BIOLOGY OF THE COCKROACH

The Cockroach is an insect pest common in cellars and kitchens, and is often called the Black Beetle, although it is dark brown in colour and is not a beetle. It is not a native of Britain, but is thought to have been introduced into this country through trade in the sixteenth century. Its original home is not known, although South Russia has been suggested. It spends the day-time hiding in nooks and crevices, and comes out to feed at night. It will eat practically anything except bricks and concrete, and makes itself a nuisance in houses by eating and spoiling food. There are two kinds of Cockroach common in houses in the British Isles, namely the common Cockroach, Blatta orientalis, and the American Cockroach, Periplaneta americana. The former is the more numerous, and can be distinguished from the American Cockroach by its darker colour and the fact that the female has no wings.

Cockroaches, locusts, grasshoppers, stick insects and leaf insects belong to an order of the Insects known as the Orthoptera.

Structure

The body of a mature cockroach is about 1 in. long and is divided into three regions : the head, thorax and abdomen. The head is joined by a narrow neck to the thorax, which consists of three segments and bears the legs and wings. The abdomen is longer than the thorax and is made up of ten segments. The whole body is covered by a dark brown skeleton, consisting of a substance called chitin, which is thick, tough and horny, except at the joints, where it is thin, soft and flexible. Each segment of the body is covered by an upper plate called the tergum, and a lower plate, the sternum.

The Head

The head is oval in shape and is attached to the body by the short and slender neck. On each side of the head is a large kidney-shaped compound eye.. Projecting beneath each eye is a long whip-like feeler or antenna, which is a many-jointed structure bearing organs sensitive to touch, smell and vibration. The cockroach is constantly feeling the ground with its antennas, and it also waves them above its head in order to test the air.

Mouth-parts

From the lower front edge of the head a flap called the labrum or ’upper lip ’hangs in front of the mouth, and at the sides of the mouth there are three pairs of jaws which work from side to side.

The first pair of jaws are called the mandibles. Each has a strongly toothed inner edge used for biting. Behind the mandibles lie the second pair of jaws called the first maxillae:. Each first maxilla is a jointed structure, and has the shape of a capital Y attached at its base, so that there is an inner and outer fork. The base of a maxilla consists of two joints, and the outer fork of the Y is a five-jointed feeler called the maxillary palp. The inner fork consists of two structures which are a hard, pointed blade and a soft, blunt flap, acting as a sheath to the lacinia. These blades work against each other and break up the food masticated by the mandibles into even finer particles. The third pair of jaws or second maxilla; are constructed on the Y-like plan like the first maxillae, but the base of each second maxilla is fused with the other to form a plate called the labium or ’lower lip, ’ which lies behind the mouth. The outer fork of the Y is a three-jointed feeler, the labial palp, while the inner fork is divided into an inner and outer part, as in the case of the first maxilla.

Thorax

In the cockroach, as in all insects, the thorax consists of three segments, called the prothorax, mesothorax and metathorax. The tergum of the prothorax is larger than that of the other two segments, and overlaps so as to hide the neck in front and part of the mesothorax behind. The mesothorax and metathorax each bear a pair of wings. The first pair of wings are thick chitinous plates, which extend backwards so as to cover two-thirds of the abdomen. They are called elytra or wing cases, and protect the second pair of wings when these are not in use. When the insect is flying, the elytra are held out stiffly at right angles to the body. The second pair of wings are delicate folds of chitin, strengthened by thicker ridges or veins, and when at rest are folded fan-wise beneath the elytra. In the female of Blatta oricntalis these wings are vestigial structures. A pair of legs are attached to the under surface of each thoracic segment. Each leg consists of a number of joints, and ends in a pair of claws between which is a soft pad.

Abdomen

Only eight of the ten segments of the abdomen are visible, since the 8th and 9th are overlapped by the tergum of the 7th segment. The last tergum is grooved along its posterior edge, and the anus opens beneath it. At the sides of this segment are a pair of jointed, spindle-shaped structures, the cerci, which are supposed to be sensory in function. In the male the 9th segment bears a pair of short styles, and in the female the 7th segment has a boat-shaped sternum which projects backwards so as to form a pouch in which the egg case is formed.

