Insect Biology

Insect Biology


Basic External Insect Anatomy

Insects are all related, they share a common ancestor at the base of their family tree. From this ancestor all insects inherited a basic anatomy and body plan. The diversity in form, and ultimately function, found in insects is a result of changes made to basic anatomical elements such as legs or mouthparts. A consequence of this is that the basic anatomy of one insect is the same as the basic anatomy of all insects (there are exceptions when parts are occasionally lost or added). Below is a very brief overview of external insect anatomy. For a more exhaustive overview visit Invertebrate Anatomy OnLine .

Overall Body Plan

Insects have three major body regions: head, thorax, and abdomen (see Insect Body Regions, right).

The head is made of 5-7 fused segments and bears the eyes, antennae, and mouthparts.

The thorax consists of three segments called the pro-, meso-, and metathorax. Appendages used for movement are attached to the thorax. Each of the segments of the thorax bears one pair of legs and if wings are present they are found on the meso- and metathorax only. The top of the prothorax is called the pronotum.

An insect’s abdomen consists of 11 or fewer segments that generally do not bear any appendages, except for segments near the rear which may have appendages associated with reproduction.

Head and Mouthparts

The head can be divided into general regions (see General Insect Head Regions and Mouthparts, left): the top of the head is the vertex, the side or cheeks are gena, the front of the face is the frons, and below the frons is the clypeus. These regions may be highly modified or lost in some groups of insects. Adult insects may have two types of eyes, larger compound eyes that consist of many facets (ommatidia), and eyes that occur as a single facet, ocelli. The number and placement of ocelli can be important for identification.

The mouthparts of humans consist of five layers or horizons; upper lip, upper jaw, tongue, lower jaw, lower lip. Insect mouthparts also consist of five horizons and are made of appendages that have been modified for food handling (see General Mouthparts, right). The labrum is similar to an upper lip. It is not divided but may have a notch on the outer (distal) edge. Below the labrum are the mandibles which are paired structures generally made of strong material (heavily sclerotized) and used for cutting or grinding. The specific shape and various features found on the mandibles may be very important for understanding what and how an organism eats. The hypopharynx is an internal structure located below the mandibles and has a tongue-like function. Below the mandibles (externally) are paired appendages called the maxillae. Generally each maxilla bears an appendage, the maxillary palpus that is used for food handling and may contain taste or smell organs called sensillae. The bottom horizon of insect mouthparts is the labium which is made of two fused maxilla-like structures and bears labial palps.

All insect mouthparts are modifications of this basic plan. A mosquito’s proboscis contains all five mouthpart types, see the cross section in Mosquito Mouthparts, B., right. In cases of extreme modification some mouthparts may become fused, reduced, or lost. Mouthpart arrangement can be very important when studying the potential an insect has to vector a disease, access a portion of a plant, etc.

Insect Legs

Insect have three pairs of legs, one pair on each of the three segments of the thorax and are generally called the fore-, mid-, and hind legs. Any of the pairs of legs may be heavily modified and are important for locomotion, prey capture, mating, etc. Thankfully, just like mouthparts, all insect legs contain the same basic parts. From proximal (toward or against the body) to distal (away from the body) the parts of an insect leg are: coxa, trochanter, femur, tibia, and tarsus. The tarsus almost always has one or two claws at the type used to grasp the substrate. The figure Insect Legs, right, shows legs modified for numerous purposes: A, running; B, jumping; C, digging; D, grasping; E, catching; F, walking and digging; G, reduced leg used for walking and digging; H, male leg modified for grasping females during mating.

Basic Internal Anatomy

The internal anatomy of insects is amazingly complex. A good sized caterpillar has more muscles than a human. The internal anatomy of insects differs from vertebrates (including humans) in several major ways.

Digestive/excretory system: Insects have a complete digestive system just like vertebrates (tube from the mouth to the anus) but it differs in a very important way (see Digestive System, left). The insect digestive system has three major regions, foregut, midgut, and hindgut.[1] The foregut and the hindgut are lined with chitin, the same stuff that makes up much of the exoskeleton of the insect. When an insect molts (sheds it’s «skin», see below) it also sheds the internal lining of the fore- and hindguts. Loss of the gut contents is a problem if the insect relies on gut microorganisms (gut fauna) to help with digestion. The gut fauna often lives in the hind gut (termites, for example). Suddenly the gut fauna is lost and must be replenished with every molt.

Insects do not have kidneys. Instead, metabolic wastes are removed with the Malpighian tubules[2].

Respiratory (ventilation) system: Insects don’t have lungs. They obtain oxygen and dispel carbon dioxide through a series of tubes called tracheae (see Respiratory System, right). The tracheae are attached to openings on the body called spiracles. The number and placement of spiracles varies and smaller insects may not have any. Traditionally, the view has been held that respiration in insects is passive, but recent evidence has demonstrated that some insects actively expand and contract trachea to ventilate their bodies.

Circulatory system: Insects do not have blood, or blood vessels that are part of a closed circulatory system. Instead insects have an open circulatory system where a substance called hemolymph bathes the organs directly. Some insects have a long heart-like organ along the dorsal side of the internal organs that helps circulate the hemolymph through the body. It comprises a single sheath of tissue and a series of muscles, and in many insects includes a tubular portion that functions as a dorsal aorta. Hemolymph also circulates through the legs, wings, and antennae via a series of simple one-way valves.

Life Cycle

Three general lifecycles occur in insects, but some insects (e.g. aphids, blister beetles, telephone-pole beetles, etc.) may have additional steps or variations. Most insects have direct internal fertilization, like mammals. This means they do not need to return to water to mate, nor do they need to worry with spermatophores like the arachnids. Most insects lay eggs, although some retain the egg inside the body until it hatches and then give «birth». Immature insect growth occurs through shedding of the skin called molting. Immature phases between molting are called instars and the growth sequence is denoted first instar, second instar, etc. Most insects grow through a specific number of instars between hatching from an egg and becoming an adult, but some insects have an indeterminate number of instars that depend on environmental temperature and food availability. In some cases appendages that were lost can be re-grown in immatures. Once an insect molts to adulthood it cannot molt again (except in some cases, such as silverfish). Adults mate (or not, many insects are parthenogenic), lay eggs (or not) and the cycle starts again.


Ametabolous Life Cycle: egg, multiple instars, and adult. Immatures look very similar to adults, but tend to be smaller, and lack fully formed reproductive structures. The primitively wingless orders have this life cycle: Protura, Collembola, Diplura, Microcoryphia, and Thysanura.


Hemimetabolous and/or Paurometabolous: egg, multiple instars, adult. «Simple metamorphosis» is the common term used to describe this life cycle. The immatures tend to look like miniature versions of the adults, except in the immature the head is larger in proportion to the body, wings are not fully formed and appear as wing buds, and reproductive structures are not developed. The Grasshopper Life Cycle and Squash Bug Life Cycle (right) are good examples. Insect orders with this life cycle are grouped under the term Exopterygota because of visible wing buds on the immatures. Major orders of insects in this group include Orthoptera (grasshoppers and crickets), Blattodea (cockroaches), Isoptera (termites), Plecoptera (stoneflies), Thysanoptera (thrips), Hemiptera (true bugs), and Phthiraptera (lice).

Internal and external structure of insects

The most diverse class is insects, externalThe structure and internal organs of which are well researched. From other types of arthropods, insects are distinguished by dividing the body into three sections: the head, chest and abdomen. As a rule, the external structure of insects is studied using the example of a May beetle or grasshopper.


