1. Form and Function of Systems
Insect Physiology
Form and Function of Systems

2. Outline
Digestive
Excretory
Circulatory
Respiratory
Endocrine

3. An insect uses its digestive system to extract nutrients and other substances from the food it consumes.   Most of this food is ingested in the form of macromolecules and other complex substances (such as proteins, polysaccharides, fats, nucleic acids, etc.) which must be broken down by catabolic reactions into smaller molecules (i.e. amino acids, simple sugars, etc.) before being used by cells of the body for energy, growth, or reproduction.   This break-down process is known as digestion.                                                                                 

4. All insects have a complete digestive system
All insects have a complete digestive system.   This means that food processing occurs within a tube-like enclosure, the alimentary canal, running lengthwise through the body from mouth to anus. Ingested food usually travels in only one direction.   This arrangement differs from an incomplete digestive system (found in certain lower invertebrates like hydra and starfish) where a single opening to a pouch-like cavity serves as both mouth and anus.   Most biologists regard a complete digestive system as an evolutionary improvement over an incomplete digestive system because it permits functional specialization -- different parts of the system may be specially adapted for various functions of food digestion, nutrient absorption, and waste excretion.   In most insects, the alimentary canal is subdivided into three functional regions:   foregut (stomodeum), midgut (mesenteron), and hindgut (proctodeum).

5. Stomodeum
In addition to the alimentary canal, insects also have paired salivary glands and salivary reservoirs. These structures usually reside in the thorax (adjacent to the foregut).   Salivary ducts lead from the glands to the reservoirs and then forward, through the head, to an opening (the salivarium) behind the hypopharynx.   Movements of the mouthparts help mix saliva with food in the buccal cavity.

6. An insect's mouth, located centrally at the base of the mouthparts, is a muscular valve (sphincter) that marks the "front" of the foregut.   Food in the buccal cavity is sucked through the mouth opening and into the pharynx by contractile action of cibarial muscles.   These muscles, located between the head capsule and the anterior wall of the pharynx, create suction by enlarging the volume of the pharynx (like opening a bellows).   This "suction pump" mechanism is called the cibarial pump.   It is especially well-developed in insects with piercing/sucking mouthparts.                                                                                 

7. From the pharynx, food passes into the esophagus by means of peristalsis (rhythmic muscular contractions of the gut wall).   The esophagus is just a simple tube that connects the pharynx to the crop, a food-storage organ.   Food remains in the crop until it can be processed through the remaining sections of the alimentary canal.   While in the crop, some digestion may occur as a result of salivary enzymes that were added in the buccal cavity and/or other enzymes regurgitated from the midgut.
In some insects, the crop opens posteriorly into a muscular proventriculus.   This organ contains tooth-like denticles that grind and pulverize food particles.   The proventriculus serves much the same function as a gizzard in birds.   The stomodeal valve, a sphincter muscle located just behind the proventriculus, regulates the flow of food from the stomodeum to the mesenteron.

8. In a developing embryo, the foregut arises as a simple invagination of the anterior body wall:   this means that all of its tissues and organs are derived from embryonic ectoderm.   In effect, the inside of the stomodeum is continuous with the outside of the insect's body.   Since exoskeleton is secreted to protect the insect externally, it is not surprising to find that cells lining the foregut produce a similar structure (known as the intima) to protect themselves from abrasion by food particles.   The hard denticles inside the proventriculus are made from this same material.

9. Mesenteron
The midgut begins just past the stomodeal valve.   Near its anterior end, finger-like projections (usually from 2 to 10) diverge from the walls of the midgut.   These structures, the gastric caecae, provide extra surface area for secretion of enzymes or absorption of water (and other substances) from the alimentary canal.   The rest of the midgut is called the ventriculus -- it is the primary site for enzymatic digestion of food and absorption of nutrients.   Digestive cells lining the walls of the ventriculus have microscopic projections (microvilli) that increase surface area for nutrient absorption.                                                                                 

10. The midgut is derived from embryonic endoderm so it is not protected by an intima.   Instead, the midgut is lined with a semipermeable membrane secreted by a cluster of cells (the cardial epithelium) that lie just behind the stomodeal valve.   This peritrophic membrane consists of chitin fibrils embedded in a protein-carbohydrate matrix.   It protects the delicate digestive cells without inhibiting absorption of nutrient molecules.
The posterior end of the midgut is marked by another sphincter muscle, the pyloric valve.   It regulates the flow of material from the mesenteron to the proctodeum.