The Internal Structure of the Cockroach

The Blood System

If the terga are carefully cut away from the back along the thorax and abdomen of the cockroach, a mass of loose white tissue is seen. This is the fat body which largely fills the space between the alimentary canal and the body wall. This space is not a coelom and so cannot be compared with the body cavity of the worm or a vertebrate, but is a blood space in which the organs lie bathed in blood. Lying just below the terga and extending along the middle of the thorax and abdomen is the narrow contractile heart , which by its action forces blood to circulate through the sinuses. Such a blood system is said to be an open system, whereas the arrangement of narrow blood vessels possessed by the worm and by vertebrates is a closed system. The blood is colourless and contains small amoeboid cells.

Heart

The Alimentary Canal

The alimentary canal lies beneath the fat body and is partly enveloped in it. The mouth opens into a narrow pa* museles^ tube, the oesophagus, which is perioardium followed by a large pear-shaped crop and the gizzard, which has thick walls and six horny teeth which grind up the food. Behind the teeth are a number of bristles which strain the food before it passes into the next part of the alimentary canal, which is a short narrow tube termed the mesenteron or mid-gut. The mid gut is followed by a longer tube, the colon, the end of which is enlarged to form the short rectum. The rectum opens to the exterior by the anus at the hind end of the abdomen. The most notable feature is that only the mid-gut is lined by endoderm and conveys digested food through its walls to the surrounding blood. All the other parts are lined by ectoderm. A pair of salivary glands lying beside the crop produce saliva which is poured on the food prior to its ingestion.

Excretory System

At the beginning of the colon the numerous fine, thread-like Malpighian tubules are attached. These, interlaced with the lobes of the fat body, form the excretory system.

Respiratory System

The respiratory system consists of a series of air tubes, the larger ones opening to the surface at pores along the sides of the body. Their branches divide into 2 tracheoles which penetrate every tissue. All parts are prevented from collapsing by a spiral thickening of chitin.

Nervous System

The nervous system of the cockroach is constructed on a similar plan to that of a worm. In the head there are two large supra-oesophageal ganglia, from which nerves pass to the eyes and antenna;. A ventral nerve cord runs along the body wall beneath the alimentary canal, and consists of a sub-cesophageal ganglion and a chain of nine ganglia united by double connectives. The first ganglia are joined to the ventral nerve cord by a pair of commissures which pass round the oesophagus.

Reproduction of the Cockroach

The male cockroach has a pair of testes which lie in the abdomen hidden by the fat body. They are only-functional in the young cockroach, and afterwards gradually disappear.

In the female there are two ovaries, each of which consists of eight tubes which have a beaded appearance due to the enclosed eggs. After fertilization the eggs are enclosed in a chitinous egg capsule which is formed from the secretion of certain glands in the abdomen of the female. The capsule contains sixteen eggs and is carried about by the female, protruding from the end of the abdomen for several days until she finds a suitable hiding-place for it. After about twelve months the young cockroaches hatch out from the capsule, and resemble the adults except for the absence of wings. The young cockroach takes four years to become an adult, during which time it casts its skin seven times.

Practical Work on the Cockroach

Examine a living cockroach. Kill it with chloroform and then make out the various parts of the body.

Examine prepared slides of the mouth-parts of the cockroach.

Pin a cockroach down with its dorsal surface uppermost and remove the wings. Then cut through the terga close to each side of the body and remove them. Look for the heart lying on the fat body and then unravel the alimentary canal and its associated organs from the fat body. Look for the nerve chain lying along the ventral wall of the body and examine the head for the supra-cesophageal ganglia.

Examine with the microscope a small piece of a trachea mounted in water.

Examine with the microscope a prepared section of the eye of an insect.

Examine the egg case of a cockroach.

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