Don't be surprised if small wasp-like or other kinds of insects emerge from the gall instead of, or along with, the gall gnat. The wasp-like insects are called insect parasites, or more correctly parasitoids, and make their living by eating the gall gnat. Normally they do not bother the gnat. You can easily find several other kinds of galls on willow and on many other plants too. The more serious investigator will want to start a gall collection.
Innumerable variation occurs and galls are attractive and worth collecting for their own sake.
Collecting galls is easy. Galls are found all year long and those with woody tissues such as those on stems require no preparation other than labeling and displaying them. Soft-galls from buds, leaves, and fruit require some preservation. Most of the leaf galls will remain intact if pressed between newspapers until dry. Large galls should be slipped Into a large envelope and allowed to dry.