Cantharis vesicatoria (Spanish fly)
Spanish fly doesn't actually come from a fly. It is made from the cantharides beetle, also called the blister beetle. The name comes from an intense irritant they generate as a defense mechanism. The beetles are collected, killed, dried and powdered, and the result is called Spanish fly.
Cantharides, being a natural inflammatory agent, can burn the mouth and throat and cause abdominal pain, vomiting, diarrhea and kidney damage even in small doses. It also strongly irritates the genito-urinary tract, and sex is one way to calm that itch. The stories regarding medical application of cantharides would be listed as "not to be believed," except that they are true. In 1752 a Frenchman was prescribed 2 drams of cantharides for a fever and in the next two months bedded his wife at least 87 times. Another man with a similar prescription "knew his wife", as the Bible puts it, 40 times in one night.
As little as one thousandth of an ounce of the active chemical in cantharides is sufficient to cause kidney failure and death, making it one of the most dangerous substances ever to be associated with sexuality. It is classified medically as a Class One poison and a blistering agent. Fortunately, it is also illegal nearly worldwide.