Googling Caenis I found this:
http://www.pbase.com/michellemahood/image/32893967

The thorax wasn't dark this picture and had no trico-like hump. The
thorax was about the same as the abdomen - certainly the same color and
about the same diameter. Also some were this
cream-with-a-tint-of-light-green color (although most were cream).

Larry

Tom Littleton wrote:
maybe Caenis, maybe some really bleached-out Pseudocleon(you did say you had
no idea how long in
the web). Both can have species that run pretty small,
say size 26 or 28 hook sized........

Tom