Secretin
Secretin was the first hormone to be identified. In 1902, William Bayliss and Ernest Starling were studying how the nervous system controls the process of digestion. It was known that the pancreas secreted digestive juices in response to the passage of food (chyme) through the pyloric sphincter into the duodenum. They discovered (by cutting all the nerves to the pancreas in their experimental animals) that this process was not, in fact, governed by the nervous system. They determined that a substance secreted by the intestinal lining stimulates the pancreas after being transported via the bloodstream. They named this intestinal secretion "secretin". Secretin was the first such "chemical messenger" identified. This type of substance is now called a hormone, a term coined by Bayliss in 1905.
Vincent Wigglesworth
In particular, he studied metamorphosis. His most significant contribution was the discovery that neurosecretory cells in the brain of the South American kissing bug, "Rhodnius prolixus", secrete a crucial hormone that triggers the prothoracic gland to release prothoracicotropic hormone (PTTH), which regulates the process of metamorphosis. This was the first experimental confirmation of the function of neurosecretory cells. He went on to discover another hormone, called the juvenile hormone, which prevented the development of adult characteristics in "R. prolixus" until the insect had reached the appropriate larval stage. Wigglesworth was able to distort the developmental phases of the insect by controlling levels of this hormone. From these observations, Wigglesworth was able to develop a coherent theory of how an insect's genome can selectively activate hormones which determine its development and morphology.