The events that take place in the egg stage of insect life cycles are known as embryonic development. Upon emerging from eggs, juvenile insects embark on an excursion of post-embryonic development that will eventually take them to their adult forms. Post-embryonic development is characterized by dramatic increases in size and by formation of new anatomical structures characteristic of adults. Argued from an ecological point of view, juvenile stages of insects are devoted to eating, growing, and accumulating energy reserves while adult stages are devoted to migrations, mate-finding and reproduction. This appears to be particularly true among some insects, such as certain species of sphinx moths, which do not feed during adulthood. But as generalizations, these broadly sweeping remarks are not strictly true. Development of reproductive organs begins early in larval life in many insect species, so even if an insect is not actually reproducing, it is preparing for reproduction quite early in life. Many adult insects spend quite a lot of time in feeding behaviors. The value of broad generalizations is that we can appreciate the major thrust of insect life stages. Certainly larval insects are largely given over to eating and growing.

Insects are invested in a more or less rigid exoskeleton, and a generally accepted wisdom is the physical constraints of a least certain of the very rigid parts of exoskeletons inhibit growth beyond set limits. Here is an example of the overall increase in weight during several weeks of development. The figure emphasizes the potential to increase in size during post-embryonic development, in this species from a few milligrams at hatching to nearly 2 grams. Fully hardened cuticle does not expand, so growth of the hardest parts can only take place during molts. The softer parts of insects can expand. The softer cuticle often occurs in folds that can be expanded, and sometimes the cuticle itself can be stretched. Another figure indicates that the soft parts of an insect grow rather continuously, while the hard parts grow in steps.

Growth can refer to an increase in cell number or an increase in cell size. The development and expression of cuticle depends on the cells of the epidermis. Epidermal cell numbers increase prior to molting in many insect species. In larvae of many Diptera, growth is due exclusively to increased size of the epidermal cells. Similarly, growth of internal organs can proceed by increased cell size or increased cell number. Both forms of growth occur in the mosquito Aedes. Cell numbers in the nervous system and fat body increase, but most other tissues have a constant number of cells. Growth occurs by cell enlargement in these tissues.

Malpighian tubules are separated into primary Malpighian tubules and secondary Malpighian tubules. The primary tubules arise from cells of the proctodeum during embryonic development. The secondary tubules emerge during post-embryonic development. The secondary tubules are reduced to buds during each molt, then develop by cell division, following by cell enlargement.

Let us turn to the endocrinology of molting. We will consider five hormones in the regulation of postembryonic development, plus a newly discovered, sixth hormone. The actions of the five hormones are summarized in a figure.

Prothoracicotropic hormone (PTTH) is synthesized in neurosecretory cells of the brain. It is stored and released from the corpora cardiaca. Again, PTTH is released from the neurohemal portion of the CC. PTTH stimulates the prothoracic glands to release ecdysone into the hemolymph. The prothoracic glands have other names. You will see the prothoracic glands called by other names, including ventral glands and ecdysial glands. In higher Diptera it is part of the ring gland. Ecdysone is not the active molting hormone. Various tissues, including the fat body convert ecdysone to 20-hydroxyecdysone, the active form of molting hormone, shown here.

The cells of the epidermis respond to 20-hydroxyecdysone with initiation of the process of molting. Apolysis refers to the detachment of old cuticle from the epidermis, the first step in molting. Molting fluid is actively transported into the space between the old cuticle and the epidermis. Epidermal growth by cell division may occur during this time. The next step involves depositing layer of new cuticle. This is followed by activation of enzymes in the molting fluid that function to digest inner layers of the old cuticle.

The amount of juvenile hormone (JH) released from the corpora allata (CA) determines the form of the new cuticle that is deposited. When JH is present in high concentrations, the new cuticle is larval; when JH is present in low concentrations, the new cuticle is pupal. Adult cuticle is formed in the absence of JH.

