>>it's plausible that trans people are at least in some cases the result of abnormal development of sexually dimorphic brain architecture that supports synergy between the sex of the body and the model of the sex of the body in the brain. >>

The best hypothesis at the moment seems to be upregulation of hormones in the womb, based on prevalence of trans individuals in families based on birth order. But it's not that strong.

Your lovely Drosophila detail doesn't actually tell us how the flies identify. So I'm not sure how applicable it is to humans.