Insect Digestive System Home Assignment PDF

Home Assignment What do you know about the digestive system of insects? Where does digestion and absorption occur in insects? Which glands are involved in providing different secretions to alimentary canal? What are the steps in the process of digestion and absorption of food in omnivorous insec...

Home Assignment What do you know about the digestive system of insects? Where does digestion and absorption occur in insects? Which glands are involved in providing different secretions to alimentary canal? What are the steps in the process of digestion and absorption of food in omnivorous insects? **Digestion of special food stuffs** Beside the digestion of general diet components or basic constituent of food, there are insects which are adapted to feed upon some special food substances like Wood, Collagen, Keratin, Pollen, Silk, Wax etc Such insects developed some special mechanism to digest them. **Digestion of Wood** The chief constituent of wood is **Cellulose** and **Lignin** where cellulose constitute about 40-62% of the dry weight whereas lignin shares 18-38% To digest this **cellulose**, the larvae of some beetles like of *Cerambyx, Hylotrupes, Macrotoma* etc are able to secrete true **cellulase enzyme** in their midgut. In *Stromatium* larvae (Col.) even the gastric juice contain **cellulase enzyme** that digest the cellulose which is present in bound form with lignin as **lignocellulose**. Beside this cellulase, some insect posses microorganisms which help in digestion of cellulose. The next constituent of wood is **lignin** which is usually not digested e.g. termites feeding on wood that contain 54.6% cellulose, 18% pentosans and 27.4% lignin. After digestion their faecal mater contain 18% cellulose, 8.5% pentosans and 75.5% lignin. This indicates that these insects have digested and utilized maximum cellulose followed by pentosans however, they could not digest lignin. Beside cellulose and lignin, the wood also contain **Hemicelluloses** (mixture of polysaccharides posses both hexosans and pentosens. Some of the insects e.g. *Lyctus, Xystrocera, Coccus* etc are not able to digest cellulose but mainly depend upon the hemicelluloses. Such hemicelluloses are digested in the presence of **hemicellulase enzyme**. The hexosans (2-14%) on hydrolysis librated glucose whereas pentosans on digestion produce xylose and arabinose. So, on the basis of digestion, three types of food preferences/ wood feeding are recognized in beetle: (i) *Those which only feed upon the **cell contents*** (part of polysaccharides intermediate between starch and hemicellulose) *and make long tunnels in wood in search of sugar, starch in sufficient amount*. (ii) *Those which feed upon and utilize both **cell contents and carbohydrates*** **(cellulose) of *the cell wall as well as the hemicellulose***. (iii) ***Those which feed upon the carbohydrates (cellulose) of the cell wall only***. The Cerambycid larva *Macrotoma palmata* which contain active cellulase can invade the heart wood of the trees and consume both cellulose, hemicellulose, starch and sugar. However, *Xystrocera* which do not posses cellulase enzyme can not penetrate deep in wood and attack only superficially. Beside cellulose, lignin and hemidellulose, the wood also contain some amount of starch (0-5-9%), sugar as glucose (0.6.2%) and proteins (1.1- 2.3) **Digestion of Collagen** Collagen is animal protein and is the main constituent of fibrous tissues. This protein is resistant to most of the protein digestive enzymes, however, in some insects, like *Lucilia* or *Hypoderma*, a specific enzyme **collagenase** is present which digest or hydrolyze the collagen and elastin. This enzyme act in alkaline medium. This enzyme is also present in the excreta of bacteria free larvae that digest the fibrous septa of the muscles. So, all those insect which feed upon muscle tissue secrete this **collagenase enzyme** for the digestion of the inert proteins. **Digestion of Keratin** **Keratine** which is the chief constituent of hair, wool and feather, is made up of long peptide chains which are held togather by disulphide (S-S) linkage and are folded in such a way that they provide elasticity. Because of the presence of these disulphide linkage, the general protein digesting enzymes can not work or digest the peptide chain. So in some insect like *Tineola, Anthrenus,* and *Mallophaga*, the mid gut secrete a strong reducing agent (i.e. **special proteinase.