NEET 2020 Study Notes: Structural Organization in Animals: Cockroach

By Noushin Chaudhary|Updated : July 15th, 2019

In this article, we are providing short notes on the Structural Organization in Animals: Cockroach which is an important chapter for NEET 2020. This is an important section to pay attention from the Unit Structural Organization in Plants and Animals as every year 2-3 questions are asked from this chapter. This chapter is completely theoretical and needs a lot of revision. In this article, Morphology and Anatomy of Cockroach is discussed in brief important for last days revision. Let's begin with a brief introduction about the Cockroach. 

STRUCTURAL ORGANIZATION IN ANIMALS (COCKROACH)

Cockroaches, scientifically known as Periplaneta americana, belong to class Insecta and phylum Arthropoda. They have been reported to inhabit the earth for the past 300 million years and are still present on earth. Cockroaches are nocturnal and cursorial inhabit. Approximately 2600 species of cockroach from all over the world are known till now.

External Morphology

The body of cockroach is reddish-brown in color. They have an elongated and dorsoventrally flattened body. Their size differs from male to female. Generally, males have body measuring up to 35 to 40 mm in length and 10 to 12 mm in breadth. Females have a comparatively smaller body build up. A hard and brownish chitinous exoskeleton covers the entire exoskeleton. The segments of exoskeleton have hardened plates termed as sclerites. Each segment is connected to one another by articular membranes. Articular membranes are thin and flexible. It has a compound eye that consists of about 2000 visual units or ommatidia present at the dorsal side of the head.

The body of cockroach is segmented and can be divided into three different regions; head, thorax, and abdomen. The head is triangular in shape and located anteriorly at the right angle to their longitudinal body axis. They have a very flexible neck enabling their head to move in all directions. Six different segments fuse to their head and it has an exoskeletal cover called as the head capsule. Membranous sockets present in front of the eyes give rise to a pair of antennae.

Antennae are long thread-like structures that are sensitive and can move in all directions. Mouth is present at the anterior end of the head and is provided with appendages that are jointly called as mouthparts. Mouthpart comprises of a pair of mandibles and maxillae, labium forms the lower lip and labrum forms the upper lip. A median flexible lobe termed as hypopharynx is present within a cavity the mouthparts enclose and it acts like a tongue. The male and female genital aperture is present on the 9th, 10th sternum, and 8th sternum, respectively.
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Fig 1: Head region (front and back view) of the cockroach with mouthparts

The thorax comprises of three segments; prothorax, mesothorax, and metathorax. The neck, a short extension of the prothorax connects the head with the thorax. A pair of walking legs is present in each thoracic segment. Mesothorax gives rise to the first pair of wings (tegmina) and metathorax gives rise to the second pair of wings. Tegmina are dark in color and cover the hind transparent and membranous hind wings. Hind wings are used in flight.

The abdomen comprises ten segments in both male and female. The seventh, eighth, and ninth sterna form the genital pouch in females whereas, in males, it is present at the hind end of the abdomen. A thread like anal style is present in male cockroach only. Anal cerci, a filamentous structure is present on the tenth segment of both male and female cockroach.

Digestive system

The alimentary canal can be divided into three segments, foregut, midgut, and hindgut. Digestive system starts with the mouth giving way to pharynx that leads to the esophagus, a narrow passage. Esophagus opens into the crop, a sac-like structure for storage of food.

Gizzard, also known as the proventriculus, follows the crop and it helps in grinding of food particles. Gizzard possesses six chitinous plates termed as teeth. The entire foregut is lined by the cuticle. A ring of tubules, called as the gastric caecae, is present at the junction of the foregut and midgut and is responsible for secretion of digestive juice. Malpighian tubules, a ring of 100-150 thin, yellow colored filaments, is located at the junction of midgut and hindgut. Hindgut opens through the anus.

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Fig 2: Digestive system of cockroach

 

Circulatory System

Blood vessels are poorly developed in cockroaches and hence an open blood vascular system is presenting them. The visceral organs are present in the hemocoel, an open space. Blood of cockroach is also called as hemolymph. It consists of colourless plasma and hemocytes. Elongated tubes with muscular wall regulate blood in the hemocoel. The elongated tube has many funnel-shaped chambers and is present mid-dorsally in the thorax and the abdomen, and hence is called the heart of the cockroach. The blood vascular system of cockroach comprises of the heart, anterior aorta (blood vessel), and a system of blood spaces.


