Digestion: An imperative process for insects.
Digestion is an imperative process in the survival of an insect as it breaks down the organic material ingested by the organisms giving rise to the essential molecules needed by the body for not only body building and maintenance, but also providing energy (Watson, 2003). The digestion process occurs in the digestive tract running from the mouth to the anus with the aid of enzymes, and this is mainly divided into three sections according to their function in the digestion process. These three regions are: the foregut (stomodeum), the midgut (mesenteron) and the hindgut (proctodeum) (Gullan & Cranston, 2010). Variations in structure may occur due to differences in the feeding habits of the class Insecta.
Fig1: The generalised digestive system of the class Insecta (Gullan & Cranston, 2010).
The Locusta migratoria possess all three sections and all three have specialised features that allows their functions to be carried out effectively. The whole digestive tract has a one cell thick epithelial layer lying on a basement membrane with only the foregut and midgut possessing a cuticular lining (Gullan & Cranston, 2010). The foregut consists of the pharynx, esophagus, crop, proventriculus and esophagal valve (Capinera, 2008). Its main functions are: ingestion and filtration of food, storage, transport, grinding and it is the also the primary site of the glands. The anterior end of the foregut consists of the upper area (cibarium) and a lower part (salivarium) which houses the labial and salivary glands (Gullan & Cranston, 2010). The crop functions as a storage organ and the proventriculus as a grinding organ (in insects that feed on solid foods). The proventriculus is highly developed in the order Orthoptera whereby the cuticle has spines and teeth from the ridges formed by a highly folded epithelium (Gullan & Cranston, 2010). The esophagus transports the food to the next section aided by the esophagal valve that prevents backflow.
The midgut contains the gastric caeca, tubular ventriculus and bladder-like pouches that increase surface area (Gullan & Cranston, 2010; Capinera, 2008), here most digestive enzymes are produced and secreted whilst absorption also takes place (Gullan & Cranston, 2010). It is lined with microvilli that allow rapid absorption to take place and contains the peritrophic membrane (Gullan & Cranston, 2010) that protect the midgut epithelium from mechanical damage, prevent micro-organism penetration, binding toxins and to separate the midgut lumen into sections (Gillot, 2005). The division improves digestion efficiency and allows recycling of the enzymes (Gillot, 2005).
The hindgut consists of the malphigian tubules that mark the opening of the section, the pylorus, ileum, rectum and it leads to the anus (Gillot, 2005; Gullan & Cranston, 2005). The pylorus facilitates the movement of material from midgut to hindgut, whilst the ileum is a channel that directs undigested material to the rectum where reabsorption of ions and water occurs. The ileum is usually dilated and its function leads to the final formation of faeces that are expelled through the anus (Gillot, 2005).
Although this is the general structure of the digestive system some parts of the digestive tract may be absent in other insects due to differences in dietary requirements. In the foregut, the orders Hemiptera and Lepidoptera have complicates glands, with the hemipterans having two types of saliva production and the lepidopterans’ labial glands secreting silk and the mandibular glands secreting saliva (Gullan & Cranston, 2010). Since the Hemiptera are plant fluid feeders, the anterior midgut and hindgut are in close proximity with the addition of filter chambers allowing direct passage of minute molecules, simple sugars and excess water to the hindgut (Gullan & Cranston, 2005).
Fig2: The structure of the digestive system of a locust (Gillot, 2005).
The Hemiptera, Thysanoptera, adult Lepidoptera and some blood sucking Diptera generally lack the peritrophic membrane of the midgut whilst some produce only at certain times, such as the mosquito that only produces after a blood meal (Gillot, 2005). In some orders absorption of water and ions may occur in the ileum. In the Isoptera and Coleoptera, the ileum is dilated forming a fermentation pouch that contains bacteria and protozoa that digest wood particles that they ingest. For a few species nitrogenous waste production and excretion occur in the ileum (Gillot, 2005).
Although the insects’ may be similar to mans’ gut in having sections throughout the gut, microvilli function as well as enzyme function, differences are also noted. The presence of a crop and ventriculus is not seen in the digestive tract of a human being, the foregut and midgut are also lined with a cuticle whilst in the human digestive tract it is not seen and lastly the insect food is held in the peritrophic membrane whilst this feature is not in the human digestive system (Purves et al., 2003).
The digestive system represents the diversity of the class Insecta and how this has contributed to its success in the terrestrial environment.
REFERENCES
Capinera, J. L. (2008). Internal anatomy of Insects. In J. L. Capinera (Ed.), Encyclopedia of entomology (2nd ed., Vol. 4, pp.2021-2022).
Gillot, C. (2005). Entomology (3rd ed.). Dordrecht: Springer.
Gullan, P. J., & Cranston, P. S. (2005). The Insects: An outline of entomology (3rd ed.). Oxford: Blackwell Publishing Ltd.
Gullan, P. J., & Cranston, P. S. (2010). The Insects: An outline of entomology (4th ed.). Oxford: John Wiley & Sons Ltd.
Purves, W.K., Sadava, D.E., Oriano, G.H., & Heller, H.C. (2003). Life: The Science of Biology (7th ed.). New York: W. H Freeman and Co Ltd.
Watson, F. (2003). Distribution of Digestive Enzymes in Cockroaches. Mini Workshops, 24, 311-316.