** This enzyme is less sensitive to --SH compound) and open the S-S bonds and thereafter, the other protein digesting enzyme digest the peptide chain, liberating the amino acids. Generally in insects most **proteinase enzymes** are of tryptic type (enzymes that work in alkalin medium) and are marked by or inhibited by --SH compounds but the proteinase which digest keratin is insensitive and are not inhibited by --SH compound. Thus because of two adaptation i.e. i. the low oxidation-reduction potential of the medium and ii. low sensitivity to --SH compounds, some insects can digest the keratin. **Digestion of Pollen and leaves** In phytophagous insects, most of cell wall and skeletal parts of the leaves remain unchanged /undigested and pass as such through the gut and only little amount is digested completely without mechanical or chemical breakdown of the wall. In bees also, the same pattern of change is seen, where some pollen grains pass through unchanged, whereas, the contents of many are completely dissolved and digested without mechanical or chemical breakdown. For example the diet of the larvae of *Phalera* contain 60% protein, 28% soluble sugars, 6% fat and 3% ash. These larvae utilized all components of the diet except polysaccharides which were excreted unchanged. The chlorophyll of plants mostly passes unchanged or undigested through the gut of the phytophagous insects but in some insects e.g. in silk worm, about 41-46% of the chlorophyll and 29.34% of the carotenoids of the mulberry leaves are digested and absorbed. **Digestion of Wax** The diet of *Galleria* larvae, which feed on honeycomb largely consists of wax that comprises all the alcohol components, part of fatty acids and esters of high molecular weight and have no paraffins. It has been observed that the **lipase** which is extracted from the gut of the larvae act only on tributyrin, olive oil but has no action on myricin (i.e. esters of high alcohols). So it is believe that the digestion of this myricin or some of the components of wax is carried by the **bacterial flora of the gut**, The further digestion is performed by other enzymes like **lipase, lecithinase** and **cholestrol esterase**. In *Galleria* the formation of phospholipids is an important step in the utilization of wax. In gut tissue about half of all the lipids are phospholipids which indicates that the digestible components of the wax penetrate the cells in the form of phospholipids. **Digestion of Silk** Silk which also made up of proteins is also digested by some insects, however, the physiology of digestion as well as the enzymes concerned are not known yet. **Absorption of Food** After digestion, the products are absorbed in the **midgut**, especially in anterior region and **hindgut** especially in rectum. The products of digestion are absorbed in solution and no phagocytosis take place. In most of the insects, the secretion of enzymes and absorption of digested material are carried out by the **same cells** where these cells go-through **alternate phases** of **secretion** and **absorption**. For example, in *Dytiscus*, the young **epithelial cells** are involved in **secretion** and later **absorption** i.e. on feeding the cells goes through: Secretion Extrusion (To remove dying or unwanted cells from an epithelium, If extrusion is defective, epithelial cells rapidly lose contact inhibition and formmasses.) Regeneration Function as absorbing Began to store its secretion After a number of these cycles, the cell undergoes degeneration. Three factors which affect the absorption are: -- i. The presence of microvilli, which increase the surface area for both absorption and digestion. -- ii. The functional differences in the membrane permeability of various regions of digestive tract. -- iii. The presence of a counter-current. **A biological mechanism designed to enable maximum exchange between two fluids.** **The mechanism\'s effect is dependent on the two fluids flowing in opposite directions,** **and having a concentration gradient between them.** The absorption in insects may be a **passive** or **active** process. The **passive absorption** depend upon the relative concentrations of the substances inside and outside of the cell as well as diffusion i.e. movement of substances taking place from **higher to lower** concentration. The active absorption depend upon some metabolic process for the movement of a substance against concentration gradient on the expenditure of some energy. Movement of water involves movement from a solution of lower osmotic pressure to higher osmotic pressure. **Absorption of Carbohydrates** Carbohydrates are mainly absorbed as monosaccharides in the midgut especially in **midgut caeca**. After digestion, the process of absorption of sugars started from higher concentrations in the gut to the lower concentration in the haemolymph. This remain continued till equilibrium reaches. As soon as the sugars are absorbed in the haemolymph it is immediately converted into **trehalose** in the fat body which surround the gut as a result the glucose concentration in the haemolymph never buildup. This facilitated conversion of glucose to trehalose in fat body is known as **facilitated diffusion mechanism**. Thus we can say that there are two factors which favours the inward diffusion or there are two factors which regulate the diffusion of sugars back into gut: -- i. The immediate conversion of glucose to trehalose, a disaccharide that involves an increase in molecular weight. -- ii. Absorption of water that results in an increase concentration of sugars in the gut consequently a greater concentration gradient. In some insects like in *Aedes* larvae, the glucose is also rapidly converted to glycogen to maintain the concentration gradient. On the other hand as in *Phormia* and other dipterous larvae the concentration of glucose in the haemolymph is normally high, so absorption of glucose from the gut to haemolymph is also take place through active process. Beside glucose, Manose and Fructose