Class Insecta
- Related Topics:
- hormone
- human endocrine system
- thyroid gland
- adrenal gland
- pituitary gland
- On the Web:
- MSD Manual - Professional Version - Overview of the Endocrine System (Mar. 28, 2025)
Neurosecretory, neurohemal, and endocrine structures are all found in the insect endocrine system. There are several neurosecretory centres in the brain, the largest being the pars intercerebralis. The paired corpora cardiaca (singular, corpus cardiacum) and the paired corpora allata (singular, corpus allatum) are both neurohemal organs that store brain neurohormones, but each has some endocrine cells as well. The ventral nerve cord and associated ganglia also contain neurosecretory cells and have their own neurohemal organs; i.e., the multiple perisympathetic organs located along the ventral nerve cord. The insect endocrine system produces neurohormones as well as hormones that control molting, diapause, reproduction, osmoregulation, metabolism, and muscle contraction.
Molting
A peptide neurohormone that controls molting is secreted by the pars intercerebralis and is stored in the corpora cardiaca or corpora allata (depending on the group of insects). This brain neurohormone is known as the prothoracotropic hormone (PTTH), and it stimulates the prothoracic glands (also called ecdysial or molting glands). In turn, the prothoracic glands release the steroid ecdysone, which is the actual molting hormone. Ecdysone initiates shedding of the old, hardened cuticle (exoskeleton).
In the 1940s Sir Vincent (Brian) Wigglesworth discovered that distention of the abdomen of the blood-sucking hemipteran bug Rhodnius prolixus following consumption of a blood meal sends neural impulses to the brain and triggers the release of PTTH. A similar mechanism has been found in a herbivorous (plant-eating) hemipteran as well. Size seems to trigger molting in lepidopterans (moths, butterflies), although the mechanism is not understood. Each molt is aided by a small amount of juvenile hormone (JH) secreted by endocrine cells of the corpora allata. Without JH during a critical time of the intermolt period of the last larval stage, either a pupa stage (diapause, or a resting state) or an adult stage is achieved. Juvenile hormone also keeps the epidermis in a larval state and causes it to secrete larval cuticle. Without JH, the epidermis changes and secretes the adult cuticle type. Three different closely related forms of JH have been isolated from seven major insect orders.
Diapause
Some insects enter diapause during development. Diapause is characterized by cessation of development or reproduction, decrease in water content (dehydration), and reduction in metabolic activities. It usually is preceded by an accumulation of nutrients resulting in hypertrophy of the fat bodies. Environmental factors (such as temperature, photoperiod, and food availability) cause storage of neurohormones, and the corpora allata become inactive. Termination of diapause can be brought about by reversing the environmental conditions that induced the diapause. Although juvenile hormone can terminate diapause, it triggers diapause in some insects. The stage of the life history may be important in determining the role of JH. For example, in imaginal diapause (characterized by cessation of reproduction in the imago, or adult), the absence of JH initiates diapause. In lepidopterans, a peptide that initiates diapause has been isolated from the subesophageal ganglion.
Reproduction
In some insects the pars intercerebralis secretes a neurohormone that stimulates vitellogenesis by the fat body (vitellogenesis is the synthesis of vitellogenin, a protein from which the oocyte makes the egg proteins). This neurohormone is stored in either the corpora cardiaca or the corpora allata, depending on the species. Uptake of vitellogenin by the ovary is enhanced by JH. In most insects, JH also stimulates vitellogenin synthesis by the fat body. There is evidence that other neurohormones secreted by the pars intercerebralis also influence reproduction. Some induce other tissues to secrete pheromones that influence reproductive behaviour of other individuals. In the live-bearing tsetse fly, Glossina, a neurohormone released from the corpora allata stimulates milk glands that provide nourishment to the developing larvae.
Osmoregulation
All insects produce a diuretic hormone and many produce an antidiuretic hormone as well. Insects feeding exclusively on a liquid diet (such as plant sap or blood) have only the diuretic hormone that allows them to eliminate excess fluid and salts through the malpighian tubules (the insect kidney). These osmoregulatory neurohormones are produced both in the brain and in the ventral nerve cord.
Myotropic and metabolic factors
One or more small peptide neurohormones are produced in the brain and ventral nervous system and are stored in the corpora cardiaca and perisympathetic organs, respectively. These myotropic factors stimulate heart rate as well as contractions of the kidney tubules and digestive tract. The corpora cardiaca were named for the heart-stimulating action produced by extracts of these organs. The glandular portion of the corpora cardiaca is thought to secrete the hyperglycemic hormone that causes a rapid increase in blood levels of trehalose, the “blood sugar” of insects. It is sometimes called the hypertrehalosemic hormone. This hypoglycemic hormone apparently is identical to the myotropic factors in at least one species, the American cockroach. An adipokinetic neurohormone released from the orthopteran corpora cardiaca (locusts, grasshoppers) causes the release of diglycerides into the blood during, and immediately after, flight. It is a peptide similar to the myotropic factors.
