One common use for pheromones is as attractants in mating. Pheromones are widely studied in insects and are the basis for some kinds of Japanese beetle and gypsy moth traps. While pheromones have not been so widely studied in humans, some interesting studies have been done in recent years on pheromonal control of menstrual cycles in women. Each hormone's shape is specific and can be recognized by the corresponding target cells. The binding sites on the target cells are called hormone receptors .
Many hormones come in antagonistic pairs that have opposite effects on the target organs.
For example, insulin and glucagon have opposite effects on the liver's control of blood sugar level. Insulin lowers the blood sugar level by instructing the liver to take glucose out of circulation and store it, while glucagon instructs the liver to release some of its stored supply to raise the blood sugar level. Much hormonal regulation depends on feedback loops to maintain balance and homeostasis. These are classified by chemical structure, not function. Together, they control many other endocrine functions.