Cockroach

7.5 Cockroach — Periplaneta americana

Cockroaches are brown or black bodied animals that are included in class Insecta of Phylum Arthropoda. Their bright yellow, red and green coloured tropical species are also known. The size of adults varies from \(\frac{1}{4}\) inch to over 3 inches, and have long antenna, legs and flat extension of the upper body wall that conceals head. They are nocturnal omnivores that live in damp places throughout the world. They have become residents of human homes; thus are serious pests and vectors of several diseases.

7.5.1 Morphology

The adults of the common species of cockroach, Periplaneta americana, are about 34-53 mm long with wings that extend beyond the tip of the abdomen in males. The body of the cockroach is segmented and divisible into three distinct regions — head, thorax and abdomen. The entire body is covered by a hard chitinous exoskeleton (brown in colour). In each segment, exoskeleton has hardened plates called sclerites (tergites dorsally and sternites ventrally) that are joined to each other by a thin and flexible articular membrane (arthrodial membrane).

Head: The head is triangular in shape and lies at right angles to the longitudinal body axis. It is formed by the fusion of six segments and shows great mobility in all directions due to flexible neck. The head capsule bears a pair of compound eyes, a pair of thread like antennae arising from membranous sockets lying in front of eyes. The antennae have sensory receptors that help in monitoring the environment. Anterior end of the head bears appendages forming biting and chewing type of mouth parts. The mouthparts consist of a labrum (upper lip), a pair of mandibles, a pair of maxillae and a labium (lower lip). A median flexible lobe, acting as tongue (hypopharynx), lies within the cavity surrounded by mouthparts.

Thorax: The thorax consists of three parts — prothorax, mesothorax and metathorax. The head is connected with thorax by a short extension of the prothorax known as the neck. Each thoracic segment bears a pair of walking legs. The first pair of wings arises from mesothorax and the second pair from metathorax. Forewings (mesothoracic) called tegmina are opaque dark and leathery and cover the hind wings when at rest. The hind wings (metathoracic) are transparent, membranous and are used in flight.

Abdomen: The abdomen in both males and females consists of 10 segments. In females, the 7th sternum is boat shaped and together with the 8th and 9th sterna forms a genital pouch (gynovum), whose anterior part contains female gonopore, spermathecal pores and collateral glands. In males, genital pouch or chamber lies at the hind end of abdomen bounded dorsally by 9th and 10th terga and ventrally by the 9th sternum. It contains dorsal anus, ventral male genital pore and gonapophysis. Males bear a pair of short, thread like anal styles which are absent in females. In both sexes, the 10th segment bears a pair of jointed filamentous structures called anal cerci.

7.5.2 Digestive System

The alimentary canal present in the body cavity is divided into three regions: foregut, midgut and hindgut. The mouth opens into a short tubular pharynx, leading to a narrow tubular passage called oesophagus. This in turn opens into a sac like structure called crop used for storing of food. The crop is followed by gizzard or proventriculus. It has an outer layer of thick circular muscles and thick inner cuticle forming six highly chitinous plate called teeth. Gizzard helps in grinding the food particles. The entire foregut is lined by cuticle.

A ring of 6-8 blind tubules called hepatic or gastric caecae is present at the junction of foregut and midgut, which secrete digestive juice. At the junction of midgut and hindgut are present another set of 100-150 yellow coloured thin filamentous Malpighian tubules. They help in removal of excretory products from haemolymph. The hindgut is broader than midgut and is differentiated into ileum, colon and rectum. The rectum opens out through anus.

7.5.3 Circulatory System

The circulatory system of cockroach is of open type. Blood vessels are poorly developed and open into space (haemocoel). Visceral organs located in the haemocoel are bathed in blood (haemolymph). The haemolymph is composed of colourless plasma and haemocytes. Heart of cockroach consists of elongated muscular tube lying along mid dorsal line of thorax and abdomen. It is differentiated into funnel shaped chambers with ostia on either side. Blood from sinuses enter heart through ostia and is pumped anteriorly to sinuses again.

7.5.4 Respiratory System

The respiratory system consists of a network of tracheae, that open through 10 pairs of small holes called spiracles present on the lateral side of the body. Thin branching tubes (tracheal tubes subdivided into tracheoles) carry oxygen from the air to all the parts. The opening of the spiracles is regulated by the sphincters. Exchange of gases takes place at the tracheoles by diffusion.

7.5.5 Excretory System

Excretion is performed by Malpighian tubules. Each tubule is lined by glandular and ciliated cells. They absorb nitrogenous waste products and convert them into uric acid which is excreted out through the hindgut. Therefore, this insect is called uricotelic. In addition, the fat body, nephrocytes and urecose glands also help in excretion.

7.5.6 Nervous System

The nervous system of cockroach consists of a series of fused, segmentally arranged ganglia 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, the head holds a bit of a nervous system while the rest is situated along the ventral (lower) part of its body. In the head region, the brain is represented by supra-oesophageal ganglion which supplies nerves to antennae and compound eyes.

In cockroach, the sense organs are antennae, eyes, maxillary palps, labial palps, anal cerci, etc. The compound eyes are situated at the dorsal surface of the head. Each eye consists of about 2000 hexagonal ommatidia. With the help of several ommatidia, the cockroach can receive several images of an object. This kind of vision is known as mosaic vision with more sensitivity but less resolution; hence it is common during night (nocturnal vision).

7.5.7 Reproductive System

Cockroaches are dioecious and both sexes have well developed reproductive organs.

The male reproductive system consists of a pair of testes lying one on each lateral side in the 4th-6th abdominal segments. From each testis arises a thin vas deferens, which opens into ejaculatory duct through seminal vesicle. The ejaculatory duct opens to exterior through male genital pore 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 male gonapophysis or phallomere (chitinous asymmetrical structures, surrounding the male genital pore). The sperms are stored in the seminal vesicles and are glued together in the form of bundles called spermatophores which are discharged during copulation.

The female reproductive system consists of two large ovaries, lying laterally in the 2nd-6th abdominal segments. Each ovary is formed of a group of eight ovarian tubules or ovarioles, containing a chain of developing ova. Oviducts of each ovary unite into a single median oviduct (also called vagina) which opens into the genital chamber. A pair of spermatheca is present in the 6th segment which opens into the genital chamber.

Fertilisation takes place in the genital chamber. The fertilised eggs are encased in capsules called oothecae. Ootheca is a dark reddish to blackish brown capsule, about 8 mm in length. They are dropped or glued to a suitable surface, usually in a crack or crevice of high relative humidity, near a source of food. The average number of oothecae produced by a female is 9-10 and each ootheca contains 14-16 eggs. The hatched nymphs (young ones of cockroach) are pale white, and after one hour, become darker, looking like adult except absence of wings.

The development of Periplaneta is paurometabolous, meaning there is development through nymphal stage. The nymphs grow by moulting about 13 times to reach the adult form. The next to last nymphal stage has wing pads but only adult cockroaches have wings. This kind of development with gradual change is called paurometabolous development.

7.6 Worked Examples