Digestive System in Cockroach (Invertebrates)
Nutrition in cockroach is holozoic and it is an omnivore, feeding on different kinds of organic matter. It takes in pieces of food and has to grind them before digesting them. Thus its mouth parts are modified accordingly for chewing the food.
The digestive system includes the mouth parts, a pair of salivary glands and the alimentary canal.
Mouthparts of cockroach
To understand the position of the mouth parts, observe the external features of a cockroach in the diagram below.
The body of the cockroach is segmented into three portions head, thorax and the abdomen. The mouth parts are attached to the ventral side (underside) of the head portion and surrounds the mouth or the oral cavity which faces down.
The different mouthparts are -
Labrum or the upper lip
It is a broad, roughly rectangular shaped structure. It hangs from the front edge of the head on the lower side. It covers the mouth and the mandibles.
Mandibles
They are a pair of hard, strong, large, dark coloured triangular structures found one on either side with jagged inner edges and the two mandibles move in horizontal motion and crush food between them.
Maxillae
Maxillae are a pair of structures lying outside and behind the mandibles. Each of them consists of three parts - protopodite, exopodite and endopodite. Protopodite consists of cardo and stipes, exopodite is 5-segemented and sensory, also called maxillary palp and endopodite is made up of inner lacina and outer galea. The maxillae are used to manipulate the food before it enters the mouth.
Labium
It is said to be formed by the fusion of the second pair of maxillae. It forms the broad median lower lip consisting of several parts in addition to a pair of 3-segmented labial palps on either side.
The maxillary and labial palps have sense organs that help them to choose suitable food.
The mandibles and the maxillae grind the food by moving it laterally.
The labrum and labium help to hold the food between the mandibles and the maxillae.
Alimentary canal
Digestion takes place in specialised cavities joined together to form a continuous canal. It is called the alimentary canal.
The alimentary canal is divided into three main portions-
Foregut
Foregut consists of the mouth surrounded by the mouthparts. The mouth cavity is called the pharynx. It continues as the oesophagus that is short, narrow and thin-walled. The canal then enlarges into the crop, which is also thin-walled. The crop opens into a short, muscular organ, the gizzard or the proventriculus. Outside and lying below the crop are a pair of salivary glands.
Each salivary gland is branched, the secretions of all the branches being poured into a common duct. For either pair of salivary glands there is a thin walled salivary receptacle or reservoir which is like a bladder. It stores the salivary secretions. The receptacles of either side have a common receptacular duct which opens into the common salivary duct. This common salivary duct opens into the mouth cavity at the labium.
The entire foregut is lined with chitin. In the gizzard, the chitin forms proventricular teeth and the plate to facilitate grinding of the food.
Midgut
Midgut forms the true gut or the mesenteron and consists entirely of stomach or ventriculus. At the junction of the gizzard and stomach are six pairs of gastric caecae ('gastric' means pertaining to stomach). These are pouch-like structures arranged in a ring-like manner around the anterior end of the stomach. The anterior lobe of each pair of the caecae extends over the proventriculus and the posterior lobe extends over the ventriculus. The caecae secrete digestive juices and pour them into the stomach. The midgut is not lined by chitin or cuticle but by a peritrophic membrane. This membrane protects the stomach wall from abrasions and is fully permeable to enzymes and digested food.
Hindgut
Hindgut is a coiled structure consisting of anterior ileum, middle colon and posterior rectum. The rectum opens to the exterior through the anus. The hindgut is lined by cuticle. At the junction of the stomach and ileum are attached numerous long tubules called the Malpighian tubules.
Mechanism of digestion
Digestion starts in the mouth with the mandibles and the maxillae chewing the food. It is also acted upon by the salivary carbohydrases which partially digests the food. The food is then swallowed with the help of lubrication provided by the salivary juice.
The food then enters the oesophagus and then into the crop. Here, the masticated food is temporarily stored.
The food is then passed into the gizzard which acts as the grinding chamber. At the junction of the gizzard and the stomach is a valve called the stomodael valve. It allows the passage of only the thoroughly digested food into the stomach and also, prevents the regurgitation of food from the stomach.
The ground food, then enters the stomach. The digestive enzymes secreted by the gastric caecae act upon the food in the stomach. These enzymes include amylase, maltase, invertase, tryptase and lipase. The remaining carbohydrates, proteins and fats are digested here.
The digested food is absorbed through the stomach walls into the surrounding space which is called the haemocoel. The haemocoel consists of a large number of fat bodies which are fat cells having fat globules, protein granules and glycogen. They form storage structures. From here, it is transported to the different body parts.
In the hindgut, absorption of water takes place and the undigested food is formed into almost dry pellets. These are excreted through the anus as faeces.
