moth id

[Rich and Lisa Flynn]

I found a large green catapillar eating my Datura (Jimson weed).   I put it in
an aquarium with the plant. It is now as large as my little finger,  has a
thin diagonal black strip with one small irridescent spot on each segment.  It
hasn't any protrusions.  This morning it has burrowed into the gravel at the
bottom of the aquarium.  I have wedged some sticks it there.   It doesn't fit
the pictorals in  my Audubon guide.  Any ideas?
Lisa Flynn
PS I assume its a moth but don't know for sure.