I'll categorize this as a quick tip, but it will be a touch longer than most. Wire bodies can be tricky to achieve the perfect taper without those gaps between wraps. I still struggle at times to get it just right, but here are some things I've learned along the way.
I believe creating a good wire body fly can be won or lost in the underbody. The smoother you can make it the easier it will be to wrap on top of it.
It's also important to pay attention to how you tie in your wire. I get the best results when I secure the material on the near side of the hook and closely follow the contour of shank.
Make each thread wrap count creating a smooth body especially in areas of transition (from the shank to the lead).
When wrapping, I angle the wire towards the tail. This allows each wrap to seat the next instead of trying to place them side by side. If you get gaps pull the tag end of the wire to bring them together. If that still doesn't work use your finger nail to push them tight. Another cheat I use, when possible, is matching the thread with the wire. In the event you do have a gap it won't be as noticeable(see photo in bottom right).