Cockroaches belong to the class Insecta under the phylum arthropoda. They belong to the genus Periplaneta. They are found living in damp and warm places like sewage, pipes, kitchen, bakeries, restaurants, warehouses, grocer's shop, etc. They are nocturnal and cursorial in habit. They possess three pairs of jointed appendages. The species available in India is known as Periplaneta americana. About 2600 species of cockroach are known from all over the world.
The body of cockroach is elongated and dorsoventrally flattened and reddish-brown in colour. Males usually measure 35-40 mm in length and about 10-12 mm in width but the females are slightly smaller. The entire body is covered by a hard and brown chitinous exoskeleton. Each segment of exoskeleton has hardened plates called sclerites that are joined to each other by thin and flexible articular membranes.
The body is segmented and divisible into three distinct regions - head, thorax and abdomen. The head is triangular and lies anteriorly at right angles to the longitudinal body axis. The head is formed by the fusion of six segments and is highly mobile in all directions due to a flexible neck. The exoskeletal covering of the head is called head capsule. The head bears a pair of sessile (without stalk) compound eyes, one on each of its sides. A pair of thread-like long antennae (sing, antenna) arise from membranous sockets lying in front of the eyes; they can be moved in all directions and are very sensitive. Anterior end of the head bears the mouth, which is provided with appendages, collectively called mouth parts, which are used in chewing, cutting and swallowing. The mouthparts consist of a pair of mandibles and maxillae, labium forming lower lip and a labrum forming the upper lip. Within the cavity enclosed by mouthparts there is a median flexible lobe called hypopharynx, which acts like a tongue.
Fig: Head Region of Cockroach with Mouth Parts (a) Front View (b) Back ViewThe thorax consists of 3 segments - the prothorax, mesothorax and metathorax. The head is connected with the thorax by a short extension of the prothorax, the neck. A large sclerite covers the prothorax and shields the mesothorax. The mesothorax and metathorax have independent coverings called sclerite. Each thoracic segment bears a pair of walking legs. Each walking leg consists of five segments:
- a short and broad coxa,
- a triangular short and rod-like trochanter, articulated with
- a long spout, spiny femur,
- a spring tibia which represents the largest segment, and
- a long tarsus.
The abdomen in both males and females consists of 10 segments. In females, sclerites of the 8th and 9th segments are overlapped by corresponding sclerites of the 7th segment. The seventh sternum is boat-shaped and together with eighth and ninth sterna forms a brood or genital pouch. In males, only 8th tergum is overlapped by the 7th segment. The tenth segment bears a pair of jointed filamentous structures called anal cerci (sing.cercus). Ventral to these in the male, the 9th segment bears a pair of short, thread-like anal styles, which are absent in females.
The body remains covered by a cuticle, which is impermeable to water. Numerous fine tubules originating from lower epidermis traverse the cuticle. The alimentary canal is present in the body cavity. It is divided into three regions: fore-gut, mid-gut and hind-gut. The mouth opens into a short tubular pharynx, which passes and bends into a narrow, tubular passage called oesophagus. Oesophagus opens into a sac-like structure, crop. The crop is followed by a gizzard or proventriculus. It has an outer thick layer of circular muscles and a thick, inner cuticle forming six highly chitinous plates called teeth. Gizzard is designed in such a manner that it helps in grinding the hard food particles. The entire foregut is lined by cuticle. The mid-gut or mesenteron is a narrow tube of uniform diameter without inner lining of cuticle. A ring of blind caeca (eight in number) called hepatic caeca, which secrete digestive juice, is present at the junction of fore and mid-gut. At the junction of mid-gut and hind-gut there is a ring of about 150 thin yellowish Malpighian tubules, which help in excretion. The hind-gut is silghtly broader than the mid-gut and is lined internally by cuticle. It opens by an anus, lying posteriorly below the 10th tergum. The digestive juices are produced not only by the wall of mesenteron and hepatic caeca but also by a pair of large salivary glands, which open into the pharynx.