11. Proctodeum
The pyloric valve serves as a point of origin for dozens to hundreds of Malpighian tubules.   These long, spaghetti-like structures extend throughout most of the abdominal cavity where they serve as excretory organs, removing nitrogenous wastes (principally ammonium ions, NH4+) from the hemolymph.   The toxic NH4+ is quickly converted to urea and then to uric acid by a series of chemical reactions within the Malpighian tubules.   The uric acid, a semi-solid, accumulates inside each tubule and is eventually emptied into the hindgut for elimination as part of the fecal pellet.                                                                                 

12. The rest of the hindgut plays a major role in homeostasis by regulating the absorption of water and salts from waste products in the alimentary canal.   In some insects, the hindgut is visibly subdivided into an ileum, a colon, and a rectum.   Efficient recovery of water is facilitated by six rectal pads that are embedded in the walls of the rectum.   These organs remove more than 90% of the water from a fecal pellet before it passes out of the body through the anus.
Embryonically, the hindgut develops as an invagination of the body wall (from ectodermal tissue). Just like the foregut, it is lined with a thin, protective layer of cuticle (intima) that is secreted by the endothelial cells of the gut wall. When an insect molts, it sheds and replaces the intima in both the foregut and the hindgut.

13. Circulatory System
Insects, like all other arthropods, have an open circulatory system which differs in both structure and function from the closed circulatory system found in humans and other vertebrates.   In a closed system, blood is always contained within vessels (arteries, veins, capillaries, or the heart itself).   In an open system, blood (usually called hemolymph) spends much of its time flowing freely within body cavities where it makes direct contact with all internal tissues and organs.

14. The circulatory system is responsible for movement of nutrients, salts, hormones, and metabolic wastes throughout the insect's body.   In addition, it plays several critical roles in defense:   it seals off wounds through a clotting reaction, it encapsulates and destroys internal parasites or other invaders, and in some species, it produces (or sequesters) distasteful compounds that provide a degree of protection against predators.   The hydraulic (liquid) properties of blood are important as well.   Hydrostatic pressure generated internally by muscle contraction is used to facilitate hatching, molting, expansion of body and wings after molting, physical movements (especially in soft-bodied larvae), reproduction (e.g. insemination and oviposition), and evagination of certain types of exocrine glands.   In some insects, the blood aids in thermoregulation:   it can help cool the body by conducting excess heat away from active flight muscles or it can warm the body by collecting and circulating heat absorbed while basking in the sun.

15. A dorsal vessel is the major structural component of an insect's circulatory system.   This tube runs longitudinally through the thorax and abdomen, along the inside of the dorsal body wall.   In most insects, it is a fragile, membranous structure that collects hemolymph in the abdomen and conducts it forward to the head.                                                                            
In the abdomen, the dorsal vessel is called the heart.   It is divided segmentally into chambers that are separated by valves (ostia) to ensure one-way flow of hemolymph.   A pair of alary muscles are attached laterally to the walls of each chamber.   Peristaltic contractions of the these muscles force the hemolymph forward from chamber to chamber.   During each diastolic phase (relaxation), the ostia open to allow inflow of hemolymph from the body cavity.   The heart's contraction rate varies considerably from species to species -- typically in the range of 30 to 200 beats per minute.   The rate tends to fall as ambient temperature drops and rise as temperature (or the insect's level of activity) increases.

16.                                        
In front of the heart, the dorsal vessel lacks valves or musculature.   It is a simple tube (called the aorta) which continues forward to the head and empties near the brain.   Hemolymph bathes the organs and muscles of the head as it emerges from the aorta, and then haphazardly percolates back over the alimentary canal and through the body until it reaches the abdomen and re-enters the heart.
To facilitate circulation of hemolymph, the body cavity is divided into three compartments (called blood sinuses) by two thin sheets of muscle and/or membrane known as the dorsal and ventral diaphragms.   The dorsal diaphragm is formed by alary muscles of the heart and related structures; it separates the pericardial sinus from the perivisceral sinus.   The ventral diaphragm usually covers the nerve cord; it separates the perivisceral sinus from the perineural sinus.

17. In some insects, pulsatile organs are located near the base of the wings or legs.   These muscular "pumps" do not usually contract on a regular basis, but they act in conjunction with certain body movements to force hemolymph out into the extremities.
About 90% of insect hemolymph is plasma:   a watery fluid -- usually clear, but sometimes greenish or yellowish in color.   Compared to vertebrate blood, it contains relatively high concentrations of amino acids, proteins, sugars, and inorganic ions.   Overwintering insects often sequester enough ribulose, trehalose, or glycerol in the plasma to prevent it from freezing during the coldest winters.   The remaining 10% of hemolymph volume is made up of various cell types (collectively known as hemocytes); they are involved in the clotting reaction, phagocytosis, and/or encapsulation of foreign bodies.   The density of insect hemocytes can fluctuate from less than 25,000 to more than 100,000 per cubic millimeter, but this is significantly fewer than the 5 million red blood cells, 300,000 platelets, and 7000 white blood cells found in the same volume of human blood.   With the exception of a few aquatic midges, insect hemolymph does NOT contain hemoglobin (or red blood cells).   Oxygen is delivered by the tracheal system, not the circulatory system.