Sir Vincent Wigglesworth, who was knighted for his research in medically important insects, first suggested the idea of an "inhibitory hormone" in the 1930's. Working with penultimate instars of the blood-sucking bug, Rodnius, Wigglesworth showed that if bugs were decapitated 5 days after a blood meal, they molted into the last larval instar. If the bugs were decapitated 3 days after a blood meal, they molted into precocious, but miniature adults. The Harvard biologist Carrol Williams gave JH its name. Williams worked with Cecropia moths. The adult moths were parabiotically joined to diapausing pupae. Parabiotically is a relatively rare word for people who work on mammalian systems. Parabiotic animals are joined in such a way that they share hemolymph circulation. Wigglesworth was famous for his "parabiotic trains", in which many Rhodnius nymphs were joined in long trains; molting would progress rearward down the trains. Williams' idea was to try to prolong the life span of the adult moths, which do not feed, by providing nutrients from the pupae. The interesting thing was this: when the pupae were joined to adult females, the pupae would continue normal development and emerge as adults. Alternatively, when the pupae happened to be joined to male adults, they molted into another PUPAL stage. A series of now famous experiments finally showed that males of some moths, including Cecropia moths, accumulate a blood-borne factor in their abdomens that exerts a "status quo" effect on pupal moths. Williams named the factor juvenile hormone because it caused molting into a second pupal stage.

Ecdysis refers to shedding the old cuticle, and is the basis of the name ecdysone for molting hormone. We note that apolysis and ecdysis are separate processes. In some insects shedding of the pupal skin by adults is triggered by another brain hormone called eclosion hormone (EH). EH is synthesized in the median neurosecretory cells of the brain, and stored in the neurohemal portion of the CC. It is released from the CC under regulation of a circadian rhythm. EH stimulates the abdominal nervous system to play out a "hard-wired" behavioral program that helps the adults emerge from the pupal case.

We consider a figure, showing the influence of EH on nerve firing in an in vitro preparation of the tobacco hornworm ventral nerve cord. EH was added to the preparation 22 minutes before the first spike occurred. The early spikes (A and B) represent the rotational bursts that begin ecdysis. The later spikes (C and D) represent peristaltic bursts. Jim Truman and Lynn Riddiford, then at Harvard, are responsible for the discovery of EH, and its role in helping insects emerge from the pupal case. Truman and Riddiford married, and took up dual positions in the Department of Zoology at the University of Washington in Seattle.

Let us now introduce a newly discovered hormone in the physiology of insect development. While molting appears to be rather abrupt, the process is long and detailed. As molting progresses, the inner layers of the old cuticle are digested away. Then a soft new cuticle is deposited. When these developmental events are complete, the shedding processes begin, and the new cuticle is expanded and hardened. The key point is noting the shedding processes are behavioral, and the role of EH is to release a pre-programmed sequence of behaviors. We now know the release of the ecdysial behaviors is more complicated.

Last year Zitnan and colleagues, then at UC Riverside, discovered the existence of a second ecdysial hormone, called Mas-ETH. Mas comes from Manduca sexta, and ETH stands for ecdysis-triggering hormone. ETH is a 26-amino acid peptide. It is produced by a system of epitracheal glands. In M. sexta there are 8 pairs of epitracheal glands, each located on the outer surface of the large tracheal tube immediately adjacent to each spiracle. The hormone is produced and released from Inka cells. Inka is the name of a fairy goddess, known from the Tatra Mountains. Zitnan takes great inspiration from Inka, and he named the cells in her honor.

Ecdysis is now thought to follow from the sequential release of two hormones. EH is released from the brain, and it primes the Inka cells to release ETH. The Inka cells release ETH, and the ETH acts on the abdominal ganglia to release the behaviors required for ecdysis. But why do we need two hormones to release ecdysial behaviors? Ecdysis involves many behaviors that are unique to ecdysis. Successful ecdysis requires that all behaviors are coordinated. Failures in coordination result in deformities and death: each ecdysis episode is a crucial event in an insect's life.

