TX hit it right on the head and if they are allowed to dry on the paint or windshield then to get them off the windshield a squeegy will not work but an ice scraper will

On paint they will eat through the paint and clearcoat. If I remember correctly something on/in their bodies contains a mild acid for defense.

I've seen cars so badly covered after driving through a swarm that they are most black. Just swatting them out of the air usually makes them "pop" as they are very delicate bugs