Is this like a composting tumbler?

It's just a plastic can with a tight lid; I "tumble" it by flipping the can around. sorry for the delay in answering

But the intent is to turn the chopped up tomato plants into compost?

Yes; hopefully.

The picture is blurry, so I can't tell whether it's a maggot (which would be fine and expected--maggots are generally good composters and harmless to living plants) or a caterpillar. Given the shape and color of the head and the indication of hairs (if that's not just an artifact on a bad photo), it really looks like some kind of moth caterpillar, like a pantry moth caterpillar, but these are not composters, AFAIK. Did you have some other kind of waste or storage of straw or other cereal-bearing material in there recently?

No, the entire can is filled with shredded tomato plants. Actually the problem seems to have gone away by itself. I added a layer of dead leaves to the top of the pile and now I don't see the critters. Basically I'll just leave it be over the winter and toss and check every now and then. Worst case scenario is that I have a can full of useless soil. Best case scenario is that I'll have a can full of nice composted soil. I have another can going that I filled with compostable food leftovers; IE banana skins, coffee grinds, egg shells and the like. This is my first attempt at composting; I'm such a newbie that I referred to it as mulching when I should have said composting.

I have a semi-knowledgeable neighbor who has been helping out with the kitchen scrape pile = mixing in some soil and he added something else, I haven't seen him recently so I haven't been able to pick his brain about the tomato pile. I'm hoping to have some success with the kitchen pile and will take whatever happens to the tomato pile. I guess I'll find out in the spring. Thank you for all the advice; I love forums

Good luck. And I highly recommend a worm bin for kitchen garbage. I have one myself, and, done right, it's a great way to dispose of plant-based food waste without having to deal with any stink.