About 1¼" inch long, these beetles are shiny black with bold yellow markings, including a yellow dorsal stripe on the head, thorax and scutellum, and transverse yellow and black stripes on the abdomen.
They were clustered on a large-leafed plant growing out of the water at the edge of a lagoon next to the Amazon River. Photo#2 is just a blow-up of #1 so you can see them better.
Note: the yellow marking on the lead beetle may be a break in the thorax covering, not a marking. [later: wrong -- that is an actual marking. I googled the ID given below and found similar markings on other Rutela beetles.]