Collecting documents where only one field term matches

[mad Cow]

Could some more experienced users suggest a solution to my problem. I have 
documents which contain multiple terms and phrases, and I wish to collect 
documents which match only the term I query for.

For example:
Doc1 contains,
  species:"homo sapien" Mammalia
Doc2 contains,
  species:"homo sapien"
I wish to collect documents ONLY with "homo sapien" but a search for 
species:"homo sapien" returns both documents as they both contain the 
phrase.
I have written code to cache every term for every field an I hoped that I 
could do the search - species:"homo sapien" -species:Mammalia. Unfortunately 
the terms homo and sapien seem to be separate.  So when I collect every term 
to use with the "-" operator I end up with a query thus
species:"homo sapien" -species:(homo Mammalia sapien)

which isn't the same.
Can anybody suggest another approach?
Many thanks
Iain
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