18. Respiratory System
The respiratory system is responsible for delivering sufficient oxygen to all cells of the body and for removing carbon dioxide (CO2) that is produced as a waste product of cellular respiration.   The respiratory system of insects (and many other arthropods) is separate from the circulatory system.   It is a complex network of tubes (called a tracheal system) that delivers oxygen-containing air to every cell of the body.                                                                            
Air enters the insect's body through valve-like openings in the exoskeleton.   These openings (called spiracles) are located laterally along the thorax and abdomen of most insects -- usually one pair of spiracles per body segment.   Air flow is regulated by small muscles that operate one or two flap-like valves within each spiracle -- contracting to close the spiracle, or relaxing to open it.

19. After passing through a spiracle, air enters a longitudinal tracheal trunk, eventually diffusing throughout a complex, branching network of tracheal tubes that subdivides into smaller and smaller diameters and reaches every part of the body.   At the end of each tracheal branch, a special cell (the tracheole) provides a thin, moist interface for the exchange of gasses between atmospheric air and a living cell.   Oxygen in the tracheal tube first dissolves in the liquid of the tracheole and then diffuses into the cytoplasm of an adjacent cell.   At the same time, carbon dioxide, produced as a waste product of cellular respiration, diffuses out of the cell and, eventually, out of the body through the tracheal system.
Each tracheal tube develops as an invagination of the ectoderm during embryonic development.   To prevent its collapse under pressure, a thin, reinforcing "wire" of cuticle (the taenidia) winds spirally through the membranous wall.   This design (similar in structure to a heater hose on an automobile or an exhaust duct on a clothes dryer) gives tracheal tubes the ability to flex and stretch without developing kinks that might restrict air flow.

20.                                        
The absence of taenidia in certain parts of the tracheal system allows the formation of collapsible air sacs, balloon-like structures that may store a reserve of air.   In dry terrestrial environments, this temporary air supply allows an insect to conserve water by closing its spiracles during periods of high evaporative stress.   Aquatic insects consume the stored air while under water or use it to regulate buoyancy.   During a molt, air sacs fill and enlarge as the insect breaks free of the old exoskeleton and expands a new one.   Between molts, the air sacs provide room for new growth -- shrinking in volume as they are compressed by expansion of internal organs.

21. Small insects rely almost exclusively on passive diffusion and physical activity for the movement of gasses within the tracheal system.   However, larger insects may require active ventilation of the tracheal system (especially when active or under heat stress).   They accomplish this by opening some spiracles and closing others while using abdominal muscles to alternately expand and contract body volume.   Although these pulsating movements flush air from one end of the body to the other through the longitudinal tracheal trunks, diffusion is still important for distributing oxygen to individual cells through the network of smaller tracheal tubes.   In fact, the rate of gas diffusion is regarded as one of the main limiting factors (along with weight of the exoskeleton) that prevents real insects from growing as large as the ones we see in horror movies!

22. Endocrine System
A hormone is a chemical signal sent from cells in one part of an organism to cells in another part (or parts) of the same individual.   They are often regarded as chemical messengers. Although typically produced in very small quantities, hormones may cause profound changes in their target cells.   Their effect may be stimulatory or inhibitory.   In some cases, a single hormone may have multiple targets and cause different effects in each target.   There are at least four categories of hormone-producing cells in an insect's body:

23. 1) Endocrine glands -- secretory structures adapted exclusively for producing hormones and releasing them into the circulatory system.
2) Neurohemal organs -- similar to glands, but they store their secretory product in a special chamber until stimulated to release it by a signal from the nervous system (or another hormone).
3) Neurosecretory cells -- specialized nerve cells (neurons) that respond to stimulation by producing and secreting specific chemical messengers.   Functionally, they serve as a link between the nervous system and the endocrine system
4) Internal organs -- hormone-producing cells are associated with numerous organs of the body, including the ovaries and testes, the fat body, and parts of the digestive system.
Together, these hormone-secreting structures form an endocrine system that helps maintain homeostasis, coordinate behavior, and regulate growth, development, and other physiological activities.