are also absorbed in the haemolymph in a similar manner but more slowly because their conversion to trehalose is less rapid. **Absorption of Proteins** Proteins are absorbed after degradation of polypeptide chain to amino acids. Absorption of proteins also take place in midgut especially in caeca. Some of the excess amino acids are also absorbed by malpighian tubules and are then released into the urine which are reabsorbed in the rectum Some times some proteins are absorbed as such i.e. unchanged or undigested which are digested intracellularly like in case of *Rhodnius* and *Pediculus* the haemoglobin are absorbed unchanged The mode of absorption of amino acids depend upon their relative concentration in food and haemolymph i.e. those which are present in high concentration in the food as compare to haemolymph, they may be absorb through passive diffusion. However, in some cases e.g. in *Schistocerca,* the concentration of some amino acids (e.g. glycine and serine) remain still high in the haemolymph therefore, water is absorbed from the gut and released into the haemolymph making it more diluted, leaving gut content more concentrated so again passive diffusion take place from the gut to the haemolymph. At some instances, the diffusion gradient is also maintained by rapid metabolism of absorbed amino acids. Depending upon the composition, availability of specific amino acids in the diet and requirement of the insect, active transport may also occur rather than diffusion alone. **Absorption of Lipids** Fats are also absorbed in different area of the midgut region. For example: in case of *Periplanata* absorption of lipids take place in caeca, *Aedes* in anterior midgut region whereas in blowfly anterior as well as in posterior region both are involved in fat absorption. The hydrolysed (digested) products of wax are absorbed in phosphorylated form (addition of phosphate) and than dephosphorylation take place in the gut epithelium. Cholesterol is esterified before absorption.**Absorption of Water & Inorganic Ions** Like fats, water is also absorbed in different area of the midgut region. For example: in case of *Schistocerca* & *Aedes* through Caeca. *Glossina* through Anterior midgut *Lucilia* through Middle zone In some insects, water is reabsorbed from the urine through **rectal pads**. Water may be absorbed passively if osmotic pressure of the haemolymph exceeds that of the gut fluids The absorption of water is also regulated by the uptake of inorganic ions in between intercellular spaces within the midgut epithelium and rectal pads. Such accumulation of inorganic ions in these spaces change the osmotic pressure as the result of which water passes passively from gut lumen into intercellular spaces. This water increases the hydrostatic pressure in these intercellular spaces due to which water & ions pass into the haemolymph. Such type of absorption of water required the continued supply of ions to maintain the osmotic pressure in intercellular spaces. These ions may be derives from lumen, from recycling within the epithelium or from haemolymph. The inorganic ions are absorbed in the midgut and reabsorbed from the fluids in the rectum. In the midgut region, there are specific zones for the absorption of different ions For example in *Lucilia* larva there is a small zone in the centre of midgut whereas in adults both anterior and posterior parts of the midgut are involved in the absorption of ions. How do insects digest some special food stuffs? The digestive system of insect consists of **(1) alimentary canal** (**gut**) and **(2) salivary glands**, is responsible for all steps in food processing i.e. **digestion**, **absorption**, and **elimination of waste product (faeces).** The insect\'s digestive system is a closed system, with one long enclosed coiled tube called the **alimentary canal** which runs lengthwise through the body. One of the major reasons for **biological success** of insects is their **ability to eat, digest and utilize** an enormous diversity of food. For the digestion of diversified foods, **structural and biochemical modifications occur** in alimentary system of **different species of insects**. Moreover, **within the same species** the diversity may occur in various life stages for example butterfly larvae feed on leaves whereas adults feed on plant nectar and even **within the same stage** structural and biochemical modifications may occur e.g. in mosquito the female feeds on blood whereas the male takes plant juice. **Foregut** **Midgut** **Hindgut** But in general, the alimentary canal system of insects consists of **three regions**: 1. **Foregut** which is also called as **Stomodaeum** 2. **Midgut** or **Mesenteron** 3. **Hindgut** or **Proctodeum** 1. **Foregut** (**Stomodaeum)** The foregut is **ectodermal in origin** and is lined with cuticle. This portion of the alimentary system is generally **concerned with storage of the food** but some time helps in crushing of the food. According to function the foregut is further subdivided into 04 regions such as: **Pharynx**, **Oesophagous**, **Crop**, **Proventriculus** **Pharynx** helps in **passing of the food** to the oesophagus. **Oesophagus** helps in **passing of the food** to the crop. **Crop** act as **storage house** as well as helps in crushing of the food material. 