Caterpillars in general irrespective of various kinds are classified as belonging to phylum- Arthropoda, subphylum-Mandibulata, subclass-Hexapoda, class-Insecta, order-Lepidoptera, family-Papilionidae, and subfamily-Papilioninae. Insects are usually considered to have three pairs of mouthpart appendages; mandibles, maxillae, and labium. Hemocoel is the cavity of the insect body, which is filled with circulating blood that immerses the tissues and transports food, wastes, and hormones (but not oxygen). The digestive system fills most of the space in the perivisceral hemocoel.
A Pair of glands opening on the labium known as labial glands act as salivary glands in insects and secrete salivary hydrolytic enzymes that are specific for the diet of the species. Caterpillars have a pair of mandibular glands that secrete salivary enzymes which help in the digestive process. The salivary (silk) glands and Malpighian tubules are close to the gut. The gut consists of anterior foregut, middle mid-gut, and posterior hindgut. The mid-gut cells of most insects secrete a multilayered peritrophic membrane consisting of chitin, proteins and proteoglycans. This membrane is non-cellular in nature and functions as physical barrier to protect the epithelium from mechanical rupture, toxic plant chemicals and pathogens and also compartmentalization (cell, ecto and endo-peritrophic spaces) of the gut lumen and the spatial organization of digestive processes.
Caterpillar's mid-gut is also lined by the peritrophic membrane, a layer that allows digestive enzymes to flow into the gut and allows the nutrients to flow from the gut out into the cells around the gut. The membrane is made up of substances such as chitin. The holes in the membrane are large enough to allow food molecules to pass through, but are too small to allow the virus particles or bacteria to pass through it. Large molecules are hydrolyzed by enzymes inside the peritrophic matrix, until they are small enough to diffuse into the space between the peritrophic matrix and mid-gut epithelium, where digestion is completed by membrane-bound enzymes which may be integral proteins of the microvillar membranes. Structural strength is provided by the meshwork of chitin fibrils, while permeability properties are determined by pore diameters in the gel-like matrix. Labeled dextrans with diameters ranging from 21 to 36nm penetrate the peritrophic matrix of several species of lepidoptera and orthoptera. So size exclusion does not explain impermeability to digestive enzymes. The evidence for compartmentalization of digestive processes is reviewed by terra and colleagues.
Soluble enzymes like amylase, carboxypeptidase and trypsin are found to be present in intracellular vesicles. Amylase, carboxypeptidase A and trypsin are found in minor amounts in the ectoperitrophic contents (both soluble and membrane-bound) and in major amounts in the peritrophic membrane with contents. The initial digestion is carried out in large amounts by enzymes in the endo-peritrophic space (inside peritrophic membrane) and in small amounts by enzymes arrested in the peritrophic membrane. Intermediate and final digestion occurs at the ecto-peritrophic space (space between peritrophic membrane and mid-gut epithelium) or at the surface of mid-gut cells. Trypsin is found in luminal contents inside and outside peritrophic membrane, with small amounts being recovered in cells. Hence, trypsin is secreted by cells and is able to cross the membrane. Amino-peptidase is not found inside peritrophic membrane, but is found outside the membrane. Thus, amino-peptidase is secreted into luminal contents but it does not pass through the membrane. Large amounts of amino-peptidase are also found in caeca cells. Amylase has a mid-gut distribution similar to trypsin, whereas maltase is restricted to mid-gut cells, mainly those of caeca. Diet was found to have a strong affect on caterpillar salivary gland, glucose-oxidase activity. Peritrophic gel differs from peritrophic membrane in lacking mechanical resistance and permeability properties. Peritrophic membrane pore sizes are less than 8 nm. So, the gel separates the mid-gut cells from the food during the forward movement of food while membrane is found in the posterior mid-gut probably to improve the compartmentalization of digestive events.
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