I don't know why mosquito bites have to be itchy. If they weren't you wouldn't even notice them and most people wouldn't even care about being bitten (if they weren't in a disease area). Therefore you'd think nature would favour mosquitos whose bites weren't as itchy as they would be able to feed more easily (people wouldn't bother with protective measures).

Perhaps the itchiness evolved in US so that we DO know that we've been bitten as many diseases are transmitted by them, and it's important to know ASAP if you're bitten to try and prevent infection.

Pretty interesting!