External Features Of Cockroach

 External Features Of Cockroach :

Periplaneta americana are elongated, dorso-ventrally flattened organisms. Their size varies from 3 to 5 cm in length and about 1.5 to 2 cm in breadth. The color is reddish-brown. They are all bilaterally symmetrical, triploblastic and coelomate. Body cavity is known as haemocoel as it is filled with haemolymph. Body of cockroach is protected by hard, waxy, water proof, chitinous exoskeleton. It is made up of polysaccharide called chitin. It also helps in providing the surface area for attachment of muscles. In each segment it is further subdivided into four plates namely dorsal tergum, ventral sternum and two lateral pleurons.

Body divisions : The body is divided into the three regions namely head, thorax and abdomen. The head is ovate, flattened dorso-ventrally and pear-shaped, attached at the right angles to the thorax by a thin, narrow, movable neck or cervicum. Hence the broad end is directed upwards and the narrow end is directed downwards. Head is formed is six sclerites. These fuse in adults and form head capsule. Head bears four important parts - compound eyes, antennae, fenestrae and mouth parts. Compound eyes are paired, dark, kidney-shaped structures placed on the lateral sides of the head and are made up of large number of ommatidia (singular-ommatidium). Each ommatidium acts as a single simple eye and forms a parts of image of an object. All such images combine together in the brain and form a complete single but blur mosaic image.

Antennae are paired, long, slender, filamentous, many jointed structures present between the compound eyes. They are lodged in the membranous pits called antennal sockets. These are the tacto receptors or organs of olfaction (sense of small) useful in localizing the food material in the vicinity. Fenestrae are also called the oscillar spots situated at the base of each antenna and they appear as white spots.

Mouthparts or trophi - Mouthparts of cockroaches are of chewing and biting type. These are movable, segmented appendages which assist in ingestion of food. The mouth parts of cockroach comprise of labrum, mandibles, maxillae, labium and hypopharynx. Labrum or upper lip is flap like or shield like movable structure which covers the mouth from the upper side. Thus it forms the anterior wall of mouth cavity. It is also useful in holding of the food during feeding. Mandibles or true jaws are two dark, hard chitinous structures with serrated margins with teeth present below the labrum on either side of the mouth. These are useful in cutting and crushing the food. Maxillae or accessory jaws are paired appendages situated on the either side of the mouth behind the mandibles. Each maxilla is many jointed structure with maxillary palp. These are also called first pair of maxillae. Labium or lower lip is also called the second maxilla which covers the mouth from the ventral side and is firmly attached to the posterior part of head. It has labial palp which is three jointed. These are sensory in function. Labium is useful in pushing the chewed food in the preoral cavity and also prevents the loss of food falling from mandibles during the chewing. Hypopharynx or tongue, is a long, prominent medially placed appendage.

Cockroach-Mouth Parts

The Cervix or the neck is membranous structures between the head and the thorax. It allows the free movement of the head. Thorax is made up of three distinct segments: prothorax, mesothorax and metathorax. Dorsally the thorax bears two pairs of wings-fore wings and hind wings. Forewings are first pair of dark, opaque, thick leathery wings which are are protective in function. Hind wings are thin, broad, membranous, delicate and transparent. Second pair of wings are attached to tergum of metathorax. These are useful in flight and hence are also called true wings. Three pairs of thoracic legs are present on ventral side. Each leg is five jointed or segmented and is covered with the sensory bristles and spines. Each leg has five podomeres namely Coxa, Trochanter, Femur, Tibia and Tarsus. Tarsus is the last segment and is made up of five movable segments or tarsomeres. Last segment of the tarsus bears a pair of claws and arolium helpful in clinging. Thorax bears two pairs of spiracles.

The abdomen is the largest of the three regions and contain most of the viscera. Its segmentation is conspicuous both dorsally and ventrally. The abdomen consists of 11 segments of which the 11th is reduced and fused with the 10th so that a maximum of ten segments are distinct and visible. Dorsally each segment is covered by a tergite and, ventrally by a sternite. Laterally, tergites and sternites are connected by unsclerotized flexible pleura. Cockroaches exhibit sexual dimorphism. In males, anal styles are present in segment 9. These are absent in females. Most abdominal segments lack appendages but segment 10 bears a pair of large, segmented cerci, which are visible in both dorsal and central views. These are sensitive to air movements, including sound, and perhaps to ground vibrations. Genital openings are present on different segments in both the sexes and are surrounded by the chitinous plates in both which are collectively called gonapophyses.