Comment by Sharlin

Mosquitoes (which are flies) don’t really deserve the exclamation mark, given that the males of all mosquito species live exclusively on nectar, with the females (of most but not all species) augmenting their diet with animal protein in order to produce eggs.

The long, thin proboscis originally evolved for nectar drinking like in many other insects (and some birds!); its use for sucking animal blood was a later adaptation.