Just to add to Hugh's comment about midges...
Midges are one of the most widely distributed groups of insects worldwide. They inhabit almost every type of freshwater aquatic environment on Earth, and there are over 700 species of midges in North America alone. In fact, in most cases, midges represent over half of the total number of macroinvertebrate (insects, crayfish, etc.) species present in a given stream.
Due to their widespread distribution, relatively high concentrations, and diverse characteristics, they are able to persist in most environments. For this reason, it is always a good idea to have a variety of midge patterns in your fly box.