3.3. The endocrine system and the function of hormones
Hormones are chemicals produced within an organism’s body and transported, generally in body fluids, away from their point of synthesis to sites where they influence a remarkable variety of physiological processes, even though present in extremely small quantities. Insect hormones have been studied in detail in only a handful of species but similar patterns of production and function are likely to apply to all insects. The actions and interrelationships of these chemical messengers are varied and complex but the role of hormones in the molting process is of overwhelming importance and will be discussed more fully in this context in section 6.3. Here we provide a general picture of the endocrine centers and the hormones that they export.
Historically, the implication of hormones in the processes of molting and metamorphosis resulted from simple but elegant experiments. These utilized techniques that removed the influence of the brain (decapitation), isolated the hemolymph of different parts of the body (ligation), or artificially connected the hemolymph of two or more insects by joining their bodies. Ligation and decapitation of insects enabled researchers to localize the sites of control of developmental and reproductive processes and to show that substances are released that affect tissues at sites distant from the point of release. In addition, critical developmental periods for the action of these controlling substances have been identified. The blood-sucking bug Rhodnius prolixus (Hemiptera: Reduviidae) and various moths and flies were the principal experimental insects. More refined technologies allowed microsurgical removal or transplant of various tissues, hemolymph transfusion, hormone extraction and purification, and radioactive labeling of hormone extracts. Today, molecular biological (Box 3.1) and advanced chemical analytical techniques allow hormone isolation, characterization, and manipulation.