reduce it. When water content is lowered to about 18%, the nectar has turned into a supersaturated sugar solution – or honey to us. Watch a clear and accurate 6’32” summary of honey bees’ amazing comings and goings to produce honey.

Incidentally, the bubbling activity has led some people to assert that honey is bee vomit. Is it? Vomiting is to eject the stomach contents through the mouth, usually as a reflex to some physiological anomaly. Bubbling and other forms of animal regurgitation are regular occurrences with some purpose other than self-protection. Moreover, nectar collected to become honey is not digested or mixed with food. Rather, it is temporarily stored in the bee’s crop (or honey stomach), a pouch located before the stomach proper. So, despite what you read in the social media, honey is not bee puke.

A nectar bubble between the mandibles of a honey bee © Weird Science.

Solitary bees, i.e., those that don’t live in colonies, don’t have large stashes of honey to care for, but several species also blow bubbles to concentrate the nectar. We don’t know for sure why they do it, but by repeated regurgitation and re-ingestion, bees make their nectar more viscous and possibly better suited for storage – just like for honey bees – but also to be transported,  mixed with pollen to be fed to their larvae, or used in nest construction (Portman et al., 2023). Watch the superbly named pure gold-green sweat bee (Augochlora pura) pausing her busy life to work on a bubble.

A dark-headed dimorphic-masked bee (Amphylaeus obscuriceps) bubbling to reduce nectar © Marc Newman, Wikimedia Commons.

Some flies, which make up another group of bubbling experts, hint at another function of the practice for bees: thermoregulation. The harshly named – despite being an excellent pollinator – oriental latrine fly (Chrysomya megacephala) lowers its body temperature by pushing a gobbet of liquid food in and out of its mouth several times before swallowing it. As the bubble comes out, evaporation lowers its temperature; when sucked in, it cools the fly’s body (Gomes et al., 2018). This form of evaporative cooling is analogous to what panting dogs and ear-flapping elephants do to keep cool.

The heat exchange dynamics in a droplet moved in and out of the mouth of an oriental latrine fly © Gomes et al., 2018.

We don’t know the extent of bubbling among bees, flies and other practitioners such as hawk moths, sawflies and mosquitoes. Whatever its frequency, bubbling is another clever trick that helps insects succeed out there in the wide harsh world.

A bubbling bee impersonator © David Haberthür, Wikimedia Commons.