Flight Of The Bumblebee

A male bumblebee about to alight on a Heuchera, with its glossa extended and ready to extract nectar. Date 24 May 2008 Source Own work Author

Sffubs Thursday was World Bee Day. There is a widespread myth about bumblebees which started in the 1930s when the entomologist August Magnan asserted that it would be aerodynamically impossible for them to fly, but they ignore this and fly anyway. It’s a good myth, because it encourages people to strive for the impossible, and is for example quoted memorably (for me) in the ‘Doctor Who’ story ‘The Robots Of Death’, where the Doctor is told that murders cannot be carried out by robots due to the Second Law of Robotics: A robot may not harm a human being or through inaction allow a human being to come to harm. It works well as an illustration of positive thinking and avoiding dragging people’s confidence down through negativity. Sarada has a story where someone tells the bumblebees they can’t fly, and all of a sudden they can’t. This story would in fact work even if they could, and it might even work better because the ignorance of the person who told them would end up doing more damage than their practical experience of the opposite. But how was the conclusion reached in the first place? Firstly, it started off in the informal context of a dinner party and was based on some back of the envelope calculations. Magnan noted that the bees concerned had very bulky bodies and relatively small and flimsy wings which didn’t look like they would be able to support their weight. This thought is actually not as slapdash as it might seem, as most people shown pictures of non-existent animals such as Dumbo and wyverns will find they look wrong if the proportions of the flying organs are incorrect for their weight. Dumbo actually would be able to fly, apparently, or at least glide. We somehow have an intuitive understanding of these things, so the entomologist’s impression shouldn’t be dismissed out of hand. However, he didn’t simply assert that it was impossible, but went on to investigate how they were in fact able to fly. The calculations were based on heavier than air craft with smooth flat wings, and assumed that the air passing the wings would be able to separate easily from their surfaces. Neither of these turned out to be correct. On examining their wings under a microscope (and I’m going to make the charitable assumption here that this was of a bee who had been found dead), Magnan found that their wings had rough surfaces, and the reason they can fly is that they undergo a process known as “dynamic stall”. Bumblebee wings, like those of most other animals, flap, and changing the angle of the wing creates a vortex at the leading edge which then travels backwards, increasing lift for a short period. Because bumblebees flap very fast, like most other insects, the brevity of the effect is not a problem as it happens with every flap and it’s thought that they get most of their lift from dynamic stall. So, that’s how they fly (bearing in mind that every equation halves readership, there are none in this post!). The trouble is, of course, that the idea that they can’t fly but they do anyway is more memorable than the explanation of how they do, which is also quite complicated. I found it difficult to follow, anyway. It always seemed to be in the spirit of “we don’t know how bumblebees fly: let’s find out” rather than “there simply is no explanation.” However, I know I like a mystery and I want the world to be weird. Ockham’s Razor could be restated as “the most boring explanation is also the most plausible”. I used to watch ‘Call My Bluff’ a lot, and pretty soon I found that the easiest way to work out which of the definitions was true was just to go for the one I found least interesting, so it doesn’t just work in science but all over the place. There’s a confusing division among animals, mainly applied to vertebrates, between “cold-blooded” and “warm-blooded”. This doesn’t always work for several reasons. One is that in a very hot climate the so-called “warm-blooded” animals may have a lower body temperature than the “cold-blooded” ones, although the latter would attempt to reduce their temperature through their behaviour such as diving into water or burying themselves. Another is that hibernating mammals such as hamsters and bats sometimes have such a low internal body temperature that it’s practically at freezing point. It’s therefore better to describe them as homoeothermic or pokilothermic – they either regulate their temperatures internally or externally. There are also animals, only bony fish as far as I know, who warm specific parts of their bodies such as particular muscles, their eyes and their brains, which need to work faster than the rest. It’s generally believed that any animal breathing water with gills cannot be entirely homoeothermic because heat would be constantly lost to the water passing over the blood vessels in them, but I would beg to differ because testicles and the feet of birds who live in cold climates have a countercurrent multiplier system whose blood heat is transferred from one side of the circulation to the other to ensure that either the testicles stay colder than the rest of the body or that heat is not constantly lost to the cold water or ice at the birds’ feet, and I can’t see that this wouldn’t happen with gills. However, it never seems to have done so, and there are disadvantages to having an internally heated body, notably that you need a lot of energy from food, there are certain body shapes which are impossible for a warm-blooded animal to have, such as a snake-like one, and a minimum size due to the relative increase in surface area and therefore heat loss (or gain). The reason I bring this up here is personal scepticism about the idea that only mammals and birds are warm-blooded throughout their bodies. Muscle activity involves the generation of heat simply because that’s how thermodynamics work, and insects flap their wings very fast – six hundred times a second in the case of mosquitoes – so it could be expected that their bodies would heat up as they fly, although some of that heat will dissipate because of their small size and the fact that air is rushing past them carrying it away. Nonetheless, flying insects show up on heat-detecting cameras and their bodies are constantly generating heat as they fly. Moreover, their bodies are usually very small so they would be expected