The horses on the farm were surrounded by annoying shoofly flies.

shoofly 是由 shoo（驱赶动物的拟声词）和 fly（苍蝇）组成的复合词，字面意思是"驱赶苍蝇"。 shoofly fly 特指一种骚扰牲畜的蝇类，中文常译为"牛蝇"或"马蝇"，它们会围着牲畜打转并叮咬。