Real Monarch or Imposter?

Aren't these great photos my granddaughter shot of the caterpillar munching away on the milkweed plant? I keep hoping to raise Monarch Butterflies. I got hopeful when this plant came back and the butterflies started laying eggs. We even saw a couple of caterpillars but they keep disappearing. Then we saw this impostor. He looks very much like the real thing which is his camouflage. Apparently monarch caterpillars taste bad. I think someone forgot to tell my local birds because they aren't surviving here. The big giveaway is that this impostor has an extra set of 'spikes' in the middle while the real thing just has two at each end.

Actually if you look closely at the first photo, (click on it for a full size version), you can see the real monarch caterpillar peeking through a hole in the leaf to the upper left of the impostor.