Middle caterpillar has successfully formed its chrysalis.
I can always tell when a Monarch caterpillar is ready to form it's chrysalis.
It attaches itself to (hopefully) a firm surface - sometimes they walk off the plant for this, sometimes they remain on it. Next it creates a small fibrous button on the surface and eventually attaches its end 'feet' into the button. Then it hangs downwards in a "J" shape for about 24 hours and, after this time, you notice that its quivering.
The next sign for me, is that it looks as if the caterpillar has completely withdrawn from its feelers and they are now empty skin hanging limply. Then you notice that it seems to be trying to expand itself outwards and downwards and suddenly its skin begins splitting behind its head. Now the green inner section is becoming more and more visible as it slides the skin up, up up.
Once the skin has been slid right up to the top, the forming pupa twists and writhes in order to attach the cremaster (shiny black section that hooks into the button it is hanging from) into the fibrous button attaching it to whatever surface it has attached to. The cremaster will hold the chrysalis firmly until the butterfly is ready to emerge.
During all this writing and squirming (I call it the Houdini act) the expelled skin will usually drop away.
Then it grows still and, slowly but surely, pulls itself back up into the chrysalis shape it will remain in for about 10 days, until it starts to emerge as a butterfly.