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Fig 3: Circulatory system of cockroach

Respiratory System

The respiratory system of cockroach has a network of the trachea that opens through ten pairs of spiracles present on the lateral side of their body. Sphincters regulate the spiracles and gaseous exchange takes place by diffusion. The gaseous exchange takes place by means of diffusion through tracheoles.

Excretory System

Excretion in a cockroach is performed by malpighian tubules. Each tubule is lined with ciliated and glandular cells that absorb nitrogenous waste products. The waste products are then converted into uric acid and then are excreted out via the hindgut. As such, the cockroach is called uricotelic. Moreover, the fat body, nephrocytes, and the uricose glands aid in excretion.

Nervous system

The nervous system of cockroach consists of fused ganglia arranged segmentally and is present throughout the body. The thorax region possesses three ganglia while the abdomen possesses six ganglia. The head region of cockroach houses a little bit of nervous system with the majority of it situated on the body’s ventral side. The supra-oesophageal ganglion gives way the nerves to the antennae and compound eyes. The sense organs of cockroach include the antennae, eyes, maxillary palps, labial palps, and the anal cerci.

Reproductive system

The male reproductive organs consist of a pair of trilobed testes that lie on each lateral side of the fourth and the fifth abdominal segments. Vas deferens, a thin tube arises from each tube and opens into the ejaculating duct via the seminal vesicle. The ejaculating duct opens into male gonopore located ventrally to the anus. The accessory glands of male consist of a mushroom-shaped structure located in the sixth and seventh abdominal segments.

External genitalia is characterized by an asymmetrical, chitinous structure that surrounds the male gonopore, present at the end of the abdomen, and termed as male gonapophysis or phallomere. Sperms are glued together in bundles known as the spermatophores and stored in seminal vesicles. The wall of spermatophore consists of three layers. Utriculi majors secrete the inner layer, ejaculatory duct secretes the middle layer, and the phallic gland secretes the outer layer.  

Two large ovaries located laterally in the fourth, fifth, and sixth segments make up the female reproductive organs. Each ovary consists of a group of eight ovarian tubules that contain a chain of developing ova. The oviducts of each ovary unite into one median oviduct, termed as a vagina that opens into the genital chamber. The accessory glands of female consist of a pair of branched, collateral glands that open into the dorsal side of the genital chamber.

The sixth segment houses a pair of spermatheca that opens into the genital chamber on small papillae. The external genitalia of female cockroach includes an ovipositor made by two gonapophyses. The ovipositor functions in the conduction of fertilized eggs to the oothecal chamber.

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Fig 4: Reproductive organs of a) male and b) female cockroach.

 

Fertilization and Development

Fertilization in cockroach occurs in the vestibulum of the female genital pouch. The secretions of the collaterial glands are poured over the vestibulum’s inner surface area and thus form the ootheca. A single ootheca house sixteen eggs arranged in a double row, supported by the ovipositor valves.

 

The sperms fertilize the eggs when they pass from the vagina into the vestibulum for annexation in the ootheca. The secretions from the collaterial glands strengthen the ootheca. Initially, ootheca is white in colour, later turning into pink. It is oblong in shape and possesses a keel on the outer dorsal fringe. Female cockroach carries the ootheca for numerous days as a protrusion from the tip of the abdomen. It is generally laid in a sheltered, warm, and dark place.

The laid ootheca contains fertilized eggs that are slightly curved and contain a diploid nucleus and yolk. The yolk serves as food for the developing embryo. Each egg first goes through cleavage consequently giving rise to a blastula, followed by the formation of a gastrula. The gastrula gets differentiated into an early embryo that gets parted from the blastoderm via an amniotic cavity. The ectoderm, mesoderm, and endoderm (the three germ layers of the embryo) form various parts of the nymph.

The dorsal keel of ootheca splits during hatching and the nymphs emerge out from the egg membranes present within the ootheca. Nymphs that are freshly hatched are delicate, colourless creatures with black eyes. They have practically all characters of an adult, however, differ in colouration, size, lack wings, and are sexually immature.

After hatching, the nymphs undertake ecdysis or first moulting, followed by successive five or six moults. With nymphal development, there is an increase in body size, development of wing pads, dark colouration, and consequently takes its final form with fully developed wings and genitalia.

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Fig 5: Life cycle of cockroach

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