This is a very easy costume, which is great for costume parties. (I wore it at a party, and the costume was a big hit.)

Basically, you wear all black clothing, and you make a few accessories.

Constructing the wings:

To make the wings, you will need some wire. You can just use an old wire hanger, and change its shape. For each wing, bend the wire into, roughly, a tear shape. Either use one piece of wire for both wings, so that they are attached, or use two pieces and attach them together. The two points of the "tears" should point toward each other.

For my wings, I used two wire hangers. I changed the triangular shape of each hanger to a rounder, more tear-shaped form. I straightened out the hook part of each hanger somewhat, and then I twisted the two hook parts around each other to attach the wings. This way, the wings were firmly attached.

Next, cover each wing with clear cellophane. Take a large piece of cellophane for each wing, wrap it around the wing, and gather it at the tip of the "tear" shape. You can secure it by tying it tightly, or by wrapping some clear tape around it a few times. Secure any runaway bits of cellophane with little pieces of clear tape as well.

To wear the wings on your back, you need some elastic. Take two pieces elastic and tie them in loops. Each loop should be large enough to put your arm and shoulder through comfortably. Take the two loops and attach one to the base of each wing. (The "base" is the inside part, the point of the "tear".) Now you can wear your creation on your back by putting each arm through a loop (sort of like you would wear a backpack.).

Decorating the wings:

On each wing, draw some veins with black permanent marker. If you use a regular marker, it will come off. Look at the way the veins appear on a real fly, (in the encyclopedia, in a nature book or on the Internet,) and mimic them. Note how they branch off of each other - this is what makes them look like veins rather than just stripes.

To make the veins glow in dark, get some glow-in-the-dark sparkles. (They are available at craft stores.) Go over the black lines with some glue (such as Elmer's Glue), sprinkle the sparkles over the area, and then shake the excess off.

I suggest applying the sparkles one section, or one vein, at a time. Work over a large piece of paper. After sprinkling the sparkles, gather up the paper at both sides so that the excess sparkles fall to the center of the paper, and gently "pour" them into a small container. This way they aren't wasted, and you can reuse them for the next section/vein. If you try to apply sparkles to an entire wing at once, or if you don't use a paper to gather up the excess sparkles, you might run out of sparkles in the middle of the project.

Additional accessories:

Technically, flies don't have antennae. However, I chose to add antennae to my costume to inhance the insect-like appearance. You can make these easily with a hair band and two pipe cleaners. Black pipe cleaners look good, or maybe silver tinsel ones. You can even use a regular piece of wire if you prefer. Attach each antenna to the hair band simply by wrapping its end around the hair band once or twice. Try attaching a tiny piece of cardboard or construction paper to the end of each antenna, and putting a glow-in-the-dark sticker on the front and back. This way, the ends of your antennae will glow as well as the veins on your wings.

For a finishing touch, look for something fly-ish to put over your eyes. You can try bottle green or dark gray swimming goggles, or maybe some weird looking dark glasses.

I chose to make my own "fly eyes" by making some wire circles and covering them with bottle green cellophane. (I used some more wire to make a dome shape for each eye before attaching the cellophane.) I attached a piece of elastic and wore this creation like one would wear swimming goggles. It looked great, very fly-like, but I have to admit it was a bit hard to see through! Maybe you will have some smarter ideas yourself...