Just because you have walls, cliffs, and water, it doesn't mean you are safe.

The Flier is a new enemy type that come to invade the world of Nauvis. As the name implies, it has the ability to fly over the terrain and obstacles to attack players and structures. It comes in four variations of the small, medium, big and behemoth sizes with the larger sizes being able to spew forth fire. This creature spawn from the spitter spawner once the evolution level reaches 10% and from their they will evolve and become more powerful as the evolution factor increases. The graph in the gallery more or less depicts the spawning chances of the evolution factor. Though it's not accurate due to it sharing the spawns of spitters as well and the mod settings.

Mod Settings

There are a few different configuration settings implemented to adjust how they spawn and what they attack with.

Mainly the ability to change how often they spawn compared to spitters on the spawner. The higher the chance the higher the odds of it spawning with even higher spawns making them more likely to spawn over spitters. By default it's set to 75%.

The second is the ability of the them to spew forth fire or acid. Due to how fire damage works in Factorio, fire damage is extremely deadly to the players structures that don't have immunity and even a small swarm can cause massive amounts of damage even to something like a spidertron. Though as long as you keep moving you should be fine, but at the same time, they tend to cause a bit of friendly fire to fellow bugs....

However, if you want them to not be as powerful you have the option to change it to a spitter like acid attack that isn't as insane in it's damage output compared to fire.

By default, the Small and Medium Fliers spew acid, and Big and Behemoths spew forth fire.

Compatibility

Armoured Biters

I've tested it with Armoured Biters and Armoured Biters are fine as it doesn't mess with spitter spawner code.

Rampant

Rampant technically causes some issues. Since rampant replaces the vanilla spitter spawners with their own after a bit, the fliers wont be able to spawn most of the time if at all, you might occasionally see some due to them spawning before the spawners got replaced, but in terms of errors or crashes there doesn't seem to be any with the mod, just don't be surprised if Fliers aren't common.
(I might resolve this in the future, but for now I'm going to leave it as is)

First mod upload yippee!