The hint for "fly" only gives verb forms of fly and not noun forms
Why isn't the masculine form correct as well?
Because for the word fly in Portuguese, there is no masculine form. Mosca doesn't necessarily mean a female fly, it's either. If you want to specify, you have to add in the Portuguese word for either "female" or "male". Hope this helps!
You forgot to mention those words are "fêmea" (female) and "macho" (male) :)