24. In insects, the largest and most obvious endocrine glands are found in the prothorax, just behind the head.   These prothoracic glands manufacture ecdysteroids, a group of closely-related steroid hormones (including ecdysone) that stimulate synthesis of chitin and protein in epidermal cells and trigger a cascade of physiological events that culminates in molting.   For this reason, the ecdysteroids are often called "molting hormones".   Once an insect reaches the adult stage, its prothoracic glands atrophy (wither away) and it will never molt again.
Prothoracic glands produce and release ecdysteroids only after they have been stimulated by another chemical messenger, prothoracicotropic hormone (PTTH for short).   This compound is a peptide hormone secreted by the corpora cardiaca, a pair of neurohemal organs located on the walls of the aorta just behind the brain.   The corpora cardiaca release their store of PTTH only after they receive a signal from neurosecretory cells in the brain.   In a sense, they act as signal amplifiers -- sending out a big pulse of hormone to the body in response to a small message from the brain.                                                                

25. The corpora allata, another pair of neurohemal organs, lie just behind the corpora cardiaca.   They manufacture juvenile hormone (JH for short), a compound that inhibits development of adult characteristics during the immature stages and promotes sexual maturity during the adult stage.   Neurosecretory cells in the brain regulate activity of the corpora allata -- stimulating them to produce JH during larval or nymphal instars, inhibiting them during the transition to adulthood, and reactivating them once the adult is ready for reproduction.   The chemical structure of juvenile hormone is rather unusual:   it is a sesquiterpene compound -- more similar to defensive chemicals found in pine trees than to any other animal hormone.

26. The neurosecretory cells are found in clusters, both medially and laterally in the insect's brain.   Axons from these cells can be traced along tiny nerves that run to the corpora cardiaca and corpora allata.   The cells produce and secrete brain hormone, a low-molecular-weight peptide that appears to be the same as (or very similar to) prothoracicotropic hormone (PTTH) manufactured by the corpora cardiaca.   Insect physiologists suspect that brain hormone is bound to a larger carrier protein while it is inside the neurosecretory cell, and some believe that each cluster of cells may produce as many as three different brain hormones (or hormone-carrier combinations).   Large numbers of neurosecretory cells also occur in the ventral ganglia of the nerve cord, but their function is unknown.
Many other tissues and organs of the body also produce hormones.   Ovaries and testes, for example, produce gonadal hormones that have been shown to coordinate courtship and mating behaviors.   Ventral ganglia in the nervous system produce one compound (eclosion hormone) that helps an insect shed its old exoskeleton and another compound (bursicon) that causes hardening and tanning of the new one.   There are still other hormones that control the level of sugar dissolved in the blood, adjust salt and water balance, and regulate protein metabolism.

27. When an immature insect has grown sufficiently to require a larger exoskeleton, sensory input from the body activates certain neurosecretory cells in the brain.   These neurons respond by secreting brain hormone which triggers the corpora cardiaca to release their store of prothoracicotropic hormone (PTTH) into the circulatory system.   This sudden "pulse" of PTTH stimulates the prothoracic glands to secrete molting hormone (ecdysteroids). (                                                            

28. Molting hormone affects many cells throughout the body, but its principle function is to stimulate a series of physiological events (collectively known as apolysis) that lead to synthesis of a new exoskeleton.   During this process, the new exoskeleton forms as a soft, wrinkled layer underneath the hard parts (exocuticle plus epicuticle) of the old exoskeleton.   The duration of apolysis ranges from days to weeks, depending on the species and its characteristic growth rate.   Once new exoskeleton has formed, the insect is ready to shed what's left of its old exoskeleton.   At this stage, the insect is said to be pharate, meaning that the body is covered by two layers of exoskeleton.

29. As long as ecdysteroid levels remain above a critical threshold in the hemolymph, other endocrine structures remain inactive (inhibited). But toward the end of apolysis, ecdysteroid concentration falls, and neurosecretory cells in the ventral ganglia begin secreting eclosion hormone. This hormone triggers ecdysis, the physical process of shedding the old exoskeleton. In addition, a rising concentration of eclosion hormone stimulates other neurosecretory cells in the ventral ganglia to secrete bursicon, a hormone that causes hardening and darkening of the integument (tanning) due to the formation of quinone cross-linkages in the exocuticle (sclerotization).

30. In immature insects, juvenile hormone is secreted by the corpora allata prior to each molt.   This hormone inhibits the genes that promote development of adult characteristics (e.g. wings, reproductive organs, and external genitalia), causing the insect to remain "immature" (nymph or larva).   The corpora allata become atrophied (shrink) during the last larval or nymphal instar and stop producing juvenile hormone.   This releases inhibition on development of adult structures and causes the insect to molt into an adult (hemimetabolous) or a pupa (holometabolous).
At the approach of sexual maturity in the adult stage, brain neurosecretory cells release a brain hormone that "reactivates" the corpora allata, stimulating renewed production of juvenile hormone.   In adult females, juvenile hormone stimulates production of yolk for the eggs.   In adult males, it stimulates the accessory glands to produce proteins needed for seminal fluid and the case of the spermatophore.   In the absence of normal juvenile hormone production, the adult remains sexually sterile.