The Scourge of Flies

I was at Beamer Falls Manor yesterday - a heritage home bed and breakfast in Grimsby on the escarpment.  Its target market is weddings.  We were looking at the garden together for planting ideas.  It is situated at Beamer Park, where the hawk migration is celebrated each year - the park has a commanding view of Grimsby and the Lake.   My question was:  Are these black flies? There were so many flies all around us that we swatted continuously.   There is nothing like this below the escarpment. What causes them to be in one place and not another in close proximity?  I didn't find any stories on this, but did enjoy the questions from foreign visitors on whether they should come to Ontario at all because of the 'horror stories about black flies'.   We do consider black flies to be a 'scourge'. Black flies can be a serious health problem - they can cause sickness and death.  There are stories of cattle being severely affected - an outbreak in Alberta in 1971 resulted in almost 1,000 animals dying, and others to lose significant amounts of weight. The term commonly used is: 'black flies cause human suffering and are a scourge to livestock.'  Returning down the escarpment, I considered myself lucky to return to my black flie-less garden.  Today we look at Strasburg Rail Road which we visited last Friday while in Pennsylvania.