Weird question: I've been trying to find a short story I read years ago, which I think is by Sturgeon. Is there any chance you have run across it?

In it, an engineer is brought into a war zone, where giant tank-like creatures are on the edge of overrunning the front lines. The lines must be maintained in a cave system to keep extracting a valuable mineral. The generals keep pressuring the engineer for weapons, but he wants to understand the problem first. After lots of analysis, he directs the troops to destroy a certain structure. Within days all the tank creatures die. Then the engineer explains that the cave system is actually the inside of a caterpillar on a cosmic sized leaf, and the tank creatures were its immune system. Killing the caterpillar allowed the troops to loot the caterpillar's body in peace.

Ever heard of it? I'm not sure that my memory is completely accurate.