A great deal of experimental work has been carried out on this subject, particularly on cockroaches, locusts, grasshoppers, and most noteworthy is the work on the Heteropteran bug Rhodnius, by V.B. Wigglesworth. As might be expected, there is considerable variation among the insect orders with regard to endocrine structures, but certain general principles have been well established. In all insect, there are neurosecretions from four groups of cells in the brain, two medial and two lateral. The axons of the medial groups pass to the corpora cardiac, and those of the lateral groups pass through the corpora cardiac to terminate in the corpora allata.
There are minute groups of nuerosecretory cells in each ganglion; their axons lead to small nuerohaemal organs closely associated with the ganglia. The first epithelial endocrine organs yet known are the thoracic glands in the first thoracic segment.
An antagonistic hormone neotenin, from the corpora allata, prevents metamorphosis during the immature stages. There is progressive differentiation during each instance due to the influence of neotenin. During the last one or two moults, production of neotenin gradually ceases, and ecdysone stimulates metamorphosis.
Another neurosecretion, released from the corpora cardiac, is a growth hormone which promotes protein synthesis; there are also two hormones which affect the concentration of blood sugar. In several groups of insect, rhythmic behaviour, e.g. in light and in darkness, is concerned with a neurosecretion from the suboesophageal ganglia; this is itself controlled by a tropic hormone released by the corpora cardiac. A diuretic hormone from the brain, and probably also from the segmental ganglia, promotes the movement of fluid through the malpighian tubules. From the corpora allata, gonadotropic hormones affect the maturation of the gonads and accessory structures; the gonads themselves secrete hormones which stimulates ripening of the gametes and also control the reproductive process.
Probably many details of the insect endocrine system are not yet known, but the general pattern is strictly comparable with that of vertebrates
Insects must respond quickly to a multitude of internal and external stimuli that can threaten survival. The nervous system and the endocrine system mediate different types of short-term responses through dedicated pathways between receptors and effectors.
In the case of insects this involves neuroendocrine peptide hormones. Examples include prothoracicotropic hormone (PTTH), a brain neurohormone that stimulates the release of ecdysone by the prothoracic glands in insect larvae and thus controls the molting process.
What are insect hormones?
Like all multicellular organisms, insects have evolved to respond to a great variety of external stimuli and contingencies. To do so effectively, they have developed two systems of internal communication and integration: the nervous system which mediates rapid short-term responses through direct and dedicated pathways between receptors and effectors; and the endocrine system which orchestrates longer-range physiological, developmental, and behavioral events.
The latter system uses a class of hormones to trigger a broad range of reactions in the insect body, by virtue of their ability to act as ligands for various receptors. Insect hormones are generated and used by glandular tissues, including endocrine organs, such as the prothoracic glands that secrete steroid hormones; neurosecretory cells in the brain; and the corpora cardiaca, a paired gland that produces juvenile hormone (JH). JH has the ability to promote growth and development by acting as a ligand for the FOXO transcription factor.
In addition to regulating growth, JH also stimulates the secretion of replacement cuticle in order to prepare the insect for molting or metamorphosis. Once sensory receptors determine that the insect has grown enough to require a new cuticle, a chemical signal is transmitted from the brain to the corpora cardiaca via the neurosecretory cells and it is then released into the circulation. The sudden pulse of PTTH then activates the prothoracic glands to secrete molting hormone, a steroid known as ecdysone. Molting hormone acts on the epidermis to stimulate cellular growth, and it also triggers the breakdown of the hard parts of the old cuticle, which is then shed by the insect, leaving a soft, wrinkled layer of new cuticle called an exocuticle and epicuticle.
Similarly, metamorphosis is regulated by JH, which is secreted by the corpora cardiaca in the larval and pupal stages. Once JH is present in the insect’s circulatory system, it triggers the ovaries and testes of females to produce steroid hormones that stimulate a series of physiological events that lead to the formation of an adult wing and legs. Insects that lack JH, or whose expression of this hormone is inhibited by mutations in the gene BR-C, have a much more limited capacity for growth and development, exhibit abnormal muscle development, retain larval structures that should degenerate as they develop into adults, and show a number of other developmental anomalies.
What are insect neurosecretions?
The neurosecretory, or neuroendocrine, system of insects is a complex that controls and elicits a wide range of physiological and developmental events. Although insect endocrine glands exist, the vast majority of the osmoregulatory hormones and many other neurohormones are secreted by neurons rather than from conventional glands. These neurosecretory compounds are called neuropeptides or neurohormones, and they act by modifying the activity of conventional endocrine hormones or directly by themselves.
Neurosecretory cells can be found in all ganglia of the central nervous system (CNS), but most are concentrated in the brain and its two paired corpora cardiaca or corpora allata. These neurohemal structures store the peptide-containing neurosecretions from neurosecretory cells in the brain and release them when needed. Together, the brain-corpora cardiaca neuroendocrine system controls molting, diapause, reproduction, osmoregulation, and metabolism.
