Question

under the name of buffalo - fly the black - fly is found in the west, where, on the prairies, it has been known to wage war on horses until death ensued - death of the horses, not of the fly. it is a small fly about one - sixth of an inch long, thick - bodied, and black. it is said to have broad silvery circles on its legs, but no one ever stops to look at these. its proboscis is developed to draw blood freely, and it is always in working order.
the only virtue the black - fly seems to have is its habit of quitting operations at sundown and leaving to other tormenters the task of keeping you awake at night. when the black - fly bites you will know it, and it will leave its mark, when it does leave, which must generally be by your help, for it holds on with commendable persistence. if you would learn more of this charming insect, look for simulium molestum in a book which treats the subject scientifically.
what is the central idea of this passage?
black - flies can be charming and are the subject of many scientific texts.
the black - fly, also known as the buffalo - fly, is found in the adirondack and north woods regions as well as on the prairies.
the black - fly has been known to attack horses and draw their blood, resulting in the death of the horse.
the black - fly is an unpleasant insect that torments horses and humans and is difficult to eliminate.

Explanation:

The passage describes black - flies attacking horses to the point of death, biting humans and leaving marks, and being persistent (hard to eliminate). It also mentions its only 'virtue' is stopping at sundown, indicating overall unpleasantness.

Answer:

D. The black - fly is an unpleasant insect that torments horses and humans and is difficult to eliminate.