Which insect flies the fastest?

The Australian dragonfly, Austrophlebia costalis, 36 mph.

Minnesota's answer sounds plausible. Dragonflies (and other members of order Odonata) are very strong fliers.

However, this site suggests that the horsefly (Hybomitra hinei wrighti) "was recently clocked at 145 km/h" = 90 mph. (No attribution).

We may have to arrange a race. Check this out, The deer bot fly also claims 90 mph.
Another challenger !

The questionable record of the bot fly

"In 1927, Charles Townsend took photographs of the botfly in flight. He used the blurred images of the flying insect and the shutter speed of the camera to calculate the botfly's speed. His results, published in a respected scientific journal, declared that the insect could fly 820 miles per hour - a little faster than the speed of sound!

Scientists took his claim seriously for years. But in 1938, Nobel Prize-winner Irving Langmuir challenged Townsend's findings. His own studies showed that the air pressure on an insect at that speed would be so great that it would be squashed in flight. (He was right; the botfly goes only a small fraction of that speed.)

Today, scientists gather more accurate measurements by using sound recordings as the insect passes between two points. High-speed filming is another popular way to gauge speed. Using these methods, the fastest insect known today goes about 60 m.p.h.

Some scientists report clocking insects at 90 m.p.h., but these are unpublished results. Until results are published, people tend not to take them seriously. And even so, as in Charles Townsend's case, even published results may not be right."

From:
http://www.csmonitor.com/durable/2000/09/05/p22s2.htm

Haha, I'd have to say flea myself, even though I know its not correct... They can get to ya and you not even notice them

The dragonfly must be the fastest only over a very short distance.I wonder how far the test was. Ours fly in quite a leisurely manner. Around here the dragonfly is called, in full, 'the Devil's darning-needle' because of is hunting habit; it flies up and down the same line all the time, patrolling back and forth,just like the needle of someone darning or sewing, but when it attacks it certainly darts like lightning!

This question was asked in Trivia, and ColdFuse and I both researched it fairly well. In fact, one University of California site changed their Insect Facts page in response to what we found. (Just another example of the power of AnswerPool!) See here for the thread.