Jo Ann-

Not aware of any websites that specialize in aquatic macroinvertebrates to the exclusion of aquatic insects.  However, some do a good job of covering all aquatic macroinvertebrates.  Those appear on my Recommended Websites page in the Aquatic Macro’s folder.  Other than that, I’d suggest you Google or Google Images on a key phrase, like “aquatic mite”.  Also, I have found that use of quotation marks surrounding the phrase will help cut down on unwanted query results.

Sincerely,
Roger