caterpillars eat zucchini

Day before yesterday I saw the one milkweed plant was almost completely eaten. I tried giving them cucumber, they ignored it. I gave them zucchini, they started to chow down.

Not the best for their health, maybe (it's a big unknown if it has long-term detriments) and from what I've read, you can only do this for 5th instar- when they are a few days from pupating. Better than starving right now.

I found a very small one- first or second instar, not sure. I doubt this little one will survive- no leaves left. (They have to eat milkweed when young).

Little one next to a big one:

What I guess is a 4th instar, above the stem, 5th instar below.

I now spend some time just sitting watching the caterpillars, (as well as sitting watching my fishes). They are so busy, busy munching. Every now and then one will trundle up a stem to its tip, wave its head around as if searching, trundle back down. Or make its way to the base of milkweed stem at ground level, and back up another. They freeze completely, as if playing 'statues', when I jostle the stem. Stay absolutely motionless, in whatever position they were in, for quite a few minutes (I didn't time) until all seems clear, and then they move around again.

I think I saw some vying for position on the stems, or around the food, as well. One caterpillar would approach another on a leaf or zucchini slice, and the caterpillar already eating would swing its head around and shove at the approaching one until it moved off. I saw some jerking their heads repeatedly at each other, as if having an argument. And they are kind of fastidious, too. I figured it's better to tie the zucchini on vertically, so their waste drops off easily onto the ground. They tend to lift their rears in the air when voiding, probably to make it fall free. Once I saw a caterpillar do this and the frass didn't fall off. The caterpillar swung its head around to its rear, nudged the frass until it fell, turned right back to eating again.