Curled-winged houseflies are a really good food for various types of small lizards, spiders and of course the main diet for many types of the slightly smaller praying mantis. Curled-winged houseflies are basically just the same flies that you probably run around screaming after with the electric fly swatter when they accidentally float inside and disturb. Fortunately, these flies are curled so that they cannot fly, i.e. their wings do not unfold properly after they have come out of their cocoon, as wild flies do. However, there will almost always be 1 or 2 flies in a bucket that can fly, but rarely particularly well. This makes them somewhat easier to feed and work with. Flies themselves are probably not a huge nutritional bomb, but they contain a lot of calcium, and animals such as anoles or chameleons spend a lot of time chasing them, since the flies can crawl anywhere, they quickly get to the animals that will eat them. However, the flies can be made a little healthier if you want by gut-loading them, you can do this by giving them some time to eat something. Some use a thin slurry of, for example, infant formula, which you can give them a small amount of.