Complex sentence: They feed almost exclusively on bottom living animals (snails, sedge fly larva and freshwater shrimp) with the exception of late summer when they come to the surface to feed and may be caught on the dry fly.
Split into simpler sentences: They feed almost exclusively on bottom living animals (snails, sedge fly larva and freshwater shrimp) with the exception of late summer. It is at this time that they come to to surface to feed and may be caught on the dry fly.