Insects lack an internal skeleton. Its role is performed by a solid, dense surface of the body — the cuticle. It performs a protective and supporting function, creates a kind of skeleton.

The exoskeleton is attached to the muscles, and itsThe surface is a barrier separating body cavities from the environment. The cuticle can be hard or soft, turning into a shell. In some cases, the cuticle on the head and chest is stiff, on the abdomen — soft.

The larvae, because of the rapid growth, have a flexible,stretching cuticle. They can shed several times, dropping the old shell. Some parts of the body of an insect can be additionally protected by plates and shields.

The outer structure of insects will begin to be studied from the head. At first glance it seems that the head is a single whole, but evolutionarily it was formed by merging 5 segments.

On the head there are antennae and three pairs of oralextremities. They are divided into upper, lower jaws and lower lip (a pair of fused jaws). Oral extremities in different insects differ and are divided into several species depending on the type of nutrition:

  • gnawing, for solid food, for example, as in predatory bugs;
  • piercing-sucking, if you need to pierce the food substrate, there are mosquitoes, bedbugs, cicadas;
  • tubular-sucking, if piercing is not required, as in butterflies;
  • gnawing and licking for liquid food in bees, wasps;
  • Mousseoid for feeding liquid and solid food in flies.

On the sides of the head are complex eyes, and between them from one to three simple eyes. Before the eyes — antennae, which are also divided into several types.


We continue to study the external structure of insects. Breasts of insects can be divided into three large segments, in which even smaller ones are distinguished. At the bottom of the chest are legs. The trochanter and the basin ensure mobility of the limb. The thigh is the largest and strongest part of the leg, equipped with powerful musculature.

Then follow the knee and shin, which is suppliedspurs and thorns. The paw itself is divided into several small segments, at the apex there are claws and suckers. Features of the external structure of insects depend on the species. Legs can also have specialization and are divided into types.


The external structure of insects is interesting to studydue to the diversity of species. The wings of butterflies and mosquitoes differ externally, but they have a similar structure. Most often the wings are two pairs, they are outgrowths located on the back. They consist of the finest plates, reinforced with rigid veins.

The external structure of the body of an insect depends onlifestyle. In connection with the performance of various functions, the wings have undergone a number of changes. In Diptera, the hindwings were transformed into hind wings, and in the wingwing — the anterior ones. In beetles, the front wings evolved into the elytra, the mantis and cockroaches became leathery, etc. For some insect species, the wings are absent in the same sex or completely in all individuals.


We finish studying the external structure of insectsbelly. This part consists of a set of identical segments, usually ten. On the 8th and 9th segments, sexual appendages and openings are located. In the abdomen are located almost all internal organs.

There are no extremities on the abdomen, but larvae canto place false legs. In the posterior segments are located the copulatory organ in males, the ovipositor in females and the anal opening. Table «External structure of an insect» will help to better understand the features of the structure of these representatives of the animal world.

Respiratory and circulatory system

The external and internal structure of insects dependsfrom the way of life that they lead. The respiratory system consists of trachea, they permeate the entire body. They are opened by spiracles, regulating the flow of air. In insects breathing air, the respiratory system is open. In water it is closed, spiracles absent. Larvae may have gills.

Air enters through the holes of the spiracles and penetrates the trachea, entangling the internal organs. Trachea end in branched tracheal cells and tracheoles, the tips of which penetrate into the cells.

Hemolymph does not participate in gas exchange, this role is performed by the trachea. Hemolymph is pumped by the heart located on the back. It looks like a muscular tube.

Hemolymph enters this tube through a hole andmoves in the direction from the abdomen to the head. At the other end, the hemolymph freely enters directly into the body cavity and flows around the internal organs, supplying them with the necessary substances.

Digestive and excretory systems

Let us continue our study of the external structure of insects andtheir internal organs. The digestive system begins with the oral cavity, where the ducts of the salivary glands enter. Saliva contains enzymes for the breakdown of food. Then follows the esophagus, goiter, stomach. The intestine is divided into three sections by means of two valves and ends with an anal opening. In some species the digestive system in the adult state is not developed. For example, the pods do not have jaws, the intestines are reduced. They live for several days and do not eat.

The organs of insect secretion are represented bymalpighian vessels and the hindgut. Malpighian vessels are tubes located between the middle and the back of the intestine. The products of vital activity are filtered out by the walls of the vessels and are discharged into the hindgut.

Endocrine and reproductive systems

The organs of the endocrine system secrete hormones in the hemolymph that regulate such physiological processes as metabolism, reproduction, behavior, etc.
Insects are dioecious animals. The reproductive system of males is represented by two testes, the vas deferens and the ejaculatory duct. The reproductive system of females is represented by the ovaries and oviduct.

During mating, the seminal fluid enters theThe female spermatheca is stored there. Mating can last up to several days, most species immediately part. During the laying, eggs are lubricated with sperm and fertilized. All body forces are spent on reproduction, therefore females either actively feed on or die.

Nervous system and sense organs

The nervous system of insects has a complex structure. It consists of neurons. In the nerve cell, you can identify the body, dendrites and axon. Through the dendrites, the cells receive signals, and through the axon exchange information.

The central nervous system is representedthe supra-oropharyngeal nerve and the ventral chain, which consist of ganglia. These organs control the activity of all organs and tissues. The peripheral system is the motor and sensory nerves that connect the central nervous system to organs and tissues. The vegetative system consists of separate ganglia that regulate the management of organs.

Through the sense organs, the nervous system receives information.

Vision is represented by faceted eyes, several simple eyes or larval eyes.

Hearing organs can be located on variousparts of the body. They are presented by vibro-receptors in the legs of terrestrial insects, which feel the vibration of the substrate. Sounds through water and air are perceived by phonoreceptors, and Diptera are heard with johnstones. The most complex organs of hearing are tympanal organs.

The organs of taste are located on the legs, abdomen and in the oral cavity. The organs of touch are located throughout the body. Organs of smell — on antennae.

The internal and external structure of insects canvary greatly from species to species. It depends on the lifestyle and type of nutrition. The table «External structure of an insect», which is placed in this article above, will help to systematize the received knowledge.

Essay on Cockroach

In this essay we will essay about Cockroach. After reading this essay you will learn about:- 1. Habit and Habitat of Cockroach 2. Geographical Distribution of Cockroach 3. External Structures 4. Integumentary System 5. Body Cavity 6. Alimentary System 7. Respiratory System 8. Circulatory System 9. Excretory System 10. Nervous System 11. Endocrine System 12. Reproductive System 13. Development.

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Essay Contents:

  1. Essay on the Habit and Habitat of Cockroach
  2. Essay on the Geographical Distribution of Cockroach
  3. Essay on the External Structures of Cockroach
  4. Essay on the Integumentary System of Cockroach
  5. Essay on the Body Cavity of Cockroach
  6. Essay on the Alimentary System of Cockroach
  7. Essay on the Respiratory System of Cockroach
  8. Essay on the Circulatory System of Cockroach
  9. Essay on the Excretory System of Cockroach
  10. Essay on the Nervous System of Cockroach
  11. Essay on the Endocrine System of Cockroach
  12. Essay on the Reproductive System of Cockroach
  13. Essay on the Development of Cockroach

1. Essay on the Habit and Habitat of Cockroach:

The cockroaches belong to the class Insecta or Hexapoda and under the order Dictyoptera. The word cockroach has prob­ably originated from the name of a Spanish fruit, “cucaracha”, having disagreeable taste. There are several types of cockroaches. The one which is described here is known as Periplaneta americana.

