"Biting you! They have not been biting with teeth: they are doctors, boys; they have only been bleeding you, and cupping you."
"And what have they been bleeding us with?"
"Why, with a lancet, to be sure; what should a doctor use but a lancet to let blood?"
"And has the gnat really a lancet?"
"Yes, it has: this instrument forms a part of what you may call the tongue of the gnat: it is made up of five pieces, which are shut up in a case, split from one end to the other; these give steadiness to the lancet when it is used. But the reason of the pain is not so much the wound of the lancet, as it is the fluid or poisonous juice which the gnat puts into the wound to make the blood thin enough for the insect to suck it up through a tube or case, which makes part of its mouth. Here is a drawing of part of a gnat's mouth.
And here is a picture of the lancet or knife of a horse-fly.