Periplaneta americana is the common species of cockroach. An adult cockroach is about 34 – 53 mm long. Wings extend beyond the tip of the abdomen in males. The body is segmented into three distinct regions, viz. head, thorax and abdomen.
The whole body is covered by a brown coloured hard exoskeleton which is made up of chitin. In each segment, the exoskeleton has hardened plates, called sclerites. The sclerites on dorsal surface are called tergites, and those on ventral surface are called sternites. The ventral and dorsal sclerites are joined together by a thin and flexible articular membrane. This is called arthrodial membrane. The arthrodial membrane facilitates movement between tergite and sternite.
The head is triangular in shape. It lies anteriorly at right angles to the longitudinal body axis. The head is formed by the fusion of six segments. The flexible neck facilitates movement in all directions. A pair of compound eyes is present in the head capsule. Membranous sockets lie in front of eyes and a pair of thread-like antennae arises from them. Antennae have sensory receptors.
The mouth parts are at the anterior end of the head. The mouth parts consist of a labrum (upper lip), a pair of mandibles, a pair of maxillae and a labium (lower lip). A median flexible lobe lies in the cavity which is enclosed by the mouth parts. This lobe acts as tongue (hypopharynx).
The thorax consists of three parts, viz. prothorax, mesothorax and metathorax. The head is connected with the thorax by a short extension of prothorax, called neck. Each thoracic segment bears a pair of walking legs. The first pair of wings arises from mesothorax and the second pair arises from metathorax. Forewings are called tegmina. They are opaque dark and leathery and cover the hind wings when at rest. The hind wings are transparent, membranous and are used in flight.
The abdomen consists of 10 segments. In females, the 7th segment is boat shaped. The 7th sternum, together with the 8th and 9th sterna, forms a brood or genital part. The anterior part of the genital part contains the female gonopore, spermathecal pores and collateral glands. In males, the genital pouch lies at the hind end of abdomen. It is bound dorsally by the 9th and 10th terga and ventrally by the 9th sternum. It contains the dorsal anus, ventral male genital pore and gonapophysis. A pair of short, thread-like anal styles is present in males. In both sexes, the 10th segment bears of pair of jointed filamentous structures called anal cerci.
The alimentary canal is divided into three regions, viz. foregut, midgut and hindgut. The mouth opens into a short tubular pharynx. The pharynx opens into a narrow tubular oesophagus. The oesophagus opens into a sac-like structure called crop. Food is stored in the crop. The crop is followed by gizzard and proventriculus. The gizzard has an outer layer of thick circular muscles and thick inner cuticle, forming six highly chitinous plates called teeth. Food particles are ground in the gizzard. The entire foregut is lined by cuticle. At the junction of foregut and midgut, a ring of 6 – 8 blind tubules are present. They are called blind because eacth of them has only one opening. These are called gastric or hepatic cecae and secrete digestive juice. The hindgut is broader than the midgut. The hindgut is differentiated into ileum, colon and rectum. The rectum opens through anus.
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