The word Periplaneta (Gk. Peri = about; planeta = wandering star or planet) indicates the worldwide distribution of this insect. This was possible through its travelling by ships. The other cockroaches (Fig. 18.49) are Blatella germanica, Leucophaea maderae, Blaberus craniifer, Ectobius pallidus and Ectobius panzeri.

Different species of cock­roaches differ in size and other characteris­tics but all of them exhibit certain common features. Well-developed wings are present in both sexes. In males, the wings extend beyond the abdomen. In females, keel-like ovipositor valves are present.

The cockroach, Periplaneta americana, is a common nocturnal, omnivorous household pest which also acts as a scavenger. It prefers dark warm corners of kitchens, godowns, underground drains and places where food and humid atmosphere are available.

Canni­balism is often seen amongst cockroaches. The ability to walk rapidly and the produc­tion of a pungent secretion from the abdomi­nal glands are regarded as the defensive mechanisms of this insect.

2. Essay on the Geographical Distribution of Cockroach:

The cockroaches are now distributed throughout the world. The cockroaches prob­ably originated in Southern Asia or Africa and became the dominant fossils of the Car­boniferous age (some authors have called it as “The age of cockroaches”).

3. Essay on the External Structures of Cockroach:

The body is dorso-ventrally flattened, elongated and reddish-brown in colour. The cockroach can easily creep inside the small cracks and crevices. The males are usually 35-40 mm in length and females are 29-37 mm.

The body is segmented (Fig. 18.50) and the segments are organised to form three distinct specialised parts, each of which is called a tagma. The three tagmata are— Head, Thorax and Abdomen. All the three tagmata are enclosed by exoskeletal cover­ings. Several structures occur in each tagma.

It is the anterior most part of the body and is more or less triangular in shape. It is formed by the fusion of six segments which have lost their external demarcations. The exoskeletal covering of the head is called the head capsule and the top of the head capsule is called vertex.

The head capsule includes:

(1) A pair of epicranial plates or occipital sclerites, covering dorsal and poste­rior parts;

(2) A single piece formed by the fusion of two exoskeletons, frons (which lies below the vertex), clypeus (ventral to the frons) and labrum, hangs in front of head and

(3) Two exoskeletal pieces, the genae, cover each side of the head. On each side, the gena is separated from the frons by a fronto-genal suture and the labrum is attached with the distal border of the clypeus.

Following struc­tures are present on the head region:

One pair of prominent sessile eyes are present, one on each side of the head and the two eyes occupy much of the anterolateral wall. Each eye is bean-shaped and compound in nature. The external surface is marked with several polygonal facets, each of which de­notes a single visual unit—ommatidium. The eyes are larger in males than in females.

A pair of thread-like elon­gated antennae is present in the antero-medial indentation of the eyes. Each antenna articu­lates on a ring-like sclerite at two points—one, which is rigid, is known as antennifer and the other is flexible, called surantennifer. The an­tennae can be moved freely in all directions.

The ocelli are paired circular areas one on each side of the eye, near the base of the antennae. Each area acts as a simple eye and is responsible for detecting light.

It is placed at the anterior end of the head. It is provided with several appendages and other associated structures, which are collectively known as mouth parts or trophi.

2. Mouth parts or trophi:

The mouth parts (Fig. 18.51) are composed of paired append­ages like mandibles, maxillae and the la­bium. The other structures which participate in the formation of mouth parts are the hypopharynx and labrum.

Each part of the trophi is discussed below:

These paired appendages are present one on each side of the mouth. There is a soft cuticular area in the base of the mandible, called the sub-molar region. The remaining part of the mandible is hard. Each mandible bears two surfaces, one on each outer basal angle. These are called condyles.

One condyle articulates with the clypeus of the head and the other one articulates with the gena. The inner border of the mandible is sharply serrated. The teeth of the left mandible lie dorsal to the teeth of the right mandible and both the mandibles work like a saw to cut the food into pieces.

These paired appendages are present one on each side of the mandible. Each maxilla is a many-jointed structure and contains following pieces from proximal to distal end—cardo, stipes, lacinia and galea. When the mouth parts are not working, the two galea completely enclose the laciniae.

The basal tip of cardo bears a condyle for articulating with the exoskeleton of the head. An inner groove separates the cardo from stipes. A membrane from the middle of Ate stipes extends up to the head. From the distal end of each stipe arise a many-jointed lateral maxillary palps. This appendage is used for holding the food and thus assists during ingestion.

This median single structure is, in fact, formed by the fusion of two append­ages. It is divisible into a proximal part, called submentum and a distal part, men­tum.

The submentum articulates with the head immediately near the articulation of maxillae. The free end of mentum carries following paired parts from outer to inner side—many-jointed labial palps, short paraglossae, and small glossae with curved claw at the tip.

This fleshy central part is bounded dorsally by mandibles, ventrally by the labium and laterally by maxillae. A membrane from the inner border of the labium is continuous with its ventral side. The salivary duct opens in the middle of the ventral side and near the base. The hypopha­rynx is also termed as tongue or lingua.

It articulates with the distal end of clypeus in the head region. Its dorsal side is hard but ventral side is soft and is known as the epipharynx.

The head is connected with the next tagma, the thorax, by a short neck or cervicum. A large membrane connects the head with the thorax. The thorax is divisible into three segments—prothorax, mesothorax and metathorax. The neck is the forward extension of the prothorax.

The dorsal exoskeletal plate or each segment of insects is called tergum and ventral exoskeletal plate of each segment called sternum. The lateral plate of each segment which joins tergum and sternum respectively is called pleuron (p1. pleura). Each plate is subdivided into separate plates and these subdivided plates are called tergites, sternites and pleurites.

A large sclerite covers the prothorax dorsally. It extends anteriorly to cover the head and posteriorly to shield the mesothorax. This sclerite also extends laterally. The central part of this sclerite is lighter in colour than its periphery. A slender line runs along the middle and bifurcates posteriorly. The mesothorax is covered by a round sclerite having a central triangular marking, called the scutellum.

A transverse line above the pointed apex of the scutellum divides the sclerite into an anterior and a posterior part. The anterior part is called prescutum or prealar sclerite, which looks like an inde­pendent sclerite but actually it is a part of the mesothoracic sclerite. Posterior to the prescutum lies the scutum which on each side bears one anterior and one posterior tergal processes for the articulation of the fore wing.

The exoskeleton of metathorax resembles that of the mesothorax. But here the prescutum is not so well-marked. The anterior and posterior tergal processes are present on the lateral side of the scutum for the articulation of the hind wings. Each tho­racic segment carries a pair of walking legs. All the legs are of similar shape and struc­ture. The first leg is the smallest and the third leg is the largest.

Each thoracic leg (Fig. 18.52) consists of following parts:

Proximal part of the leg which is broad and flat.

Small part which serves as a joint between the coxa and the next part, femur.

Long and flat portion with outer spiny border.

Narrow and long portions with numerous spiny projections called tibial spurs.

This is the distal part of the leg. It is five-jointed and each part is called a podornere. Beneath the joints there are soft pads, called plantulae. The terminal podomere is known as pretarsus, which bears two claws and a thin hairy pad, called arolium or pulvillus. The mesothorax and metathorax of an adult cockroach carry a pair of wings on the dorsal side of each segment.

