Answer 1:
Most flying insects have two pairs of wings (think of a dragonfly). In flies, the second pair of wings has evolved into special organs used for orientation in flight. These organs are called "halteres", and they function as balancing gyroscopes. All flies have halteres, including fruit flies and house flies. You can see the halteres easily on some crane flies because the flies are so large. In an interesting (but cruel) experiment, you could remove one or both of the halteres from a crane fly and watch how it's flight pattern changes.
An interesting discussion of halteres:
http://www.berkeley.edu/news/media/releases/98legacy/04_09_98a.html
Some ideas for organized experiments removing halteres:
http://www.uky.edu/Agriculture/Entomology/ythfacts/4h/leader/experim.htm
General information on flies:
http://insected.arizona.edu/flyinfo.htm
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Answer 2:
The wand-like structures to which you are referring are called "halteres" and as you have proposed, these structures are indeed balancing organs.Halteres or "balancers" are derived embryologically from the second pair of wings. They beat with a frequency close to that of the forewings and serve as in-flight stabilizers. Your question regarding their limitation to tipulids (the family to which crane flies belong) is a good one, as they are clearly visible in this group. Halteres, however, are found in all members the order Diptera, ("di" meaning two and "ptera" meaning wings, indicating that members of this group only have two wings, or one pair). This order includes the common houseflies, garbage flies, mosquitoes, midges, gnats, and their allies. Most other flying insects to which you are probably familiar, including dragonflies, butterflies, bees, etc., all have two pair of wings. An interesting family of flies that most people have probably unknowingly encountered are the syrphids, or "bee-flies". Members of this family are quite diverse, but many mimic bees in both behavior and color pattern, serving as a very successful defensive strategy. The next time you see something that looks like a small bee, carefully catch it (as it may in fact be a bee), and count the number of wings present and you may be surprised to discover that you have collected a harmless syrphid.
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