Subject: Caterpillar ID
Geographic location of the bug: SW North America (AZ desert)
Time: 12:11 PM EDT
Your letter to the bugman: We found this Sept, 27 early evening in New River, AZ (2,000 ft elevation). I have not found anything like it in my searches. The shiny silver barbs on its back come out when agitated.
How you want your letter signed: DC
This is a Silkmoth Caterpillar in the genus Syssphinx, possibly a Hubbard’s Small Silkmoth Caterpillar. Here is a BugGuide image for comparison.