By convention followed by the Entomological Society of America, as
mentioned in another post, honey bee is given as two words because it is
a true bee (superfamily Apoidea). House fly would also be given as two
words because it is a member of the order of true flies (Diptera), in
contrast with, for instance, dragonfly (one word), the many species of which
belong to an entirely different group of insects unrelated to the true flies.
 
Tom Culliney
Hawaii Dept. of Agriculture
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