1. **Proventriculus** act as **valve to check the backflow** of the food. This portion has circular muscles which form a sphincter that regulate the movement of the food. 2. **Midgut** or **Mesenteron** The midgut is **endodermal in origin** having 4-8 caeca. This portion does not have cuticular lining but simply lined with a delicate **Peritrophic membrane**. The peritrophic membrane is simply a loose detachable layer between the midgut cells and midgut lumen. It **protects the midgut cells** from abrasion by hard fragments in food. It is simply just like mucous membrane or mucous lining in vertebrates that lubricate the gut. It is made up of muco-protein and chilo protein. This membrane is selectively permeable and allows the movement of digestive enzymes, digested food and small particles through it. **Physiologically** this region of the alimentary canal is very important because it is responsible for: secretion of **digestive enzymes**, **absorption** of digested food materials, **intermediary metabolism** of food and some other functions. The **cells** of the midgut epithelium are **tall**, **columnar** having **microvilli**, forming a **striated border** adjacent to the lumen. These cell generally **contain** extensive **endoplasmic reticulum** with large number of **ribosomes.** They are supposed to concerned with the **synthesis as well as secretion** of digestive enzymes. This secretion may be **Merocrine or Holocrine**. In **merocrine secretion** the secreting cell remains intact throughout the process of formation and discharge of the secretory products where as in **holocrine secretion** the secretory cells disintegrate in order to deliver their accumulated product. All cells shows morphological changes during secretion. These cells are also concerned with **absorption of the digested products**. In some insects, in between the columnar cells, some **goblet cells** are also present which are concerned with pumping of excess potassium from the haemolymph derived from food. In most of the insects, the midgut is generally a simple tube like structure, but in some cases like plant fluid feeders, this region get modified as its anterior portion form a thin walled bladder like structure for rapid elimination of water. This thin walled bladder like structure wraps around the posterior midgut and proximal ends of the malpighian tubules forming a chamber called as **Filterchamber**. The malpighian tubules produces a **hypertonic fluid** which is rich in potassium thereby establishing an osmotic gradient from anterior midgut to filter chamber and then to malpighian tubules. Thus water is passes directly to the midgut, absorption of nutrients take place in the central region of the midgut. The elimination of water is necessary to avoid excessive dilution of the haemolymph and to concentrate food to facilitate enzyme activity. **3. Hindgut** or **Proctodeum** The portion is again **ectodermal in origin** and is also lined with cuticle. According to function this portion is again divided into 3 regions: **Pylorus,** **Ileum and** **Rectum** The anterior portion of hindgut is the **Pylorus** from where the malpighian tubules are arises. This portion act as valve to check the movement of materials between midgut and hindgut. the middle portion i.e. **Ileum** is which joined with rectum. In some insects e.g. in termites it form a pouch like structure where flagellates lives and helps in digestion of cellulose in some insects it is concerned with the removal of water from haemolymph and even in some other insects it is concerned with excretion of ammonia. In *Ostrimia*, the cells of ileum are concerned with secretion of hormone called **proctodone** (an insect developmental hormone). The wider, posterior section of the hind gut is **Rectum** which is a thin walled sac like structure except the region which have rectal pads. Usually there are **six rectal pads** that extended longitudinally along the rectum. These rectal pads are associated with absorption of water, salts and amino acids from the urine. In some insects like Odonata and Orthoptera, the rectal pads consist of a single layer of cells but in others like Hymenoptera, Lepidoptera and Neuroptera, two cell layers are present. These pads have good supply of trachea because of high level of metabolism and even in some aquatic insects the rectum posses the tracheal gills. In insects, water nconservation is a priority. Because of this, blind-ended ducts called **Malpighian tubules** come into play. These ducts emerge as evaginations at the anterior end of the hindgut and are the main organs of osmoregulation and excretion. These extract the waste products from the haemolymph, in which all the internal organs are bathed. These tubules continually produce the **uric acid**, which is transported to the hindgut, where important salts and water are reabsorbed by both the hindgut and rectum. Excrement is then voided as insoluble and non-toxic uric acid granules. The excretion and osmoregulation in insects are not orchestrated by the Malpighian tubules alone, but require a joint function of the ileum and/or rectum. All the three regions of the alimentary canal i.e. **foregut, midgut