Yes, great images, and also puzzling!

Where were the adult Polistes apachus wasps when their offspring were being eaten? I think of wasps as fierce warriors, I've seen even the relatively docile Polistes dominula decapitate Odonata! How could this happen?

Arthur: the caterpillars are very well protected by extremely tough webbing they spin. Even if the wasps tore apart their own nest they probably would not get to them. When I dissected the nest it was extremely difficult to tear the webbing even with two pairs of forceps.

Thanks for taking the time to explain. Polisties wasps do not seem to be aggressive with the one exception of disturbing their nests. For that reason I stay a ways back from an occupied nest. When I look at an empty nest the caps have been chewed through.

1) So it would seem that the caps made by the wasps are not very difficult obstacles.

2) The caterpillars can make very tough caps that when in place form a extremely tough barrier that the wasps can't get through.

But how do they get past the adults tending the nest, in order to make the caps! You mention that three adults wasps were at the nest when you were there. Presumably, there were even more when the nest was had more wasp cells occupied. The part I don't get is who deposits the eggs in the nest? The caterpillars are not adult, and I don't see how adult lepidopterans could parasitize an adult wasp.

If the young caterpiller is chewing through the cap to get at the wasp larva, why dont the wasps kill them? I've seen wasps kill caterpillars before. I have photographed them doing so. My impression was the wasps (Ancistrocerus spilopterus?) were actively seeking them out.

And these caterpillars would have to be small to fit in the cells! I guess the bristles and mandibles and skin must protect them? I can try to Google it, but that can be a challenge. I'd appreciate any clarification. I know it is not a question really worthy of your expertise!

And I totally love your story here because I am seeing caterpillars in and entirely different light. I have heard some could really hurt you with their "quills" and chemical excretions. But this is California. I'm not aware of any that could be called dangerous.

So, thanks for posting this fascinating struggle!