When the Continental Congress met on July 4th, 1776, it was a very hot day in Philadelphia. They settled in for a long day of arguing over the final version of the draft Independence document in a humid, sticky room. Tempers ran high.

Someone opened the windows to let in a breeze... but as they were down the street from a stable, what actually came into Congress was blood-sucking flies.

Congressional Precedent established.

From Parton's 1874 Life of Jefferson: (page 191):

"During the 2d, 3d, and 4th of July, Congress were engaged in reviewing the Declaration. Thursday, the fourth, was a hot day; the session lasted many hours; members were tired and impatient. Mr. Jefferson used to relate, with much merriment, that the final signing of the Declaration of Independence was hastened by an absurdly trivial cause.

Near the hall in which the debates were then held was a livery-stable, from which swarms of flies came into the open windows, and assailed the silk-stockinged legs of honorable members. Handkerchief in hand, they lashed the flies with such vigor as they could command on a July afternoon; but the annoyance became at length so extreme as to render them impatient of delay, and they made haste to bring the momentous business to a conclusion. "

As another historian put it, "treason was preferable to discomfort", and members closed debate, hustled to sign, and exited stage left, pursued by horse flies.

The dangerous end of a horse fly (Tabanus atratus). USGS Sam Droege/Flickr

Horse flies (Subfamily Tabaninae) are still common in the US. Horse and deer fly mouthparts are basically scissors and a sponge; they slash you and then lap up the blood that pools up. This is extremely painful. The amount of blood a horse fly can suck up varies, but 40 to 200 mg of blood seems to be the range of a single fly.

Happy 4th of July for my American readers. (Also, the full document wasn't signed by everyone until August 2nd, 1776.)