and hindgut** are innervated by the nerves of stomatogastric nervous system. This innervations varied, in some cases some of the regions like foregut and hind gut are highly innervated whereas midgut is less innervated. In addition to the alimentary canal, insects also have paired salivary glands and salivary reservoirs which are usually reside in the thorax. The main function of salivary glands is to produce saliva. Saliva mixes with food which travels through salivary tubes into the mouth, beginning the process of breaking it down. In invertebrates digestion occurs only in one area of the gut whereas in case of vertebrate there is regional differentiation due to presence of different enzymes in different regions. This localization of enzymes is concerned with more intense metabolism. But in invertebrate same area of gut is responsible for secreting of digestive enzymes, absorption of food and storage. Among invertebrate, insects have evolved some compartmentalization in gut by **peritrophic membrane**.**Secretions of the alimentary canal** There are different glands which provide secretion to the alimentary canal like: **(i) Mandibular glands** **(ii) Maxillary gland** **(iii) Pharyngeal glands and** **(iv) Labial glands.** Among these, the **labial gland** also called as **salivary gland** is most important one and is connected with the digestive tract. Presence of all types of these glands in one species is not necessary and depend up on the nature of food. **(i) Mandibular glands** These glands are usually **sac-like** structure present near the base of mandibles. Their size varies and mainly depend up on the function. For example **in *Apis***, the queen have the larger gland, which is **concerned with the production of pheromone** that regulate the whole colony. However in worker bees have smaller gland because they simply secrete the **alarm pheromone**, while the drone possess very small. In lepidoptera during larval stage, these glands produce **saliva** for lubricating the mouth parts whereas in *Sphodromantis* the gland secret an **amylase enzyme** which digest the starch. **(ii) Maxillary Gland:** These glands are usually small in size, present near the base of the maxillae. Such glands are found in Protura, Collembola, Heteroptera, and in some larvae of Neuroptera and Hymenoptera. Generally these glands are concerned with **lubrication of mouth parts** but in some cases like carnivorous heteropterons, they produce toxin like substance that kills the prey. **(Iii) Pharyngeal Glands** On each side of head, there is only one gland, each consisting of a long coiled tube to which large numbers of **solid lobules** are attached. By separate ducts **each gland open** at the base of the hypopharynx. Such glands are present in social insects specially in Hymenoptera where they undergoes changes during the developmental stages. For example in newly emerged workers, they are poorly developed but soon after feeding on pollen they become bigger and on 5 th day of adult stage they **started producing** **brood food**. Thus such bees **act as nurse bee** feeding the young larvae. Such glands also produce the "**Royal jelly**" on which the developing queen feed, and also play important role in caste determination. Beside this these glands also **secrete invertase enzyme**. iii. **Labial Gland:** In majority of insects, **labial or salivary glands** are present. They are large extended back into the thorax. They are generally of **two types:** (i) **Acinous type** and (2) **Tubular type** In majority of insect the labial glands are **acinous type** which possess two types of cells: (1) large cells that are present in the central region of the gland called as **Zymogen cell** and (2) small cells present in the peripheral region called as **Parietal cells**. The **zymogen cells** contain large amount of rough ER which are responsible for the secretion of **salivary enzymes** but in some cases like in *Schistocera* they also produce mucoproteins or muco polysaccharides. The **parietal cells** of the glands are concerned with transfer of materials from the haemolymph to the central cells i.e. zymogen cells for the synthesis of enzymes. But in some cases like in Diptera, lepidoptera and fleas these **glands are** **tubular** in nature have large number of ducts that swells gradually to form the terminal glandular parts. The production of saliva from the acinous glands is under nervous control. On the basis of functions two types of cells have been identified in these glands: (1) cells having **secreting** function (2) cells have **reabsorptive** functions. These glands are not under **nervous control** but are **regulated by hormone**. Anteriorly these glands open into a narrow duct on each side which join to form a single median duct that open into a salivarium. In liquid feeding insects, this salivarium is modified to form a pump. In different liquid feeding insects, this pump again modified and form varied structure: For example in some Hemiptera it is made up of several lobes and each lobe has quite different structure and function. **Functions of the labial glands:** The **main function** of the labial gland is to produce the saliva that lubricates the mouth parts. The saliva is