Of Crane Flies and Newton's First Law of Motion

Mating Crane Flies

My friend, Jim, knows that I like to work late and sleep in. So he emailed this morning to tell me about the mass of at least 50 crane flies in a mating frenzy behind his garage. Jim's a fisherman so he knows his flies, and he also knows my interest in anything bug-related. But by the time I arrived on scene we could only find three flies, one coupled pair and one exhausted male. They looked to be the back-spotted crane fly (Tipula dorsimacula).

The coolest thing about crane flies isn't that they look like huge mosquitoes. They're just harmless flies that don't bite, and most adults don't even eat. Nor is it that the adults only live for 10 to 15 days. They sometimes form breeding leks (like Jim found) for a brief mating frenzy, soon followed by egg-laying and death. (In this photo, the precocious female at top has just emerged from the leaf litter larval stage, and her new wings haven't even expanded fully!)

The coolest thing isn't even the fact that you can tell a crane fly's sex just by watching how it flies. Males fly with a bouncing up-and-down motion while females fly more straight-line.

No, these flies have something special that any science geek would love.

The coolest thing about crane flies is a special pair of club-like appendages, called "halteres," that work in conjunction with Newton's first law of motion to assist in their flying skills. (Remember that one? It says that an object in motion remains in motion unless acted upon by some force.) Although, it is puzzling that in spite of their scientifically-aided flight, crane flies are probably the slowest and clumsiest of all fly species (excluding flightless flies, of course).


Close-up showing a crane fly's left haltere, the club-like appendage below the wing and above rear leg

In ancient Greece, halteres were a special pair of hand-held weights that athletes swung back and forth to help them leap ("fly") farther in long-jump competitions. In some modern-day fly species, halteres are paired appendages that flap rapidly up and down with the wings, functioning as tiny gyroscopes.

Obeying Newton's first law, each flapping haltere maintains its plane of vibration until the crane fly changes direction. When the fly turns even a tiny bit, the halteres exert pressure on the fly's body that is detected by sensory organs (called campaniform sensilla) that are located at the base of each haltere. In this way the halteres provide rapid feedback to the flight muscles, help stabilize the head and steer the body in flight. At least, better than without halteres.


View of haltares from behind wing

But wait, there's more. It turns out that the formation of halteres in flies is controlled by a single gene. When this gene was experimentally turned off in the science lab, during metamorphosis from larva to adult, a pair of fully functional wings formed instead of halteres. Holy Batman!

Learning about amazing crane fly features might not be enough to morph you into a fly fan, unless you were previously susceptible to curiosity about bugs. But still, you've gotta' admit that flying gyroscopes that could be wings instead are pretty darned cool. And in this particular case, pretty funny as well.

The funny part? My friend, Jim, has an elderly uncle who is very creative. Uncle wants to patent a hat that he invented for elderly people to wear. It features a pair of gyroscopes that, according to uncle, helps older folks walk more effectively and prevent accidental falls. Maybe he should name his invention the "Haltere Hat."