During rest the dark coloured leath­ery mesothoracic wings, called wing cover or tegmina or elytra, cover the thin and membraneous metathoracic wings which are used for flight. The wings are exoskeletal modifications. Each wing is composed of double layers of chitin with branched tracheoles in between.

The abdomen is the longest tagma of the body, which is divisible into ten segments. The entire abdomen is dorso-ventrally flattened. Last few segments are short and closely set. The last part of the abdomen in both the sexes is modified to take part in the formation of an area where reproductive ducts open and the area is provided with certain genital appendages (Fig. 18.53).

A female cockroach bears following struc­tures in the last part of the abdomen:

It is formed by the close apposition of the sternites of both the sides of seventh abdominal segment towards the posterior direction. In between these two sternites, a fleshy lobe is placed with one small anterior and a large posterior folds. Dorsally the two folds bear a structure, called paraprocts, within which opens the anus.

The tergite belonging to the tenth abdominal segment extends posteriorly as incompletely bifurcated flexible projec­tion, carrying three gonapophyses or ovi­positors having sclerites, called valvifers.

These paired structures are borne by the lateral side of the paraproct and articulate distally with the anterior end of the epiproct and the tergites of the tenth abdominal segment.

In addition to the above mentioned struc­tures, the eighth abdominal segment bears three sclerites—two laterals (also called basisternite) and a single median. The spermathecal apertures open within the pos­terior projection of median sclerite and vulva or female gonopore is present on the ventral side of the two basisternites.

The posterior end of the abdomen in a male cockroach also exhibits specialisation. Here the sternum of the ninth abdominal segment is drawn anteriorly up to seventh abdominal segment and it bears following structures—cerci, epiprocts, styles and gonapophyses. The first two structures cor­respond to the identical structures of fe­males.

A pair of rudimentary undivided stylets is present on the ninth segment. Three gonapophyses, two laterals and one ventral are placed within a chamber formed dorsally by paraprocts and ventrally by the sternum of ninth abdominal segment. In between lobes of the gonapophyses opens the duct from conglobate gland and the left gonapophysis bears a slender pseudopenis.

4. Essay on the Integumentary System of Cockroach:

The body of cockroach is covered by a cuticle which is impermeable to water. Numerous fine tubules originating from the lower epi­dermal cells traverse the cuticle.

The cuticle is divisible into:

(a) Inner thick precuticle and

(b) Outer thick epicuticle.

The precuticle is divided into two sub-layers:

(i) The outer pigmented exocuticle and

(ii) And inner endocuticle.

The innermost part of the epicuticle is formed by a substance, called arthroidin and its outer is a thin polymer layer.

This polymer layer is coated externally by a substance, called amphion, which is formed by a combination of wax and ce­ment. The amphion makes the cuticle imper­vious to water. The most important integu­mentary glands are cervical glands and ab­dominal glands. The cervical glands are present within the membranes which cover the neck and its product is called ‘Periplanetin’.

The abdominal glands include dorsal abdominal gland in between the terga of 5th and 6th abdominal segments and ventral abdominal gland in between the sterna of 6th and 7th abdominal segments. These abdominal glands produce a substance having pungent smell which is used for defence.

The muscles of cockroach may be classi­fied into two broad groups—skeletal mus­cles and visceral muscles. There are nearly 370 pairs of skeletal muscles, of which 51 pairs are present in the head.

The skeletal muscles supply the mouth parts, thoracic legs, wings and genital ap­pendages. In males, the wing muscles are opaque and pink but in females these mus­cles are hyaline and white.

When compared with the histology of muscles of vertebrates, the skeletal muscle fibrils of insect show that Z-membrane, I-band, and A-band are prominent but M-line is absent and H-band is inconspicuous. The mitochondira are arranged on the opposite sides of the I-bands.

The visceral muscles include gut and heart muscles. The wall of the gut contains an outer coat of circular muscles and an inner coat of longitudinal muscles.

In the poste­rior region of crop and in the mid gut the longitudinal muscles are narrow but strongly developed in the anterior region of crop, colon and rectum. The most important heart muscles are fan-shaped alary muscles. The other visceral muscle of heart is a thin circu­lar layer around heart with distinct nuclei.

The histology of heart muscles exhibits the presence of intercalated discs in be­tween the muscle cells, a plasma-lemma hav­ing intimate connection with endoplasmic reticulum and complex mitochondria be­tween the myofilaments.

5. Essay on the Body Cavity of Cockroach:

The body cavity in the form of coelom is present only in the embryonic condition. In adult the body cavity is formed by the fusion of embryonic blastocoel with the embryonic coelomic space and is called mixocoel. The wall of the embryonic coelom is used in the formation of different organs. The mixocoel in cockroach is obliterated by a loose tissue called fat bodies.

The rest of the space is occupied by the digestive, excretory and reproductive organs. The blood flows through the mixocoel and for this reason the mixocoel is also called haemocoel and the circulating fluid is called haemolymph.

Two types of cells are present in the fat bodies—one type is binucleated and the other type is with an elongated nucleus. These cells act as storehouse of reserve food which remain in the form of glycogen and are used during starvation.

6. Essay on the Alimentary System of Cockroach:

The alimentary system which is respon­sible for nutrition includes alimentary canal and digestive glands (Fig. 18.54).

The alimentary canal is about 6.7 cm in length.

It is divisible into three distinct regions:

It is also known as stomodaeum. It is lined internally by cuticle and includes the mouth, pharynx, oesophagus, crop and gizzard. The mouth denotes the beginning of the alimentary canal. This ap­erture leads to a small chamber, called the buccal cavity, between the mandibles and maxillae on either side.

The labrum serves as upper lip and the labium acts as lower lip. A short tongue or hypopharynx is present on the floor of the buccal cavity. The buccal cavity opens into a short pharynx which is a small tube. The salivary duct opens within the pharynx near the base of hypopharynx.

The pharynx leads into the next part of the fore gut, which is called the oesophagus and the opening between the two is thick, mus­cular and guarded by a sphincter. The short and narrow oesophagus is lined externally by a layer of circular muscles and the inner wall contains cuboidal epithelial cells, mus­cle cells and tracheae.

The oesophagus ex­tends up to the prothorax and is followed by the crop. The dilated sac-like crop consti­tutes the largest part of the fore gut. The wall T5f the crop is composed of epithelial layer, circular and longitudial muscle layers. The crop extends within the abdominal cavity and acts as a temporary reservoir of food, where ingested food may be retained for two months.

The crop leads into a short thick- walled gizzard or proventriculus, which de­notes the last part of the fore gut. It is a bulb­like structure and divided into an anterior and a posterior parts.

The wall of the gizzard is highly muscular and contains a thick outer circular muscles and its anterior part con­tains in its inner wall six chitinous teeth extending towards the cavity of the gizzard. The posterior part of the gizzard possesses two circular hairy cushions. The teeth are used for crushing the food and the hairy cushions work as sieve to permit only the finer par­ticles of food to go inside the midgut.

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This undivided part of ali­mentary canal is also known as mesenteron. It is a slender tube having an internal lining of columnar epithelium. Near the junction of the fore and the midguts, there are eight hollow slender tubes, called hepatic caeca or digestive diverticula. The diameter of each hepatic caecum is nearly 1/3rd of the midgut and histologically it resembles the midgut.

All caeca open within the midgut and are believed to produce digestive juices. A loose network of muscle cells is present on the outer wall of the midgut. In the inner wall, the epithelial cells throw fine filaments within the lumen of midgut. The junction of the midgut and the hindguts is marked exter­nally by the presence of numerous threads called Malpighian tubules which are excre­tory organs.

