That speedy botfly

I revived this excerpt from my out-of-print book The Speed of Nearly Everything when the image on the right turned up in my Facebook feed, coming from Science Humor.

In case you don't look out for details, the hole that the fly was caught in (or poked into) was actually made by a projectile coming from the other side, so the accompanying question about the fly's speed is, at the very least, just a bit misleading, but somebody is going to cite a legend.

When you enquire about fast animals, more often than not, you will read that the fastest animal of all is the deer botfly, which is credited with an amazing 1287 km/hr, though if you convert this to miles per hour, it comes out as a round 800 mph, a figure that smells a little bit like fudged science—and rightly so.

The story begins with a 1927 article by an entomologist called Charles Henry Tyler Townsend, who reported a speed like this in the Journal of the New York Entomological Society. He actually claimed that the fly was clipping along at 400 yards per second, which works out at 818 mph or 1316 km/hr in metric units. As we will see shortly, any preciseness in the conversion is hardly justified.

Townsend reasoned that these flies passed in a blur, and so must have been travelling very fast. On that scientific basis and no other, he credited them with a nice round 400 yards/second.

That story should have been questioned right away, but people wrote it down, passed it on, quoted it uncritically, and never stopped to wonder what would happen if flies were tearing around at supersonic speeds.

As we will see later (next entry in this blog), some people would stop to prove that the bumblebee could not fly, but nobody stopped to consider and demonstrate the impossibility of the botfly claim until 1938, when Irving Langmuir, a Nobel laureate in chemistry, having given it some thought, tested the assumptions.

First, the air pressure on the fly at that speed would be more than half an atmosphere, surely enough to crush it. The energy needed to maintain the flight would be 370 watts, half a horsepower, which would be quite an ask. Aside from anything else, the botfly would use up its own weight in fuel every second, so it would need to be a voracious feeder.

Next, Langmuir had been hit by these flies, and while it hurt, that weight of fly at 1300 km/hr would have left a significant hole, rather like that of a soft bullet, and the fly would have been mashed inside the wound. Instead, the fly bounced off.

Langmuir mocked up a model of the botfly, using solder to make a pellet that was 1 cm long and 0.5 cm wide. He attached this to a string, and whirled it around his head, timing it so he could work out its velocity. He reported that at 13 mph it was a blur, at 26 mph it was barely visible, at 43 mph an observer could not tell which way it was going, and at 64 mph, it was completely invisible.

He concluded that the blur Townsend had seen came from a fly travelling at 25 mph (40 km/hr). His results were published in Science and reported in Time magazine, but legends are tough things, even when they are debunked by Nobel Prize winners. So even today, the same old values keep emerging from the woodwork.

By a curious chance, Langmuir’s name crept into the record books in an entirely different way in 2006 when plasma physicists used a specially designed holographic-strobe camera to capture pictures of matter waves that were travelling at 99.997% of the speed of light.

Known as Langmuir waves, they are generated by intense laser pulses, and may one day lead to “tabletop” versions of high-energy particle accelerators. One step along the way was taking photographs of the waves to see if they behaved the way scientists thought they would. They did, which is more than we can say about the botfly's behaviour.