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, Behemoths and Ultras 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.

Enemy Race Manager

NOTE:
With Factorio 2.0 released Enemy Race Manager will be updated in the future! So for now expect issues while running ERM.

Added compatibility to ERM so they can now spawn while that mod is installed. They will still be apart of the biter race and spawn from the spitter spawners, but they also grow stronger as with the rest of the enemies. They will also pathfind over water and encouraged to move over water on their own compared to without.

They fly over water fine, but in the base game they follow ground biters in groups and will only fly over terrain on their own if moving between different spawners or chasing a target. With ERM I gaves them actual Flying Squads so they they can prepare attacks on their own and attack in waves without requiring a valid pathway.

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!