There are a lot of these flies around these days. If you are going to feed a chameleon an outdoor fly, does this seem like a safe one? He's eaten one already and seemed eager for another.
It's some sort of horse fly - we have a different kind here. I'll say what I usually say and it's up to you if you're totally sure there is no pesticide and you gutload your staples and have a good vitamin schedule etc. then a few wild caught flies for variety and fun will do about as much harm as letting kids have a friday night burger and chips.....i.e. none. They enjoy them as much as I enjoy my burgers too......