3. Hind gut or Proctodaeum:

It is divi­sible into following parts—Ileum, Colon, Rectum and Anus. The ileum is the first part of the hindgut and has small narrow lumen having epithelial lining. Its outer coat is composed of scattered muscle fibres. The ileum leads to colon, which is broad and slightly coiled.

The inner lining of colon is thrown into irregular folds and is formed by slender epithelial cells having a chitinous covering. The colon continues into a small sac-like rectum. The inner wall of the rectum is raised in the form of papillae.

A special kind of glands, called rectal glands, is present in the rectal wall for absorbing water. Thus the rectum not only stores the residual parts of the food but also helps in osmoregu­lation. The rectum opens to the exterior through an opening, called the anus. The anus is present in between the two podical plates and is provided with a sphincter muscle.

The salivary glands, the inner lining of mid gut and hepatic caeca are the digestive glands of cockroach. A pair of salivary glands lie one on each side of the thoracic cavity. Each gland consists of two leaf-like diffused lobes and a reservoir. The secretory lobes, reservoirs and their ducts together constitute the salivary apparatus (Fig. 18.55).

The lobes of the salivary gland open within the reservoir. From each reservoir a salivary duct runs anteriorly. The salivary ducts of two sides unite to form a common duct which runs along the oesophagus to open into the pharynx and near the base of hypopharynx.

Each lobe of the salivary gland is made up of secretory acini, which are made up of two types of cells:

(a) Cells which are packed with secretory granules. Under electron microscope, the endoplas­mic reticulum of the cells appears to be distinct at the time of granule formation,

(b) Cells with an intracellular duct (lined by chitin) and with numerous microvilli.

These cells have very little secretory granules but abundant mitochondria, coarse endoplasmic reticulum and vesicular bodies. The internal lining of the midgut and the hepatic caeca also produce digestive juices.

Feeding and Digestion:

The cockroach is a macrophagous and omnivorous creature. It feeds on vari­ous kinds of organic matter like sugar, cere­als, fruits, flesh and plant materials.

The food is detected by sight, smell and touch. The touch receptors include the antennae and maxillary palps. The food is procured by the maxillae and cut into pieces by the mandibles and is passed into the mouth cavity.

Within the buccal cavity, the food comes in contact with saliva and passes through the oesophagus into the crop. Both peristalsis and antiperistalsis take place in the crop. Such activity of the crop is more intense in males than in females. The pas­sage of food from the crop to the gizzard depends upon the ingested fluid.

From the crop, the food passes to the gizzard, where the cuticular teeth crush the food and the hairy cushion permits only finer particles to enter the mid gut. The lining of midgut and hepatic caeca act both as secretory and absorbtive areas. Following enzymes are present in the secretion of these regions— amylase, maltase, invertase, lactase, β-glucosidase, protease and lipase.

The cellulase obtained in the midgut is synthesised by the micro­organisms residing there. Most of the digested foods are absorbed only in the midgut. Glu­cose is absorbed by the caeca.

After the absorption of digested food, the rest passes within the hindgut, where water and salts are absorbed. Residual matter is temporarily stored in the rectum and are periodically rejected through the anus. Food requires nearly 33 hours traveling the entire length of the alimentary canal.

It may be mentioned here that from the junction of gizzard and midgut the epithelial cells constantly throw membranous struc­tures, called peritrophic membrane of un­certain function. These membranous struc­tures are torned up in the anterior region of the hindgut by the internal spines.

Electron microscopic studies have revealed that the peritrophic membranes are made up of sev­eral layers and resemble the structures present in saliva. Such similarity in structures indi­cates that probably peritrophic membranes from midgut mix up with saliva at the time of regurgitation.

7. Essay on the Respiratory System of Cockroach:

The respiration in cockroach is aerial. Our knowledge about the respiratory struc­tures in cockroach is based primarily on the findings from Blatella, because little is known about the same in Periplaneta americana.

The respiratory system (Fig. 18.56) includes:

(a) Ten pairs of spiracles or stigmata,

(b) Three pairs of longitudinal trunks,

(c) Several seg­mental tracheae and

(d) Branched tracheoles.

Ten pairs of spiracles (or called stigmata) are present on the lateral sides of the body. Each spiracle is bounded by an annular sclerite, called peritreme, having a filtering apparatus formed by the bristles to eliminate dust particles from the inflowing air.

The first pair of spiracles is the largest and is present on the mesothorax. The sec­ond pair is on the metathorax and the re­maining eight pairs are on the first eight abdominal segments.

The mesothoracic spiracle has two lips—the anterior lip is rigid and the posterior lip is movable. The two metathoracic lips are united ventrally. No lip is associated with the abdominal spiracles. The thoracic spiracles open directly within the segmental trachea, but the abdominal spiracless open first within a chamber, called atrium, and from this chamber segmental tracheae originate.

Longitudinal tracheal trunks:

Three lon­gitudinal tracheal trunks are present on each side of the abdominal cavity. The dorsal and ventral longitudinal trunks are present near the middle line and the lateral longitu­dinal trunk is present on the lateral side of the abdominal cavity.

Each lateral longitudi­nal trunk is divisible into two parts—the anterior part is present between mesothoracic, metathoracic and first abdominal spiracle, and the posterior part extends from second abdominal spiracle to eighth abdominal spiracle.

Each dorsal and ventral tracheal trunk originates from a trachea given by first abdominal spiracle and extends up to a seg­mental branch from eighth abdominal spiracle. Six tracheae originate from each mesothoracic spiracle which supplies head, prothorax and mesothorax.

From the remain­ing spiracles on each side three segmental tracheae are given. The longitudinal trunks and the segmental tracheae are swollen at several places and are known as air sacs. The tracheae branch and rebranch to form a net­work of fine cuticular tubules, called tracheoles, which distribute over the tissue cells.

Large tracheae are internally supported by a spiral ring of chitin, called taenidia or intima. The taenidia prevent from collapsing of the tracheae. In addition, chitinous fibrils of 10 to 30 mm thickness and an epicuticle of lipoprotein nature lines the lumen of the trachea.

The lumen of trachea is often seen to be filled up with a substance of unknown nature. Smaller tracheae and tracheoles are devoid of taenidia and other chitinous structures. The opening of each tracheole within the tissue is immersed within the body fluid which conveys respiratory gases to and from the cells.

Mechanism of respiration:

During in­take of air (inspiration) the abdominal mus­cles relax to open the anterior four pairs of spiracles, through which air rushes in. The air reaches up to the intercellular spaces through the tracheoles. During expiration, the abdominal muscles contract to drive the air out of tracheal spaces through the last six pairs of spiracles.

According to another view, air flows in and out through all the spiracles and probably there is no direct circulation of air along the longitudinal tracheal trunks. The working of spiracles is under the control of central nervous system. The cockroaches can close all the spiracles and may suspend its respiratory activity for a considerable period of time.

The opening and closure of the spiracles depend upon the carbon dioxide concentra­tion. Usually the exhaled air contains 4% of carbon dioxide and if there is slight increase in its concentration rapid ventilation move­ment starts. The width of the spiracular opening increases with the rise of tempera­ture from 20 °-33 °C.

8. Essay on the Circulatory System of Cockroach:

The circulatory system of Periplaneta is open or lacunar type as blood opens into spaces, called lacuna, among viscera. The blood or haemolymph flows freely with the haemocoel. It has heart and aorta but no capillaries and the blood bathes tissues di­rectly. The circulatory system consists of blood or haemolymph, heart and aorta.

