The hymen remains a mystery to much of modern medicine. Some of the better studies on hymens have only been done in the last couple of years, and some of these contradict each other.
The hymen is a thin membrane that is located at the front of the vagina. Hymens are like dresses: some are nearly nonexistent, others cover everything. Most are in between. A recent study has shown that the hymen isn't necessarily torn after intercourse. 19% of the sexually active women in this study had no visible tearing of the hymen. Most physicians would have declared these women to be virgins—even if they had been boinking their brains out the night before. This study concludes: "So-called rupture and bleeding of the hymen is not to be routinely expected after first sexual intercourse."
Some women are born without hymens. Hymens can also be torn during exercise or heavy petting. Imagine that, losing your virginity to a Precor Stair Climber! Studies also show that tampon usage does little to stretch or tear a hymen. For most women, the first intercourse causes little hymen-related discomfort and/or bleeding. What usually causes the pain is tension and lack of sexual arousal. Plus, there's rearranging that goes on in the pelvis during the first intercourse.
A hymen can start bleeding for the first time years after a woman has been having intercourse. This would probably be due to a tear in a "hymenal tag," which is a remnant of the hymen. These tags are like any of the other folds of skin inside the vagina, except they might look like pointy bits where there would otherwise be smoothness. Hymen tags are fairly common, but most women never detect them because they are rather far up and they don't feel any different from other parts of the vagina.
Folklore has it that a woman's hymen stays intact until she has her first intercourse, during which it supposedly bleeds. In some cultures, the marriage could be called off and the woman even stoned to death if there was no blood on the sheets of the wedding bed. To this day, there are physicians who perform minor surgeries where the edges of the hymen are stitched together, creating the appearance of "virginity." One medical journal describes a surgery where a small capsule of bloodlike substance is attached to the hymen, assuring that there will be blood on the sheets the morning after. Women in the olden days would insert a blood-soaked sponge or a small fish bladder full of blood—several times a day in brothels that promised virgins!
For those of you with a bee in your bonnet, there is the Journal of Hymenoptera Research. Hymenoptera refers to a class of insects known as bees, wasps, and ants, and would relate to your genitals only if you had ants in your pants.