generally clear, watery having approximately 6.9 pH i.e. neutral in nature, but in some cases like in *Circulifer* it is highly alkaline (pH 10.0) on the other hand in honey bee it acidic (pH 5.0). Besides lubricating the mouth parts the saliva also contain some **enzymes**. The presence of a particular enzyme is related with **diet** but **in general** the saliva contain **Amylase** (that convert starch into sugar) and **Invertase** (that convert sucrose to glucose and fructose). The salivary glands of jassid *Empoasca* secret amylase which is injected into the plants on which they feed. The saliva of hemipteran *Eurygaster* contain amylase and a tryptic proteinase. In those insects which digest their prey extraintestinally, the saliva contain **proteases and lipase**. In blood sucking insects, the saliva contain a anticoagulant **Hyaluronidase** which prevent the clotting in the mouth region of the insect. Thus, the saliva of insects contains 4 main types of enzymes: **Amylase** **Invertase** **Protease** and **Lipase** But their presence depends up on the nature of food on which they feed, for example the saliva of *Oncopeltus* contain all 4 types of enzymes, however, the saliva of *Notonecta, Naucoris, Nepa* contain **protease** and **lipase** only, the saliva of *Gerris, Nabi, Anthocoris* contain **protease** alone. the saliva *Lygus* contain both amylase and invertase, Whereas, the saliva of *Metacanthes, Corizus* contain amylase only however, saliva of *Pentatoma, Gastrodes, Dysdercus* contain invertase only. Beside the **lubrication and secretion** of enzymes, the labial gland in some insects produce **silk** that form a protective covering of the larvae which transform into **cocoon**. In some insects, large quantity of saliva is produced by male scorpion flies *Panorpa* (Mecoptera) which is eaten by the female during copulation. In aphid which feed up on plant sap, e.g. *Myzus*, its saliva contains a pectinase enzyme which breaks down the middle of the plant cells and help in penetration. The saliva of *Aphis pomae* contains large amounts of **alanine** and **glutamic acid**, little amount of **aspartic acid**, **valine** and **serine** and traces of **leucine** and **histidine** which are unutilized dietary product excreted from the haemolyph. In aquatic insects, e.g. heteroptera, the salivary gland produces a **toxin** that kills and partially digests the prey. The salivary glands of honey bee are more complex and very in their degree of functions in different castes and even within the same caste like inworkers. **Digestive enzymes** The digestive enzymes in insects are mostly adapted to the diet on which they feed, e.g. cockroaches which feed upon all type of food (**Omnivorous**) secrete all type of enzymes like **amylase**, **invertase**, **protease** and **lipase** to digest all the major constituents of food like carbohydrates, proteins and fats. Likewise, **phytophagous** insects such as orthoptera, coleopteran, larvae of lepidoptera and honey bee also secrete same type/set of enzymes. However, those insects which have some **specificity of diet** and their diet contain some specific substances than these insects generally produce that **appropriate enzyme** in bulk. e.g. the saliva of caterpillars contain **amylase**, carnivorous insects like *Lucilia* secrete **protease** and **lipase**. These digestive enzymes are chiefly secreted by **midgut** and its caeca, although the salivary glands/labial gland also contributes some amount of invertase and amylase enzymes. **Control of Enzyme Secretion** The level of enzyme activity in the gut lumen **varies** and depends upon the presence of food in the gut e.g. in *Melanoplus,* the **protease activity** remain lowered in gut during absence of food but rises when food is ingested. Even in those insects which feed at frequent **internals** likecaterpillars,grasshoppers etc., the enzyme activity fluctuates with the state of feeding The enzyme activity also changes with development or with season e.g. in *Bombayx* larva the enzyme activity double in the first six day of the 4 thinstar or in *Apis* invertase activity is little in early spring and in autumn. Thus the synthesis and secretion of enzyme is **regulated physiologically**, whereas, in some insect it is regulated independently e.g. in *Locusta* the carbohydrases accumulate in the midnight during starvation period. Thus three possible regulatory mechanisms are involved in the syntheses and secretion of enzymes: **Secretogogue Mechanism** **Nervous Mechanism** **Hormonal Mechanism** **1. Secretogogue Mechanism** When the food itself or its product induced to stimulate the midgut cells to secrete the enzyme is called secretogogue mechanism. In Secretogogue mechanism, a chemical stimulus is given out by the chemical constituent of the food or the partially digested food stimulate the further secretion. In continuous feeders like in *Periplanets*, *Blatella*, *Tenebrio* food itself is responsible for secretion of enzyme. During feeding period there is abundant secretion but during starvation very little quantity is secreted. Likewise in discontinuous feeder like in blood sucking insects *Stomoxys* where second meal is taken after a number of days of the first and the midgut become completely empty, there is very little secretion of digestion enzymes. Similar process occurs in *Locusta* and *Dysdercus.