Biology Class Eleven  

Structural Organization In Animals



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 are called tergites; dorsally; and sternites; ventrally. The ventral and dorsal sclerites are joined together by a thin and flexible articular membrane. This is called arthrodial membrane.

Body Segments in Cockroach

Head: 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.

Mouth Parts: 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).

Thorax: 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.

Abdomen: 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.


Alimentary Canal: 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 plate 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. 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.

Blood Vascular System: The blood vascular system is an open type. Blood vessels are poorly developed. They open into space (haemocoel). Visceral organs located in the haemocoel are bathed in blood (haemolymph). The haemolymph is composed of colourless plasma and haemocytes. The heart consists of elongated muscular tubes which lie along mid-dorsal line of thorax and abdomen. The heart is differentiated into funnel-shaped chambers; with ostia on either side. The blood from sinuses enters the heart through ostia and is pumped anteriorly to the sinuses again.

Respiratory System: The respiratory system consists of a network of trachea. The tracheae open through 10 pairs of small holes; called spiracles. The spiracles are present on the lateral side of the body. Tracheal tubes are subdivided into tracheoles. They carry oxygen to all the parts. The opening of the spiracles is regulated by sphincters. Exchange of gases takes place by diffusion.

Excretory System: Malpighian tubules are the excretory organs. Each tubule is lined by glandular and ciliated cells. They absorb nitrogenous wastes and convert them into uric acid. Uric acid is excreted out through the hindgut. Additionally, fat body, nephrocytes and urecose glands also help in excretion.

Nervous System: The nervous system of cockroach consists of a series of fused, segmentally arranged ganglia. The ganglia are joined by paired longitudinal connectives on the ventral side. Three ganglia lie in the thorax and six in the abdomen. The nervous system in cockroach is spread throughout the body. In the head region, the brain is represented by supra-oesophageal ganglion. It supplies nerves to antennae and compound eyes.

Sense Organs: Antennae, eyes, maxillary palps, labial palps, anal cerci, etc. are the sense organs in cockroach. The compound eyes are situated at the dorsal surface of head. Each eye consists of about 2000 hexagonal ommatidia. Presence of several ommatidia gives mosaic vision to the cockroach. This gives more sensitivity but less resolution. This type of vision is common during night.

Reproductive System:

Cockroaches are dioecious.

Male Reproductive System: The male reproductive system of cockroach consists of a pair of testes. The testes lie on each lateral side in the 4th – 6th abdominal segments. A thin vas deferens arises from each testis. It opens into ejaculatory duct through seminal vesicle. The ejaculatory duct opens into male gonopore which is situated ventral to anus. A typical mushroom-shaped gland is present in the 6th-7th abdominal segments. It is an accessory reproductive gland. Male gonapophysis or phallomeres represent the external genitalia. These are made up of chitin. They are asymmetrical structures and surround the male gonopore. The sperms are stored in the seminal vesicles. The sperms are glued together in the form of bundles called spermatophores. Spermatophores are discharged during copulation.

Female Reproductive System: The female reproductive system of cockroach consists of two large ovaries. The ovaries lie laterally in the 2nd – 6th abdominal segments. Each ovary is formed of a group of eight ovarian tubules or ovarioles. They contain a chain of developing ova. Oviducts from each ovary unite into a single median oviduct. This is also called vagina and it opens into the genital chamber. A pair of spermatheca is present in the 6th segment which opens into the genital chamber.

Fertilization: Sperms are transferred through spermatophores. The fertilized eggs are encased in capsules; called ootheca. An ootheca is a dark reddish to blackish brown capsule. It is about 8 mm long. The oothecae are dropped or glued to a suitable surface; usually at a place with high relative humidity or near a food source. On an average, 9 – 10 ootehcae are produced by a female. Each ootheca contains 14 – 16 eggs. Development is indirect and is paurometabolous. Development through nymph stage is called paurometabolous. The nymph resembles the adults. The nymph grows by moulting about 13 times to reach the adult form. Wing pads are seen in the penultimate stage of development but wings are present only in adults.

Significance for Human: Most of the species are wild and have no economic importance. Some species live in and around human habitat. They destroy food and contaminate food with their excreta. Many bacterial diseases can be transmitted by food contamination by cockroaches.

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