Translation:He saw a spider with a butterfly.
At first I thought "aaww a pretty butterfly" then I realised it was probably being devoured
Does this mean that they were just together? If so couldn't you use bei einem meaning to be with one another?
Perhaps the spider was doing something with the butterfly - rolling it, biting it, eating it....
of course but bei sein usually work if you are just with someone or something rather than mit? I know with activities mit is fine