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 bloodsucking bug Rhodniusprolixus (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.

3.3.1 Endocrine centers

The hormones of the insect body are produced by neuronal, neuroglandular, or glandular centers (Fig. 3.8). Hormonal production by some organs, such as the ovaries, is secondary to their main function, but several tissues and organs are specialized for an endocrine role.

Neurosecretory cells

Neurosecretory cells (NSC) (also called neuroendocrine cells) are modified neurons found throughout the nervous system (within the CNS, peripheral nervous system, and the stomodeal nervous system), but they occur in major groups in the brain. These cells produce most of the known insect hormones, the notable exceptions being the production by non-neural tissues of ecdysteroids and juvenile hormones. However, the synthesis and release of the latter hormones are regulated by neurohormones from NSC.

Corpora cardiaca

The corpora cardiaca (singular: corpus cardiacum) are a pair of neuroglandular bodies located on either side of the aorta and behind the brain. As well as producing their own neurohormones, they store and release neurohormones, including prothoracicotropic hormone (PTTH, formerly called brain hormone or ecdysiotropin), originating from the NSC of the brain. PTTH stimulates the secretory activity of the prothor-acic glands.

Prothoracic glands

The prothoracic glands are diffuse, paired glands generally located in the thorax or the back of the head. In cyclorrhaphous Diptera they are part of the ring gland, which also contains the corpora cardiaca and corpora allata. The prothoracic glands secrete an ecdysteroid, usually ecdysone (sometimes called molting hormone), which, after hydroxylation, elicits the molting process of the epidermis (section 6.3).

Corpora allata

The corpora allata (singular: corpus allatum) are small, discrete, paired glandular bodies derived from the epithelium and located on either side of the foregut. In some insects they fuse to form a single gland. Their function is to secrete juvenile hormone (JH), which has regulatory roles in both metamorphosis and reproduction.

3.3.2 Hormones

Three hormones or hormone types are integral to the growth and reproductive functions in insects. These

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