Blood or haemolymph:

The circulating fluid is called blood or haemolymph. It contains a colourless fluid, called plasma, in which are suspended many haemocytes.

Following particulars are known about the blood of cockroach:

Sp. gravity—1.029 (in mymph)

Total volume—19% of the body weight (in 24 hours old nymph)

Sodium—246 mg per 100 g

Potassium—67 mg per 100 g

Calcium—17 mg per 100 g

Phospholipids, Sterols and Triglycerids

145 mg per 100 g blood (before flight)

250 mg per 100 g blood (after flight)

Proteins and amino acids:

Total protein—740 mg nitrogen per 100 ml of blood

Amino acids—Alanine, Cystine, Glutamic acid, Glutamine, Glycine, Leucine, Methionine, Proline, Serine, Tyrosine and Valine

Uric acid—14.3 mg per 100 g of blood (before flight)

22 mg per 100 g of blood (after flight)

The total number of haemocytes in a 24 hours old adult is 9 million. The haemocytes are usually uninucleated but in pathological condition they may be multinucleated. Three types of haemocytes are known in cockroach— Prohaemocytes, Transitional haemocytes and Large haemocytes.

They are small (6-9 µm in diameter) actively divid­ing cells. The population of these cells is 23% of the total haemocytes. The nucleus is fairly large in comparison to the basophilic cyto­plasm. The nucleus contains much chromatin materials and divides by mitosis. These cells are phagocytic in nature.

(2) Transitional haemocytes:

They constitute 68% of the total haemocytes. Each cell is 9-18 µm in diameter and is phagocytic in nature. These cells divide mitotically only under special condition.

(3) Large haemocytes:

These cells are 18-23 (am in diameter. Each cell has a nucleus with distinct nucleolus and a network of chromatin material. The cytoplasm shows very weak basophilic stain.

According to the work of Jones, there are only two types of haemocytes in Periplaneta americana. They are plasmatocytes and coagulocytes or cystocytes.

The plasmatocytes constitute 60- 95% of total haemocytes. These cells are poly­morphic, amoeboid and have spindle-shaped inclusions. The number of coagulocytes var­ies and the nuclei are large and round. The intranuclear material is bar-shaped.

The blood of cockroach, which is free from the burden of carrying oxygen, serves following functions:

(1) Transportation of dissolved substances.

(2) Transmission of hydrostatic pressure from one end of the body to the other.

(3) Acts as a reservoir of water.

The haemocytes which are present in the blood are responsible for phagocytosis, coagulation and wound healing. During phagocytosis, the haemocytes engulf invad­ing micro-organisms or decaying tissues. After phagocytosis, certain haemocytes fully laden with ingested materials aggregate to­gether.

A few haemocytes form a capsule around them. The coagulation is the result of dual role of haemocytes and plasma. The process begins with one of the proteins in plasma and probably it is the lipoprotein which reacts to form a network.

According to one view, the coagulocytes, being disfigured, break open to release certain granules in the plasma around it. The plasma precipitates around the haemocytes to form islands of coagulated bodies.

Another view about the process of coagulation explains that in the beginning some haemocytes round up and throw thread-like pseduopodia which be­come sticky and thus lead the cells to agglu­tinate. The plasma material around them precipitates and the entire island dries up to become hard and ultimately black.

At the time of wound healing, the haemocytes move towards the injured site and form a clot. After 8-10 days the adjoining epithelial cells start to enlarge and invade the haemocyte mass. The mass splits into an inner and an outer layers. The inner layer continues as a connective tissue layer and after a year be­comes fibrous. The outer layer disintegrates and becomes melanised. The epithelial cells form the epidermis.

Heart and Pericardial cavity:

The heart of cockroach is an elongated, muscular and contractile tube, placed along the mid-dorsal line of thorax and abdomen (Fig. 18.57). The wall of the heart is composed of outer con­nective tissue and median muscle cells.

The cavity of the heart is lined by the sarcolemma of median muscle cells. The heart is enclosed with a pericardial sinus, the wall of which has segmented, triangular wing-like bundles of muscles, called alary muscles (Fig. 18.57).

The narrow ends of the alary muscles are inserted into the terga. The heart of cock­roaches is composed of thirteen funnel- shaped segmentally arranged chambers ly­ing one above the other. The extended parts of all alary muscles form dorsal perforated diaphragm which divides the perivisceral cavity into the dorsal pericardial sinus and ventral perivisceral sinus or haemocoel surrounding the gut.

At its anterior, the heart continues as a narrow tube, called anterior aorta which branches within the head region and opens into haemocoel on the head. In each segment heart sends a pair of segmental excurrent arteries which open into the haemocoelomic spaces.

The chambers of the heart communicate with the pericardial sinus by valvular openings, called ostia. These openings are present one pair in each segment on the ventro-lateral sides of the heart. The ostia allow the blood to enter into the lumen of the heart from pericardial cavity.

Due to the contraction of the alary mus­cles, the pericardial sinus enlarges, so that the blood of the perivisceral haemocoel flows into the pericardial sinus. The relaxation of these muscles pushes the blood into the tubular heart through the ostia.

The heart and the aorta contract in a metachronous rhythm from behind forwards, pushing the blood anteriorly into the head and then passes into body cavity. The heart beats at the rate of 100-120 per minute at 27 °C. A complete circulation of blood through the body takes 30-60 minutes.

9. Essay on the Excretory System of Cockroach:

Near the junction of the mesenteron (mid­gut) and proctodaeum (hindgut), there are numerous (70-120) thread-like, blind tubes with lumen and made up of ciliated and cuboidal epithelial cells, called Malpighian tubules (Fig. 18.58A+C)

The tubules develop from the undifferen­tiated region between the midgut and hind­gut (Tirelli, 1929).

The proximal end of each tubule opens within the lumen of the gut and the distal blind end extends within the haemocoel where they are freely bathed by the blood (Fig. 18.58A).

The tubules are arranged in six bundles. Each bundle contains 15-20 tubules and each tubule has a surface area of 2200 sq. mm.

In cross section it appears a ring-shaped structure and is made up of 3-8 ciliated or cubical epithelial large cells containing prominent nuclei (18.58B). These cells are bounded externally by a coat which contains longitudinal muscle fibres and internally by a basement membrane.

The cells towards the lumen of the tube contain de`nse mass of fine processes, called brush border. The inner cell lining of the distal region of the tubule has well developed brush border while in the proximal region they are less differen­tiated.

Types of Malpighian tubules:

The tubules are of two kinds—one kind takes darker silver nitrate stain than the oth­ers. The metabolic wastes which are collected from the haemocoelomic fluid are finally drained within the cavity of the hind-gut. Opinions differ regarding the nature of excre­tory substances eliminated by the tubules.

According to one view, various nitrogenous substances like urates, uric acid are removed by the tubules along with excess of water. But another view holds that the tubules are only osmo-regulatory in function and thus do not remove nitrogenous substances.

In addition to the Malpighian tubules, the lining of the hindgut also has excretory func­tion and the rectal papillae in the wall of the rectum regulate the exit of water. The urine is finally ejected with the faeces.

Mechanism of excretion:

Functionally the Malpighian tubules are concerned with the removal of waste prod­ucts of metabolism from the haemolymph. Distal parts of tubules are secretory, that pours nitrogenous urates of K, Na in solution into the lumen of tubules through osmosis where uric acid precipitates as crystals.

