He asks for their help and they agree. The mercenaries are actually a Fellini-esque troupe of starving performers, but Flik thinks they are fighters. The actors think Flik is going to pay them to perform, not fight grasshoppers. Eventually things get straightened out, but by then, there is no time. The colony will have to make do with what it has.
Necessity is the mother of invention.
Screenwriters John Lasseter and Andrew Stanton saw the necessity from the ants' point of view, and not from their own. The problem was how to beat the grasshoppers, not how to finish the script. I was amazed that the bugs' invention was nothing I had seen before. There was even a second-grade skit about the ants versus the hoppers. Each of these bumps in the story was a reminder of how important, and how rare, good writing is.