* **2. Hormonal Mechanism** In some insects the secretion of digestive enzymes are controlled by **hormones** or hormone factors. Such hormones or hormonal factors are synthesis in the **neurosecretory cell**, situated in pars cerebralis of **fore brain**, from their they are released into blood and taken to the secretory cells of midgut epithelium to secrete the enzymes. A number of scientist showed experimentally the involvement of hormones in the secretion of enzymes. For example in *Calliphora* where the **secretion or synthesisof proteases enzymes** does not take place when the **neurosecretory cells of brain** were taken out or damaged. The experimental removal of NSC of brain incapacitate insects to digest protein. Dadd (1961) also conducted an experiment on *Tenebrio* where he **took the blood from a fed individual** and transfused into the **starved one**. In starved insect the activity and concentration of enzyme was very **low** but after transfusion of blood from fed insect, the level of digestive enzyme increased (even when the food was not present) This experiment clearly shows that or hormonal factor presented in blood which is responsible for the initial synthesis secretion of the enzyme but later the secretion is taken up by secretogogue mechanism. **3. Nervous Control** In some insects, the **Autonomous Nervous System** (ANS) along with **Ventral Verve Cord** (VNC) and **Central Nervous System** (CNS) is involved in **enzyme secretion**. The nervous component which is supposed to be involved in the synthesis or secretion of digestive enzymes includes a **hypocerebral ganglia**, which is present behind the brain, from where two lateral nervous are arises which runs along the alimentary canal and in the foregut region. The **stomatogastric ganglia** supply the nerves to the muscles of the alimentary canal. The **salivary gland of cockroach** also shows the presence of two types of nerve fibres that control the synthesis of salivary enzymes. However in other insects the role of nervous system in the synthesis of enzymes have been ruled out. **Digestion of Food** The enzymes required for the digestion of food are mainly secreted by the **midgut** **epithelium and its caeca**, and partly by the **salivary gland.** In insects digestion may be of two types: **(i) Extra intestinal digestion** **(ii) Internal digestion** **(i) Extra intestinal digestion:** In some insects, digestion may start before ingestion because the saliva of such insects contain the digestive enzymes. e.g. the saliva of fluid feeding insects contain **pectinase** which act on middle lamellae of plant tissue and **amylase** that act on carbohydrate. Likewise, the saliva of carnivorous insect *Platymerus* contain **hyaluronidase** that act upon muco-polysaccharides of connective tisues and helps in the entry of other salivary enzymes such as **proteinase** and **phospholipase** into the tissues. In *Dytiscus* larvae, the salivary gland are absent, even though the extra-intestinal digestion take place. In such case the **midgut enzymes** are injected in to the pray through the mandible. These midgut enzymes digest the content of the pray and then the larvae absorbed the digested material through the same rout. The excreta of blowfly larvae contain **proteolytic enzyme** that digest and liquefied the meat where they live. The adult of *Bombyx* secrete a **protease enzymes** that digest sericin of silk and help them to escape from the cocoon. **Internal Digestion:** Irrespective of whether an insect is **carnivorous**, **herbivorous**, **parasitic** or **saphrophagous**, most of the digestion take place in the midgut region where arange of enzymes are present. The presence of these enzymes depend upon the specificity of the diet (e.g. larval blow fly feed on protein posses protease whereas, adult that feed upon nectar is lack of that enzyme). In general most of the digestion occur in midgut region, however, in some insects some of the digestion takes place in crop due to the regurgitation of midgut juices. In some insect little digestion take place in the hindgut region apart from cellulose digestion. **Digestion of carbohydrates** Carbohydrates is generic term that includes sugars, glycogen, starch, and cellulose. They are divided into four chemical groups based on no. of sugar units: Monosaccharides Disaccharides Oligosaccharides and Polysaccharides. In general, the **monosaccharides and** **disaccharides** are commonly referred to as **sugars.** for example: inconspicuous **grape sugar** is the monosaccharide **glucose**, **cane sugar** is the disaccharide **sucrose** **milk sugar** is the disaccharide **lactose** Carbohydrates such as **disaccharides** and **polysaccharides** are **macromolecules** and can not be absorbed by the cell. Therefore, they must be hydrolyze (digested) to simplest form. These **macromolecules** are made up of either same type of sugars (glucose + glucose) or different types of sugars (glucose + galactose). These sugars are link together either by **α-linkage** or **β-linkage** The digestion of carbohydrates partly take place in gut wall and partly in the lumen of the alimentary canal. For the digestion of various carbohydrates, specific enzyme is required e.g. the gut of *Locusta* contains 7 carbohydrates: α-1,4 amylase α -1,6 amylase α-glycosidase β-glycosidase Trehalase α-galactosidase β-galactosidase These enzymes act on disaccharides and polysaccharides. The common disaccharide (**α-- glucosides)** Maltose, Trehalose and Sucrose contain glucose residue that are link with other sugar molecules by α- linkage which are hydrolized by **α- glycosidase** that act on α-linkage.