Sentence:  We have besides for this Month a little <WSD>Dun</WSD> call'd a whirling Dun (though it is not the whirling Dun indeed, which is one of the best Flies we have) and for this the dubbing must be of the bottom fur of a Squirrels tail and the wing of the grey feather of a Drake. 
Meaning of the word marked with <WSD> tags:  A (fly) made to resemble the mayfly subimago.