All insect species produce a diuretic neurohormone and many also have an antidiuretic one. These osmoregulatory substances allow the insect to eliminate excess water and salts from its body by excretion through the malpighian tubules (insect kidney). Neurosecretory cells in the brain produce these osmoregulatory neurohormones, but they also secrete tropic or inhibitory hormones that modulate the activities of other brain-derived osmoregulatory neurons.
Insect physiologists are working hard to understand the molecular and cellular basis of these neurosecretory functions. They are gaining more knowledge about the intracellular processes that occur in these cells and the routes that they use to carry the peptide-containing neurosecretions to their targets, such as other cell or vascular fluid areas. In addition, they are discovering that a significant proportion of these neurosecretions are bound to large carrier proteins.
The 3rd International Symposium on Neurosecretion held in Bristol, England, in September 2004 focused on these issues and the ways in which they affect the neuroendocrine systems of insects and other vertebrates. A major goal is to develop a better understanding of the molecular structures and properties of neurosecretory granules that are secreted by these cells and that serve as their transport vehicle. Using readily available antibodies to vertebrate hormones, investigators are also uncovering evidence of a number of cross-reactive peptides that may be involved in a broad array of physiological actions. For example, amino acid sequencing of the neuropeptide bombyxin (H-Arg-Tyr-Leu-Pro-Thr-OH) isolated from the heads of the silkmoth Bombyx mori has disclosed sequence homology with insulin.
Biological role and significance of insect hormones and neurosecretions
Insect hormones, especially the steroid ecdysone and the neurohormone juvenile hormone, play many important roles in regulating insect growth and development. In this volume the author offers an overview of the diverse ways in which they control molting, metamorphosis, reproduction, caste determination in social insects, diapause, migration, carbohydrate and lipid metabolism, and behavior. The book is light on technical details of experimental design and results, and provides ample references to the extensive literature on insect endocrinology.
The NCC III carries neurons from neurosecretory cells in the tritocerebrum (Rabe, 1983). In most insects, except the Lepidoptera, these neurons terminate in the corpora cardiaca or corpora allata, which serve as both the storage site and the release site for the neurosecretions they carry. A variety of small peptides have been isolated from these organs and, when purified or artificially synthesized, can stimulate, inhibit, or block a number of physiological activities in insects. The most well known of these is the “myotropic factor,” which causes a rapid increase in blood levels of trehalose, or blood sugar, and also stimulates contractions of tubules of the kidneys and digestive tract.
Other neurohormones secreted by the pars intercerebralis influence reproduction. In the live-bearing tsetse fly, Glossina, for example, juvenile hormone stimulates the fat body to secrete vitellogenin, which is needed for egg proteins, and it also enhances uptake of this protein by the oocyte. In addition, the tsetse fly secretes a neurohormone that stimulates milk glands to secrete fluid that supplies nourishment to young larvae.
Several studies focus on the complex way in which the steroid ecdysone, together with a variety of peptide neurohormones, controls the development of wing scales and pteranodont tissues, and a study presents an outline of the diverse ways in which insect hormones regulate sex determination. The study ends with a funding on the fascinating phenomenon of insect polyphenism, the ability of an individual to develop into one of many alternative life-styles in response to token stimuli from its environment.
Chemical composition of insect hormones and neurosecretions
The discovery of insect hormones and the demonstration that behavioural responses could be triggered by chemical communication was an important milestone in the history of biology. It was the first time that a specific molecule was shown to have a physiological effect on the behaviour of an animal and this discovery led directly to advances in understanding the way in which animals respond to their environment, which is fundamental to the evolution of life.
The insect endocrine system is complex and highly regulated, and the different functions of various endocrine hormones are controlled by their specific biochemical properties. For example, ecdysteroid hormones such as methylecdysone and 20-hydroxyecdysone have the capacity to promote the growth of tissues. They also inhibit cellular metabolism, thereby slowing down the rate at which energy is used in cells.
Insect hormones also have the ability to change an animal’s internal environment in order to adapt to a particular situation. In many cases the change is induced by signals coming from the nervous system, while in others it is triggered by the release of neurosecretions from the corpora allata.
These glands are situated at the rear of the insect body and are connected to the brain by a nerve that also passes through the corpora cardiaca. The two corpora allata are a pair of compact glands of tightly packed cells surrounded by a tough membrane. The cell wall contains a series of lipids and a mixture of enzymes. They are able to produce the ecdysteroid hormones makisterone A and 20-hydroxyecdysone as well as the sesquiterpenoid juvenile hormone (JH).
JH is the most important of these neurosecretions. It is the primary signal that regulates molting and metamorphosis in insects. It keeps the epidermis in a larval state and causes it to secrete larval cuticle. It also stimulates the production of proteins by the fat body that make up the egg protein vitellogenin.
Another important function of JH is its role in regulating osmotic pressure and the distribution of water throughout the body of the insect. It also stimulates the secretion of a diuretic hormone that allows the insect to eliminate excess salts and fluid through its kidney-like malpighian tubules. This diuretic action also helps to control molting in some insects.