Useful information on the structure and function of digestive gland of cockroach

The digestive system of cockroach consists of alimentary canal and digestive glands. One of the peculiarity of the digestive system is that the system although mainly involved in the process of digestion, it also takes part in excretion being associated with Malpighian tubules which open into the alimentary canal at the junction of midgut and hind gut.

Alimentary canal: It is complete, coiled and like a long tube of varying diameter. It is divisible into three main parts i.e. (i) Foregut or stomodeum (ii) Midgut or Mesenteron and (iii) Hindgut or Proctodeum (Fig.).

The foregut is ectodermal in origin and is lined internally with cuticle. It includes preoral cavity, a pseudocoel bounded by mouth parts lying in front of mouth. Hypopharynx, a tongue like projection hangs in it at tip of pre-oral cavoty. The mouth is a slit like aperture. The preoral cavity has the openings of the common salivary duct at the base of hypopharynx.

Posteriorly pre oral cavity (mouth cavity) communicates with the short, tubular pharynx by a small opening. The pharynx crosses the head region and after entering the thorax it joins the oesophagus, a short, narrow and tubular structure. The oesophagus dilates to form the sac-like crop which occupies whole of thorax and a few abdominal segments. The crop serves as reservoir of food. At its porterior extremity the crop narrow down and opens into a small, conical and thick walled sac, the gizzard which is broad anteriorly and narrow posteriorly.

The anterior part of gizzard (also called armarium) has six longitudinal folds bearing six chitinous teeth and in the posterior part, six cushion like pads (also called pulville) with chitinous hair are present. When the muscular wall of gizzards contracts the teeth work against each other and grind the food , while the hairs act as sieve or strainer stomodael valve present also in the posterior partnership the of gizzard prevents regurgination of food.

The midgut is a long, narrow, and tubular structure. It differs from foregut hindgut in that its wall has glandular epithelial cells. Near the junction of gizzard and midgut 7-8 slender finger like diverticula arise which are called hepatic or Mesenteric caeca. The junction of the mid-gut and hindgut is marked by the presence of numerous slender threads like tubules called Malpighian tubules.

The midgut leads to hindgut which is divisible into a short narrow tube (ileum), followed by long wide and coiled tube, colon and finally the rectum. Rectum, the small and dilated terminal part contains fold of epithelium which project into the lumen. These longitudinal folds, called rectal papillae are involved in reabsorption of water. The rectum opens to the exterior by a slit like aperture, anus at the end of abdomen below the 10^th tergum.

Digestive Glands and Associated Structures:

The well-known digestive gland is salivary gland. Besides this gland, the lining cells of midgut and hepatic caeca are also involved in secretions necessary for digestion.

The salivary glands are a pair of whitish and lobulated glands present in the thorax one on either side of the crop. Each gland has a bipartite and lobulated glandular part and a long sac-like reservoir or receptacle. The glandular part secrets saliva which when not required is stored in reservoir. Saliva from the salivary glands is carried by a common salivary duct which opens at the base of hypopharynx in the preoral cavity.

Food and Feeding:

Cockroach is omnivorous. It feeds on a variety of organic substances like food grains, cloth, wood, paper, vegetables, fats, oils etc. It also shows cannibalism i.e., it eats its own species.

Cockroach searches for its food by olfactory and tactile antennae, tasted by maxillary and labial palps and finally solid food is captured by legs and maxillae. The mandibles masficate the food to bits and finally carry them to the mouth with the help of labium. So the mouthparts of the cockroach help in ingestion and are known as chewing type of mouthparts.

Physiology of Digestion:

The variety of food consumed by the cockroach mainly contains carbohydrates, proteins and fats. Therefore the necessary enzymes required for the digestion of these substances must be secreted in the gut for intracellular digestion. Digestive enzymes are hydrolytic enzymes as they involve water molecule for the break down of the complex organic molecules to smaller molecules. In the preoral cavity amylase enzyme of saliva digests starch into maltose and maltotriose.

The semi digested food moves by peristalsis to be stored temporarily in the crop. The food is masticated in the gizzard by the chitinous teeth and is then filtered so that fine food particles enter the midgut.

The enzymes secreted by lining of hepatic caeca and midgut are:

(a) Trypsin and Erepsin to hydrolyse proteins to amino acids.

(b) Lipase to hydrolyse fats to fatty acids and glycerol.

(c) Carbohydrates such as Invertase to convert sucrose to glucose and fructose, maltase to convert lactose to glucose and galactose.

The products of digestion are absorbed partly in mesenteron and intestine.