to be entirely heated by the activity too. Then there are bumblebees. These are stockily-framed and furry, and of course fur in mammals has an insulating function. For animals of their mass, they have a small surface area. Therefore their heat is more likely to be kept within their bodies and have less opportunity to be lost compared to an insect shaped like a damselfly. The largest bumblebee species is the South American Bombus dahlbomii, who is forty millimetres long and has a mass of up to 1 500 milligrammes. For comparison, a bee hummingbird has a mass of 1 600 to 2 000 milligrammes, is fifty-five to sixty-one millimetres in length, and is obviously extremely warm-blooded with a body temperature of 46°C, and Etruscan shrews have a mass of 1 800 milligrammes. Although the bumblebee species is the smallest and lightest, there isn’t much to choose between these figures, and it’s also significant that the hummingbirds have an extremely high body temperature because of their extreme muscular exertion. Therefore, I believe that bumblebees very probably are warm-blooded, and should be acknowledged as such. They shiver when they’re cold and this brings up their temperature to the point where their thoracic muscles can function to enable them to fly. As far as I’m concerned, that’s warm-blooded behaviour. Honeybee hives reach an internal temperature of 32-35°C, above which workers stand at the entrances and fan their wings to cool them down, so considering an insect colony as a single organism, they are also warm-blooded and regulate their temperature by internal methods. There are two types of insect flight. The first, direct flight, involves muscles which actually move the wings themselves, and is used by dragonflies, damselflies and mayflies. In the indirect method, muscles alternately pull and relax, changing the shape of the thorax, which consequently flaps the wings. I don’t know if these two techniques evolved independently, but insects with this arrangement can fold their wings flat over the backs of their abdomens, although this has been lost in butterflies. I probably don’t need to say this but will anyway: although hymenoptera, the order including bees, ants and wasps, have four wings each, they’re joined together by hooks and therefore only appear to have two. I’m guessing this gives them a larger area for lift. Bumblebees avoid stinging humans more than honeybees do, and this is probably because they aren’t as colonial. The life of a honey bee in a colony is expendable because they’re infertile and there are loads of them, but a bumblebee has a smaller family most of the time. If she dies, she either won’t pass on her genes or the genes she has passed on will die with her starving offspring. However, there are also cuckoo bees, who lay their eggs to be brought up by other species of bumblebee. Nevertheless, they don’t sting us, and I don’t really know why. However, even bumblebees have colonies, of between twenty and tens of thousands of individuals, which they make in old mammal burrows underground. Solitary bees usually look more like honeybees. In accordance with World Bee Day, attention should be drawn to their rôle as pollinators. Without pollinators, we would be confined to a much smaller range of edible plants, as would other animal species. Cereals seem to be the only bulky food plants currently eaten which don’t depend on insect pollination, and in particular anything that grows on land which might be seen as healthy, i.e. brightly coloured fruit and most green vegetables, would cease to exist or need to be fertilised by humans or perhaps some robotic method. In my more paranoid moments, I imagine that there is actually a plan to drive pollinators to extinction in order to replace them with tiny flying robots, but in general I cleave to cockup rather than conspiracy so in fact I don’t think this is happening and that Monsanto and the like are just reckless and greedy without a long-term plan. It remains the case that unicellular algæ and seaweed are very nutritious and do provide many of the nutrients which would be lacking if all the fruit and veg disappeared from our diet. In fact I’m quite keen on the idea that we proceed to leave the rest of the planet alone and just live off the likes of yeast and spirulina grown in tanks or the open ocean, which would considerably reduce the damage we’re doing to the biosphere. Still, it would be full of plastic nowadays. One thing we did a while back was to get a container with a series of straw-like tubes in it, just the right width for masonry bees, and placed it on a south-facing wall, although I don’t think we got any bees. Masonry bees, like bumblebees, don’t sting humans. British bumblebees have between three dozen and three gross individuals in their colonies which are seasonal, lasting from May to October. There only feed on flowers rather than leaves and roots, so they need more of them than other bees and many of them only forage within a kilometre of their nests. This means that a practice as seemingly trivial as fortnightly mowing of road verges can wipe colonies out. Flower meadows have also almost all disappeared now, under pressure from the supermarkets. I’m guessing that Brexit will make this worse because of having to produce more of our own food, which could be a good thing, and yes, in spite of that I’m still pro-Brexit but you have to remember that I don’t share the vision of the right wing version and I just said I’d be happy to live off microbes grown in tanks anyway. Without insect pollinators or a robotic alternative, it would cost Britain £1 800 million per annum to pollinate our crops by hand. So that’s a job creation scheme at least, but at the cost of millions of dead bees. There seem to be around two dozen species of bumblebee in Britain, of whom I recognise about a quarter. At least two have become extinct over the last century. They tend to do well with the flowers which happen to be in our own back garden, which may be to do with my mother: Mahonia, borage, comfrey, wild strawberry, sage and rosemary. They also like monkshood but obviously there is no sodding way I’m going to have that in the garden, particularly as our granddaughter sometimes visits. Finally, why ‘Flight Of The Bumblebee’? The music is from Rimsky-Korsakov’s opera ‘The Tale Of Tsar Saltan’ where the Swan Bird transforms the Tsar’s son into a bumblebee so he can visit his father. Fairly typical of operas I suppose.