Fig: Alimentary Canal of Cockroach
The blood vascular system of cockroach is of open type. It means the blood vessels are poorly developed and open into spaces rather than the capillaries. All the visceral organs are bathed in blood (also called haemolymph). Haemolymph is composed of colourless plasma and many corpuscles called haemocytes. The respiratory pigments are absent in the blood. To regulate blood flow in the haemocoel, an elongated tube with muscular wall is lying mid dorsally in the thorax and abdomen consisting of 13 funnel-shaped chambers, which are arranged segmentally. Each chamber represents one heart. At the lateral sides of each chamber two pores, ostia (sing, ostium), are present one on each side. Ostium is guided by a valve to allow blood flow only in one direction i.e. from haemocoel to the inner chamber of the heart.
Fig: Respiratory System of Insects
The respiratory system consists of a network of trachea, the opening of which is called spiracle. Air enters through spiracles, which are ten pairs in number; they are arranged segmentally on lateral sides. Two pairs lie in the meso and metathorax and eight pairs in the abdomen. The opening of the spiracles is regulated by sphincters. The tracheal tubes are sub-divided into tracheoles, which branch within the tissues. Air reaches upto tracheoles, and the body fluid in the tracheoles exchanges dissolves oxygen and carbon dioxide with the cells. The exchange of gases takes place by diffusion.
In cockroach, excretion is performed by Malpighian tubules. Each tubule is lined by glandular, ciliated cells with brush-border. They absorb nitrogenous waste product and convert it into uric acid through various biochemical processes. The uric acid is excreted through the hind-gut. Hence this insect is called Urecotelic. In addition, fat bodies, nephrocytes, cuticle and urecose glands also help in excretion.
The main nervous system consists of a series of fused, segmentally arranged ganglia joined by paired longitudinal connectives on the ventral side. Three ganglia lie in thorax and six in the abdomen. In the head region the brain is represented by supra-oesophageal ganglion, which supplies nerves to antenna and compound eyes. It joins sub-oesophageal ganglion by two circum-oesophageal commissures. These constitute a characteristic nerve ring around the oesophagus. In cockroach sense organs are antenna, eyes, maxillary palps, labial palps and anal cirri etc. The antennae, palps and cirri are very sensitive tactile sense organs, which help in detection of food and various objects. The compound eye is situated at the dorsal surface of the head in the form of black kidney-shaped patches. Each eye consists of about 2000 hexagonal facets or ommatidia (sing. ommatidium), which are covered externally by transparent cuticle called cornea. Ommatidium is composed of corneal lens, a pair of corneagen cells, refractive crystalline cone distal pigment cells around the pigment cone and retinula forming rhabdome, basal pigment cell, basement membrane and optic nerve. With the help of several ommatidia a cockroach can receive several images of an object. This kind of vision is known as mosaic vision. This is how the compound eye of the cockroach is adapted for detecting movement more efficiently than an eye of a vertebrate.
Male reproductive organs comprise a pair of small trilobed testes lying one on each lateral side in the 4th - 5th abdominal segments. From each testis arises a thin tube called vas deferens, which opens into ejaculating duct through the seminal vesicle. The ejaculatory duct opens into male gonopore situated ventral to anus. A characteristic mushroom-shaped gland is present in the 6th - 7th abdominal segments, which functions as an accessory reproductive gland. The external genitalia are represented by chitinous, asymmetrical structures, surrounding the male gonopore, at the end of the abdomen, called male gonapophysis or phallomere. The sperms are stored in seminal vesicles and are glued together in the form of bundles called spermatophores, which are discharged during copulation. Female organs comprise two large ovaries lying laterally in the 4th, 5th and 6th segments. Each ovary is formed of a group of 8 ovarian tubules or ovarioles, containing a chain of developing ova. Oviducts of each ovary unite into a single, median oviduct, which is also called vagina, which opens into the genital chamber. The female accessory glands are a pair of branched, collateral glands, which open into the dorsal side of the genital chamber. A pair of spermatheca is present in the 6th segment, which opens into the genital chamber on a small papillae.
Fig: Reproductive Organs of Cockroach (a) Male (b) Female
Interaction with Mankind
Cockroach causes damage to the household material including clothes, shoes, purses, etc. It also eats and destroys foodstuffs. The food contaminated by it gives typical smell, making it unpalatable. While living in sewage pipes and gutter holes, these also carry harmful germs of diseases like cholera, diarrhoea, tuberculosis, typhoid, etc. It is also used as a safe experimental animal for laboratory exercises and biological research work since it can be obtained easily and without causing much disturbance to the ecosystem.