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I think as long as you give prominent credit to the original authors and don't do it for commercial reasons then it should be fine. The biggest problem though I that a lot of the features that you need to illustrate just don't appear prominently in any of the normal photos of flies. You might get the in specimen photos but still it can be difficult to resolve fine differences in a 2D medium, like photos, when it is so much easier to have a specimen that you can manipulate. Even withy specimen it can be difficult sometimes ;)