**α--glucosides** Similarly naturally occurring **β--glucosidesb**such as **Salicin**, **Arbutia** and **Cellobios** are digested by β-glycosidase. The α-galactosidase hydrolise Meliobiose and β-galactosidase hydrolise Lactose So, these enzymes are group/bond specific, however there are enzymes which are substrate specific e.g. in *Schistocerca* among the α- glycosidase, there is specific α- glycosidase which act on Trehalose. **α-amylase act on α-link of Starch and Glycogen** Starch Amylase Maltose Glycogen Amylase Glucose These **amylase** work in two ways i.e. some amylase splits off maltose residues from the ends of starch molecule resulting in rapid increase in the concentration of maltose, such type of amylase are called as exoamylase, whereas, there are other amylase which hydrolyze the bonds well within the starch molecule, such amylase are called as endoamylase. The products of these amylase are further hydrolyzed in the normal ways by α- glycosidase α-glycosidase act on α-link of Maltose, Trehalose & Sucrose Maltose Maltase Glucose+ Glucose Trehalose Trehalase Glucose+ Glucose Sucrose Sucrase Glucose+Fructose β-glycosidase act on β-linkage of β-glucosides i.e. Selecin, Arbutin and cellobiose Cellobiose β-glycosidase Glucose + Glucose α-galactosidase hydrolyse Melibiose to give rise to Glucose + Galactose Likewise β-galactosidae hydrolise Lactose to Glucose + Galactose In the hydrolysis of carbohydrates, **water** is required which **act as acceptor** for the sugar residues + H2O But there are sugars which **itself act as acceptor,** thereby forming **oligosaccharides** i.e. trisaccharides like glucosucrose or melezitose. + Melezitose Such process is called as **Transglucosylation**. Structure of Melezitose 1:3 Link **Digestion of Proteins** For the digestion of **proteins**, the gut of insects also secret number of **protease** enzymes. **Protease is a general term which includes**: **i. Proteinase** **ii. Peptidase** The **proteinase** digest the natural proteins into **peptones** and **polypeptides**. These **peptones and polypeptides** are then further digested by **peptidase enzymes**.Polypeptide chain **Peptides (50 units)** **Peptones** The Peptidase are of two types: -- **(i) Exopeptidases** -- **(ii) Endopeptidases** The **exopeptidase** attach and hydrolyze the polypeptide from **out side** and liberate the amino acids, whereas, the **endopeptidase** attack peptide bonds **within** the protein molecule. The **exopeptidases** are again of two types: **i. Carboxypolypeptidases** **ii. Aminopolypeptidases** The carboxypolypeptidases attack the peptide chain from the carboxyl group (- COOH) whereas the aminopolypeptidases hydrolize the chain from the amino group (- NH2) end. The **endopeptidases** are of three types: i. Serine proteases (e.g. Trypsin and chymotrypsin) ii. Cysteine protease iii. Aspartic protease After the digestion of natural proteins, most of the **peptones** and **polypeptides** are absorbed by the epithelial cells of the midgut, whereas some of them remain in gut lumen. Therefore, most of the polypeptidase are found in the epithelial cells to further hydrolize the digested product of proteinase i.e. peptones and polypeptides. Beside the common proteins, some insects are also able to digest the inert animal proteins like **keratin** which is found in wool, hair, feather etc and **collagen** present in animal tissue. In *Mallophaga* the inert protein **keratin** is digested by **keratinase enzyme**. The keratin is made up of polypeptide chain which consist of sulfure containing amino acids that are linked togather by disulphide bonds. These disulphide bonds are hydrolized by keratinase enzyme thus librating the amino acids. The larvae of *Hypoderma* (Diptera) produce collagenase enzyme that digest collagen of animal tissue. **Proteinase** Protease **Peptidase** **Natural Protein Proteinase** **Peptones & polypeptides** **Peptones & polypeptides** Peptidase Amino Acids **Exopeptidases** Carboxypolypeptidase Peptidase Aminopolypeptidase **Endopeptidases** Serine proteases Cysteine protease Aspartic protease **Digestion of Lipids** Lipids are ingested mainly in the form of **triglycerides**. These **triglycerides** are digested to **diglycerides** and then **monoglycerides** and finally to **free fatty acids**. For the digestion of lipids, 3 types of enzymes are found in insects: \(i) **Lipases** which act upon the insoluble glycerides of long chain fatty acids \(ii) **Esterases** that digest glycerides of short chain fatty acids \(iii) **Phosphatases** that digest phospholipids In insects a number of esterases are present butwhich enzyme is secreted in the gut lumen and which one is secreted in the epithelial cell is not clear.