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Proximal part of tubules are absorptive that takes out water and inorganic base as bicarbonate which in turn returns to the haemolymph (Fig. 18.59). In this way water and bicarbonates are used again and again. This is a mechanism which maintains the osmoregu­lation of haemolymph. The urine which is produced in the tubules passes into the hindgut where the composition of urine is modified.

10. Essay on the Nervous System of Cockroach:

The nervous system of cockroach includes:

(a) Central nervous system,

(b) Peripheral nervous system,

(c) Sympathetic nervous system and

(a) Central nervous system:

The central nervous system (Fig.18.60) consists of several specialised parts which control the working of the different struc­tures of the body through the peripheral nervous system.

Each of the specialised parts of the central nervous system is discussed below:

1. Brain or Supra-oesophageal ganglia:

These are paired and large ganglia, present in the posterior side of the head region and on the dorsal side of the oesophagus. Each ganglion consists of a bilobed protocerebrum, a deutocerebrum and tritocerebrum.

2. Sub-oesophageal ganglion:

This gan­glion is present in the mid-ventral region of the head and just ventral to the oesophagus. It is formed by the fusion of two ganglia.

3. Circum-oesophageal connectives:

These are short and broad, paired nerves which originate one from each supra- oesophageal ganglion and encircle the oesophagus to unite with the sub-oesopha­geal ganglion. The two connectives are con­nected by one large and one small transverse commissures.

4. Ventral nerve cord:

It is formed by two solid nerves, which begin from the posterior end of the sub-oesophageal ganglion and run posteriorly along the mid-ventral line of the body cavity. In each thoracic segment it bears a prominent ganglion and in the abdo­men there are six abdominal ganglia. First four abdominal ganglia are present one in each of the first four abdominal segments.

The fifth abdominal ganglion is present near the junction of fifth and sixth segments. The sixth abdominal ganglion denotes the termi­nation of ventral nerve cord. It is twice the size of other abdominal ganglia. This last abdominal ganglion is more or less present on the eighth segment.

(b) Peripheral nervous system:

These are nerves which are given out from the ganglia of the central nervous sys­tem. The name and distribution of different peripheral nerves which are given from the brain and sub-oesophageal ganglia are shown in the enclosed table (Table 18.2—Arthropoda).

Particulars about the Peripheral Nerves from Brain and Sub-Oesophageal ganglia of Cockroach:

Table 18.2: Arthropoda:

Each thoracic ganglion has eight pairs of nerve roots. With the first pair the ventral nerve cord communicates. The remaining seven pairs of thoracic nerves supply the muscles of the thoracic segment, legs and wings. The metathoracic ganglion in addi­tion sends peripheral nerves to the first abdominal segment.

Each of the first five abdominal ganglia sends out pair of stout nerves to supply different parts of the ab­dominal segment posterior to it. The sixth abdominal ganglion sends out seven to eight pairs of nerves to the different structures present in the seventh, eighth, ninth and tenth abdominal segments.

(c) Sympathetic nervous system:

It is represented by a slender ganglionated sympathetic nerve, which begins from circumoesophageal connectives to innervate the involuntary muscles of the alimentary canal.

The sense organ of cockroach may be grouped according to their functions as mechanoreceptors, photoreceptors, thermo-­receptors and chemoreceptors. In addition, another kinds of receptors, for responding to humidity changes, are recorded in the cock­roach, Blatella. These are called hygroreceptors.

Following sensory structures are considered as mechano­receptors because they are concerned with the reception of stimuli in the form of touch, pressure, vibration and air current.

(a) Cuticular hairs:

These are present in the cuticle either in the form of bristles, spines, plates or collection of hairs. Tactile bristles are present in the cercus and are extremely sensitive to touch. Large spines on the femur and tibia of leg perform similar function.

A collection of hairs, known as hair plates, are present on certain parts of the body for receiving stimuli in the form of touch. A group of specialised hairs which are lodged in sockets occur in the cercus for receiving air movements and low frequency sounds. These are called auditory hairs.

(b) Campaniform stress receptors:

They are present as thick ridges on the segments of the leg, on the labial palps and its adjoin­ing areas. These receptors are believed to respond to different pressure on the cuticular surface.

(c) Chordotonal organs:

These specia­lised sense organs meant for receiving vibra­tions are distributed on the legs. These are aggregates of specialised cells which remain arranged either parallelly or in the shape of a fan.

The eyes and ocelli are the two important photoreceptors of cock­roach, in addition to the general surface of the body which can also detect light stimuli through cuticular receptors.

(a) Compound eyes:

The position and appearance of the compound eye have al­ready been stated. The external surface of the eye is covered by a transparent cuticle. Each eye is composed nearly of 20,000 visual units, each one of which is called ommatidium (Fig. 18.61A).

Each ommatidium consists of usual structures, like lens, crystalline cone, rhabdome and retinular cells (Fig. 18.61B). The cone is well developed and the rhabdomes extend up to the borders of the cone.

The retinular cells are partially strati­fied and contain rhabdomes in their inner border. These cells are separated by a slender axial canal. The pigment sheaths which sepa­rate the ommatidium are non-retractile. Thus only mosaic type of image formation occurs.

This is also known as simple eye. It is present near the base of antenna as a white area. It consists of outer flattened corneal cells, 3-5 retinular cells and one rhabdome formed by the inner margins of the retinular cells. It was formerly believed that these ocelli are of uncertain functions.

Recently the light detecting ability of these structures has been proved. It has been dem­onstrated that cockroaches can respond to the change of light even when the compound eyes are painted. But it fails to do so after the covering of ocelli.

The temperature receptive sense organs are present in pads between the first four tarsal segments of the lge.

These receptors are responsible for detecting chemical stimuli in the form of smell and taste. The long and annulated antennae are beset with two types of sensory structures—thick-walled bristles and thin-walled hairs.

These bristles and hairs are responsible for the perception of smell. The tips of the maxillary and labial palps, inner surfaces of the mouth parts and the inner border of the mouth and pharynx possess sensory structures for contact, chemo- reception and gustatory (taste) responses.

11. Essay on the Endocrine System of Cockroach:

The body of cockroach contains following endocrine organs corpora cardiaca, corpora allata, prothoracic gland and cervical glands.

These glands work together with five groups of specialised cells in the brain. Of these five groups, three groups of cells are placed anteriorly and send the first nerve to the corpora cardiaca. The remaining two groups are present at the posterior end and they send the second nerve to the corpora Cardiaca.

It is a pair of small, elongated and irregu­lar glands. At the anterior end it encloses the aorta. At the dorsal side it bears a transverse commissure which extends posteriorly as a process to give rise to the aortic nerves. Ventrally, the corpora cardiaca are connected with a rudimentary ganglion and are drawn posteriorly to connect the corpora allata.

Each corpora cardiacum receives three nerves—two from the neurosecretory regions of the brain and one from unknown origin. The electron microscopic studies have re­vealed that the cells of corpora cardiaca have profuse endoplasmic reticulum and promi­nent secretory granules of 600 nm in diam­eter.

The secretion of corpora cardiaca affects the contractility of muscles lining the gut, the Malpighian tubules and heart. It is believed that the products released by the corpora cardiacum increase the effects of prothoracic glands.

This is a pair of small glands present posterior to the corpora cardiaca. Each gland is or less oval. A transverse commissure connects it with the oesophageal nerve. It has nerve connections with sub-oesophageal ganglion, corpora cardiaca and prothoracic glands. The histology of corpora allata shows the presence of profuse mitochondria within the cells but secretory granules are present in limited areas.

