The Bumblebee Myth

I had an interesting quick discussion on Twitter earlier today where the issue of the old "bumblebees can't fly myth" came up.  This is an old story that has turned into something of an urban legend.  There are all sorts of versions running around the internet (brief overview here), but they typically suggest that an aerodynamicist of some note showed that it was mathematically impossible for a bumblebee to fly.  Obviously, bumblebees do fly, and the story is often used in a derogatory sense to put down scientists and engineers or being intelligent yet (in the views of the story-tellers) unable to grasp something obvious to everyone else.  It is, as it were, another version of the Straw Vulcan argument to deride logical approaches to problems.

I could go on for a while about how ridiculous this sort of thing is, but I imagine it is self-evident to anyone that reads this blog, so I'll hold my typing on that front.  Needless to say, the story is based upon a flawed understanding of the situation.

What was shown at some point (though the details are foggy when this was done the first time) is that a standard steady-state, oscillating foil model using a rigid wing does not accurately describe bumblebee flight.  If bees had to fly like airplanes, they would indeed by grounded.  But, of course, bees don't fly like airplanes.  For one thing, insects have flexible wings.  The shape changes in the wings are critically to their lift production.  Second, insect wings carry a significant virtual mass of air on them as they move, owing to the "stickiness" of the air at that size scale.  This means the effective profile of the wing is not the same as the anatomical profile.  Thirdly, bees and many other insects with high wing beat frequencies seem to rely heavily on unsteady effects.  This means that the air never reaches equilibrium on their wings.  The flow is kept in a constant state of imbalance; this works best at very high wing beat frequencies and small size scales and can produce very high lift coefficients (well above steady state maxima).  Small birds and bats can use some of these tricks, too, it would seem, because the measured maximum lift coefficients are often quite large in small animal flyers (about 5 for flycatchers, for example, compared to the 1.6 theoretical maximum for a thick feathered wing with slight camber).

So, in short: yes, bumblebees can fly.  We understand how they fly and have for a long time, though there is plenty left to learn about insect flight overall. The story that someone proved bumblebees can't fly on paper is a myth (or, at best, a major misinterpretation).  Finally, suggesting that logic is a bad way of solving problems is silly.

Insect Takeoff

At left, a small parasitic wasp (Brachyserphus sp.) launches herself into the air. This species attacks nitidulid beetles, and the photograph is again a wonderful shot by entomologist and photographer extraordinaire, Alexander Wild (as before, go to http://www.alexanderwild.com for more awesome shots).

One thing that you may note, if you are feeling observant, is that the wings are just coming down as the walking limbs are completing their push off of the substrate.  This seems to be a rather general trend - the legs are used to initiate launch and the wings engage relatively late in the launch cycle (Nachtigall and Wilson, 1967; Nachtigall, 1968; 1978; Schouest et al., 1986; Trimarchi and Schneiderman, 1993; 1995).  A "leap first, flap second" modality is also seen in vertebrates, and the generality of the trend in flying animals probably derives from the fact that pushing off the substrate provides much greater efficiency in achieving high accelerations from rest than would be accomplished by first engaging the wings.  It is also worth noting that slow flight, which would include the first moments after takeoff, typically requires greater wingstroke amplitudes than fast flight - so clearance for the wings can be a problem for a launching animal.  The easiest solution is simply to jump first, fly second.


References

Nachtigall and D.M. Wilson. 1967. Neuro-muscular control of dipteran flight. J. exp. Biol. 47: 77–97

Nachtigall. 1968. Elektrophysiologische und kinematische Untersuchungen über Start und Stop des Flugmotors von Fliegen. Z. vergl. Physiol. 61: 1-20

Nachtigall. 1978. Der Startsprung der Stubenfliege Musca domestica. Ent. Germ. 4: 368-373

Schouest LP, Anderson M, Miller TA. 1986. The ultrastructure and physiology of the tergotrochanteral depressor muscle of the housefly, Musca domestic. J. Exp Zool. 239: 147-158

Trimarchi JR and Schneiderman AM. 1993. Giant fiber activation of an intrinsic muscle in the mesothoracic leg of Drosophila melanogaster. J. Exp. Biol. 177: 149-167

Trimarchi JR and Schneiderman AM. 1995. Initiation of flight in the unrestrained fly, Drosophila melanogaster. J. Zool. Lond. 235: 211-222

Insect Flight and Human Health

Many of the practical applications of understanding animal flight relate to areas like robotics and aeronautics.  However, there are implications for human health, as well.  Since flying animals experience intense skeletal loads and have rather strict muscle output requirements, many of them (particularly vertebrate flyers) make good models for understanding the limits of biological tissues, which in turn can play a role in future breakthroughs for biomaterials, tissue engineering, and the like.

However, there is a more direct link with human health, as well: many of the world's most serious diseases are carried by flying insects. Parasite life cycles can be quite complex, and many of them include more than one host.  It is common for one of these hosts to be a smaller, more mobile organism, and that is often an insect.  The more mobile host is typically called the vector, though in truth the definition of a disease vector is more specific than that.

The conditions under which vectors are the most mobile will therefore often be the conditions under which disease spreads most quickly. As it turns out, insect flight is rather sensitive to atmospheric conditions, on account of insect muscle and respiratory physiology, as well as their typically size range.  Many insects, such as mosquitos, sit in a size range where changes in atmospheric density or temperature can change flight performance quite abruptly.  At left is a wonderful photograph of an Aedes triseriatus eastern treehole mosquito taking a blood meal, by Alexander Wild. To see more of Alex's spectacular photography (and trust me, he has stuff you've never seen before, guaranteed) go to http://www.alexanderwild.com/

The limits on vector performance may getting more relevant in some locations. For example, in Hawaii, avian malaria has decimated many of the endemic bird populations.  Many (if not most) of the lowland species present before European habitation of the islands are now extinct, or nearly so. However, the highlands have been protected from this effect in the past because they were cool enough to keep the malarial loads inside the mosquitoes low, and also because the cooler air and lower oxygen density inhibited the malaria-carrying mosquitos (different mosquito species take over at high altitudes).  Unfortunately, Freed et al. discovered a about six years back that more mild high altitude temperatures were allowing malaria to spread up the mountains.  They attribute this mostly to malarial loads within mosquitos, but it is likely that flight performance of the mosquitoes themselves is also improving at higher altitudes.  The Freed et al. study can be found here.

Avian malaria has been used as a model for understanding human malaria for decades (in fact, two of the Nobel Prizes for malaria research were given for discoveries in avian malaria).  It is reasonable to suspect that human health will also be influenced by the impacts of climatic changes on vector mobility.  In the coming weeks, I will post more examples of how insect flight performance is critical to human health and economies - both for better and for worse.

References

Freed LA, Cann RL, Goff ML, Kuntz WA, Bodner GR. 2005. Increase in avian malaria at upper evelvation in Hawai'i.  The Condor 107: 753–764