Class Crustacea
Among the crustaceans, the major neuroendocrine system consists of the neurosecretory X-organ and its associated neurohemal organ, the sinus gland. Both an X-organ and a sinus gland are located in each eyestalk, and together they are termed the eyestalk complex. Two endocrine glands are well known: the Y-organ and the androgenic gland. As in insects, hormones and neurohormones of the crustacean regulate molting, reproduction, osmoregulation, metabolism, and heart rate. In addition, the regulation of colour changes is well developed in crustaceans, whereas only a few insects exhibit hormonally controlled colour changes.
Molting
The steroid ecdysone secreted from the Y-organ stimulates molting. After it is released into the blood, ecdysone is converted to a 20-hydroxyecdysone, which is the active molting hormone. Secretion of ecdysone is blocked by a neurohormone called molt-inhibiting hormone, produced by the eyestalk complex. The existence of several additional molting factors has been proposed from experimental studies, and the regulation of molting may be much more complicated than suggested here.
Reproduction
The eyestalk complex appears to produce a neurohormone that inhibits vitellogenesis by the fat body and blocks vitellogenin uptake by oocytes in the ovary. Older follicles in the ovary, however, may secrete a vitellogenin-stimulating hormone that overrides the effects of the eyestalk neurohormone. In shrimps and other crustaceans that exhibit sequential hermaphroditism, the androgenic gland produces a peptide hormone that is necessary to masculinize the gonad. These animals function first as males, and later with the degeneration of the androgenic gland they become females. Surgical removal of the androgenic gland causes a precocious change of a male to a female.
Osmoregulation
There are four known sources of factors that influence water and ionic balance (osmoregulation) in crustaceans. The brain factor is known to regulate function of the antennal glands (paired “kidneys” located at the base of each antenna), the intestine, and the gills. The thoracic ganglion factor affects the stomach, intestine, and gills. Both the antennal glands and the gills are affected by a factor from the eyestalk complex. Finally, the pericardial organs (neurohemal glands located in the pericardial cavity) influence salt and water metabolism by heart muscle and gills.
Myotropic factor
Heart rate is accelerated in crustaceans by a factor released from the pericardial organs. It is not known if this factor is the same one that has osmoregulatory actions mentioned above. There is evidence to suggest that the crustacean cardioacceleratory factor is identical to one of the insect cardioacceleratory factors.
Colour changes
Several neurohormones that regulate colour changes (chromatophorotropins) by pigment cells (chromatophores) have been found in extracts of the eyestalk complex. The best known are the light-adapting hormone and the red-pigment-concentrating hormone. This latter peptide is chemically similar to the insect adipokinetic and myotropic factors. Regulation of the chromatophores allows an animal to adapt to different backgrounds by changing colours or by becoming lighter or darker.
Phylum Echinodermata
Female sea stars (starfishes) are the only echinoderms that have been studied extensively. A neuropeptide called the gonad-stimulating substance (also called the gamete-shedding substance) is released from the radial nerves into the body cavity about one hour before spawning. Gonad-stimulating substance has been reported in more than 30 species of sea star. This neuropeptide contacts the ovaries directly and causes formation of 1-methyladenine, an inducer of oocyte maturation and spawning. This same hormone has been demonstrated in the ovaries of the closely related sea urchin, where it also promotes maturation of the oocyte.
Phylum Chordata
The phylum Chordata is separated into three subgroups (or subphyla). The invertebrate subphylum Tunicata consists of the marine tunicates, including the ascidians and salps. The invertebrate subphylum Cephalochordata includes the fishlike amphioxus (or lancelet). Amphioxus is a small marine animal that closely resembles the larva of the jawless fishes (class Agnatha). The subphylum Vertebrata is the largest chordate subgroup.
Subphylum Tunicata
The ascidians (also called sea squirts) have a tadpolelike larva that lives free for a short period. The larva eventually attaches itself to a solid substrate and undergoes a marked metamorphosis into the sessile adult sea squirt. The larva and adult have a mucus-secreting gland, the endostyle, that is believed to be the evolutionary ancestor of the vertebrate thyroid gland. Metamorphosis in ascidians can be induced by application of thyroid hormones.
Neurosecretory neurons in the cerebral ganglion (brain) contain the vertebrate peptide gonadotropin-releasing hormone (GnRH). Directly adjacent to the brain is the neural (or subneural) gland that may be the forerunner of the vertebrate pituitary gland. Extracts prepared from ascidian neural glands stimulate testicular growth in toads, demonstrating the presence of a gonadotropic factor in the neural gland. A protein similar to human prolactin has been found in the neural gland of Styela plicata.
Subphylum Cephalochordata
The cephalochordate brain contains neurosecretory neurons that possibly are related to a structure called Hatschek’s pit, located near the brain. Hatschek’s pit appears to be related to the neural gland and hence to the vertebrate pituitary gland. Treatment of amphioxus with GnRH or luteinizing hormone (LH) reportedly stimulates the onset of spermatogenesis in male gonads. Furthermore, extracts prepared from Hatschek’s pit can stimulate the testis of a toad. Amphioxus has a mucus-secreting endostyle like that of the ascidians. and studies have shown that the cephalochordate endostyle can synthesize thyroid hormones, too. Thus, the basic organization of the vertebrate endocrine system appears to show its early beginnings in the simple organs of these invertebrate chordates.
David O. Norris