The secretion is responsible for following functions:

1. It maintains the juvenile features in the larval stage.

2. It helps in the oocyte formation in adult females.

3. It influences the secretions of second­ary sexual organs in both the sexes.

At the anterior end of the prothoracic ganglion, lies a pair of rope-like prothoracic glands. A tracheal branch remains associated with the gland. The histology of this gland shows the presence of a central part having 6-8 muscle fibres, which remain enclosed by an outer glandular covering of 4-12 cells deep. The secretory granules are of various sizes.

The secretions of prothoracic gland facilitate moulting and after the last moult the prothoracic glands degenerate. It has been experimentally demonstrated that the implantation of two pairs of prothoracic glands in an adult induces it to moult and results into the formation of a giant cock­roach.

This is a pair of small oval glands, present in the neck region near the posterior open­ing of the head. This gland is richly supplied with trachea. The outer part of this gland is composed of large glandular cells and the inner part is made up of small cells with unusually large nuclei.

The central part of the gland is occupied by a cavity which traverses within the inner wall. The endo­crine nature of this gland has not yet been proved in cockroach but in other insects it is believed to be responsible for producing a hormone to induce moulting. The product which is secreted is called Periplanetin.

12. Essay on the Reproductive System of Cockroach:

Sexes are separate. The members of two sexes may be identified on the basis of their morphological features (Table 18.3).

A. Male reproductive system:

The male reproductive system includes the following organs:

2. Vas deferentia

3. Mushroom gland

4. Conglo­bate gland or phallic.

5. Male gonapophyses or phallomers and

6. Male gonopore.

A pair of three lobed small, white structures situated in the lateral side of the abdominal cavity beneath the terga of fourth and fifth abdominal segments. In a cock­roach 4.4 cm long, each testis is nearly 1 cm in length.

The testes are longer in the young than in order age. The testes are embedded in the fat body. Each testis is formed of 30-40 transparent testis follicles. In the young, full of sperms grow in the testis (Fig. 18.62).

2. Vas deferentia:

From the posterior end of each testis arises a delicate duct, called vas deferens. The two vas deferentia run posteriorly in the abdominal cavity and then curve towards the centre and open with a sac, called seminal vesicle or vesicula seminales, where sperm are stored. The two seminal vesicles open within a common duct, called the ejaculatory duct.

3. Mushroom gland:

Tufts of numerous blind, slender and thread-like tubules are situated at the junction of vas deferentia and ejaculatory duct. These tubules give a mush­room appearance and form the mushroom gland or utricular gland. The tubules are arranged in two groups.

The long tubules are situated at the periphery of the mushroom gland, called utriculi majores, which secret the inner layer of spermatophores. Short tubules of the mushroom gland, called utriculi brevirostris, forming the bulk of gland which secret a nourishing fluid for the sperm.

4. Conglobate gland or phallic:

A long, flat multi-lobed sac-like structure is situated be­neath the ejaculatory duct, called conglobate gland or phallic. Its anterior end is broader and taper posteriorly. The posterior end opens into the genital pouch near the male genital aperture between the ninth and tenth sterna. The function is still not known.

5. Male gonapophyses or phallomers:

A reproductive (genital) pouch is formed near the ventral of the ninth and dorsal side of the tenth segments. The pouch is surrounded by irregular chitinous hooks and plates, called gonapophyses or phallomeres. These help in copulation.

On the left side, the left gonapophysis contains a slender arm with a curved hook, called titillator and in be­tween lobes of the left gonapophyses there is a shorter arm with a hammer-like head, called peudopenis.

6. Male gonopore:

The ejaculatory duct which is unpaired and muscular, runs up to the reproductive pouch and opens through an aperture, called the male gonopore.

B. Female reproductive system:

The female reproductive system consists of a pair of ovaries, oviducts, colleterial glands, spermathecae, gonapophyses and female gonopore.

A pair of large ovaries is placed on the lateral and posterior ends of the abdominal cavity extending 4th to 6th seg­ments. Each ovary is made up of eight beaded tube-like ovarioles. Each ovariole is distaloproximally divisible into six zones (Fig. 18.63).

The distal-most ligament-like part which connects the ovarioles with the body cavity.

This is the budding zone, or zone of germarium where future oocytes are produced.

Here the oocytes grow, but are not arranged in a single file.

This is the longest region where oocytes are arranged in a single line. The distal end has smallest oocyte and the largest one re­mains at the proximal end.

Broad region of the ovariole which contains oocytes rich in yolk. This region is called the region of vitellarium.

This is the proximal part which opens within the oviduct.

From each ovary originates one short and wide oviduct which is formed by the fusion of ovarioles at the posterior end.

The two oviducts unite to form a common chamber, called vagina, which is placed in the median position.

In between the oviducts, a pair of club-shaped, unequal organs, called spermathecae, is present. The two spermathecae open within the genital pouch on a small spermathecal papilla through an independent median aperture on the ninth segment.

5. Colleterial glands:

A pair of branched tubular glands, called colleterial glands, opens on the dorsal side of the female gonopore. The left gland is larger and opaque while the right one is smaller and transpar­ent. The colleterial glands lie behind and above the ovaries. The secretion of colleterial glands forms the egg-case or ootheca.

6. Genital pouch:

The genital pouch is formed by the modification of the seventh, eighth and ninth abdominal segments. It is bounded ventrally by the sternum of sev­enth segment, dorsally by the sternum of eighth segment.

The genital pouch can be distinguished into two portions. The ante­rior smaller portion is called genital cham­ber into which female gonopore and spermathecal pore open. The posterior larger portion is called oothecal chamber or vestibulum within which oothecae are formed.

7. Female gonapophyses:

The female gonopore opens within genital pouch at the eighth segment. The opening of the pouch is provided with three pairs of chitinous rods, called the gonapophyses. These help in copu­lation and in depositing the eggs.

13. Essay on the Development of Cockroach:

The female cockroach liberates a smell to attract the male. The male cockroach on receiving the odour, raises its wings and searches the female. During pairing the male protrudes its abdominal segments to fix under the female, thus moving end to end with the female. The pairing continues for an hour. The males which are at least 2-4 days old and females of 4-5 days old are capable of pairing.

The pairing of the two sexes results into the transfer of sperm cells from males within the female as small pinhead sized packets, called spermatophores.

Within the body of the female the sperms remain tem­porarily stored within spermathecae. Sixteen eggs one from each ovariole travel towards the genital pouch through oviduct. Each egg is centrolecithal, i.e., bulk of yolk is confined to the central part of the egg. Thus the cytoplasm with nucleus is restricted to the periphery.

After fertilization, the fertilized eggs are enclosed in double row within a case, called ootheca which is formed by the secretion of colleterial gland. The ootheca is composed of a protein and contains water.

The female after laying the egg, fixes it on some suitable object with the help of an oral secretion. The development of the embryo continues within the ootheca and after cer­tain period the young hatches out of the case (Fig. 18.64).

The development requires 34-99 days and it depends upon temperature. The rate of hatching is 63% and the young takes five minutes time to come out of the inflated ootheca. After coming out, the young eats the egg membrane. The young’s resemble the adults but are soft, white and without go­nads and wings.

They are called nymphs. The nymphs undergo several moultings. The stage in between two moultings is called instar. Wings appear at the end of the last moult. The transformation from nymph to adult takes six to eight months and requires 11 moults in males and 12 moults in females.

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