The Inka cells are positioned near each spiracle, where tracheal linings are withdrawn at ecdysis. These linings are fragile, and if torn the tracheal supply to that region is blocked. It is thought the Inka cells are in best position to serve as a last inspection site before the insect is irrevocably committed to going through ecdysis.

The physiology of molting involves apolysis, depositing new cuticle and ecdysis. The circulating titre of JH at the time of the molt determines the developmental fate of the insect. Here is an example of the relationship between ecdysone and JH through 4th and 5th larval instars, the pre-pupal wandering stage and pupal stage of the tobacco hornworm. The solid line shows ecdysone titres. Ecdysone titre spikes a day or so before molting into the 5th instar, then again just before the wandering stage as a prepupa. There is a small increase in ecdysone titre before the pupal molt. Ecdysone titres reach a higher level and remain elevated during the pupal stage. The dotted line represents JH titres. There are high titres of JH in the 4th instar, which are lower in the 5th instar. A small spike of JH is associated with the wandering phase, and no JH is associated with the pupal stage.

JH is also active in adult stages of insect life. JH plays an important role of regulation of vitellogenin synthesis in adult female fat bodies. JH acts directly upon follicular epithelia of ovaries to facilitate uptake of yolk proteins. JH is also active in adult males, where it regulates development of reproductive tract accessory glands.

Hormones are transported in circulating hemolymph. Since the hemolymph bathes virtually every cell in the insect body, circulating hormones have the potential to come into contact with all tissues. Again, specific receptor sites form the connection between a circulating hormone and particular target cell. JH and ecdysteroids are not water soluble, and they are associated with specific high-affinity transport proteins. These transport proteins, then, are an important link in the overall physiology of endocrine regulation of biochemical and physiological events. These proteins are also partly responsible for regulating circulating hormone levels. They can act by protecting the hormone from enzyme degradation and by preventing rapid loss of hormone from the hemolymph through excretion.

The titre or levels of circulating hormones are regulated at several points. Biosynthesis, release, transport and degradation of hormones are regulated events. Biosynthesis of ecdysone depends upon PTTH from the brain. JH biosynthesis is partly regulated by nervous signals from the brain to the CA. Ecdysone is degraded to a biologically inactive form, 3-dehydro-alpha-ecdysone, and 20-hydroxyecdysone may similarly be oxidized to a 3-dehydro form.

There are two major pathways that change JH into biologically inactive forms. The minor pathway is hydration of the epoxide functionality to the diol. The most important mechanism is hydrolysis of the JH ester to its respective acid. JH can be broken down by general carboxyesterases, enzymes that occur throughout the insect body and by JH-specific esterases. The JH-carrier protein protects circulating JH from the general esterases, but not from the JH-specific esterase.

The endocrinology of insect reproduction is not without controversy. Mosquitoes elaborate an egg development neurohormone (EDNH) that is produced in the brain. EDNH is released after females take a blood meal. The hormone acts on the pre-vitellogenic ovary, and stimulates release of ecdysone, which is converted to 20-hydroxyecdysone. The 20-hydroxyecdysone then acts on the fat body, which results in vitellogenin synthesis, and development of the vitellogenic ovary. The interpretation of these data is that in mosquitoes the ecdysteroid produced by the mosquito ovary acts on the fat body to stimulate synthesis of yolk proteins. This, of course, runs counter to the conventional wisdom that JH is released from the CA, then acts on the fat body to stimulate yolk protein synthesis. Among people who are interested in the topic, there are two camps: one group absolutely disagrees with this idea.

Other reproductive hormone actions have been postulated for some insect species. Ken Davey and his co-workers in Canada suggested that there is an internal feedback control of ovarian development in the bug Rodnius. In this insect an antigonadotropin is thought to inhibit the action of JH on ovarian follicles to prevent from developing patency. This is regarded as a form of direct ovarian inhibition. Indirect ovarian inhibition is thought to occur in several species of Diptera. The general scheme is that the follicle cells produce an oostatic hormone that acts on the neuroendocrine system, causing a reduction of JH.