The crawlers are the babies, with the yellow infested hunches, that explode if you shoot them.
Here's a quick guide, that is simple. Use the Flamethrower. 'Nuff said.
I also wanted to add that killing the babies also helps you with the Necro Flambe achievement as well. So you are getting 2 birds with 1 stone.
The Flamethrower is a very underrated weapon, I even used it on my Zealot playthrough. If you full max out the damage you get a cool looking blue flame as well.