How do I enter the giant worm?

I've read there's four bosses located in the game. It seems obvious to me that the enormous worm that slowly snakes its way through the sky is one of these. The mouth of the worm also has a force field in it like some other bosses I've seen.

For those bosses, to drop the force field I had to find some blue crystals and destroy them on other parts of the boss. I can't find any for the life of me on the worm though, just some weird purple radar dish things that I tried shooting to no effect.

How do I drop the force field and enter the worm?

First note: the worm is the final of the four bosses, and the field that protects him will be impenetrable until all three preceding bosses have been defeated. If you haven't already defeated the three other bosses, you'll need to hunt down each of them first before challenging the worm.

If you've already defeated the other three, you'll need to find several blue segments on the sky worm's body; each of them will have a gun attached and, right near the gun, a fairly well-hidden hole that you can fly inside. In each hole will be the blue crystal target to destroy, and once you've destroyed the blue crystal in each segment the main mouth force field will be destroyed and allow the fight to proceed.

This discussion on Steam has slightly more detail on strategies for the worm boss, as well.