Medical Entomology PDF

MIDTERMS: INSECT INTEGUMENT Integumentary system - Composed of cuticle, epidermis, and the basement membrane. Functions of the Integument Protection for internal organs. Skeleton for attachment of muscles. Give the insect its form. Give chemical and physical colors. Reg...

MIDTERMS: INSECT INTEGUMENT Integumentary system - Composed of cuticle, epidermis, and the basement membrane. Functions of the Integument Protection for internal organs. Skeleton for attachment of muscles. Give the insect its form. Give chemical and physical colors. Regulates water loss. Provides a metabolic reserve. Protects against entry of foreign materials such as pesticides. Allows for modifications which provide sensory input, eyes, chemoreceptors, etc. Allows for movement and most important, flight. Chitin - a nitrogenous polysaccharide similar to cellulose: N-acetyl-D-glucosamine residues, Glucosamine residues, and ß 1,4 linkage CUTICLE The EPICUTICLE is divided into: 1. CEMENT LAYER is the outermost layer. It is probably like shellac and is not always present. 2. WAX LAYER is the next layer. It is made up Epicuticle of as many as 3 layers: - Which is outer and has Outer Layer - not no chitin. It is up to 4 always present. (wax um thick. blooms) - Consists of lipids and it Middle Layer - largest. is also complex. Mono Layer - imparts water protection. 3. POLYPHENOL LAYER (outer epicuticle ) is the next layer and is the part of  the epicuticle that has been sclerotized. This is the first-formed layer of new cuticle produced at each molt, protecting the new procuticle from the molting enzy 4. CUTICULIN LAYER (inner epicuticle) is the innermost layer of the epicuticle. Chemically complex, consisting primarily of tanned lipoproteins. During its production, phenolic substances and phenoloxidase are also present. This is the very important layer. It extends over the entire body including the ectodermal invaginations. It is a selective barrier allowing important chemicals through during molting and allows the resorption of the digested products of the endocuticle of the old cuticle. Because it is inelastic, it limits growth The PROCUTICLE is divided Procuticle into 4 layers based on staining - Is 20 to 60 percent characteristics: chitin. Most of the rest 1. EXOCUTICLE consists of protein. - In the exocuticle proteins are linked Protein of the procuticle is of 2 together by a quinone types: molecule. The resulting Watersoluble is molecule is called sometimes called SCLEROTIN and is ARTHROPODIN, very tough and really a class of non-yielding. It is a proteins. tanned procuticle. Water insoluble - Is absent from areas proteins that bind to where flexibility is  chitin and each other needed as in joints, intersemental membranes, and along ecdysial lines. 2. MESOCUTICLE - Transition layer based upon staining. Chemically similar to endocuticle. 3. ENDOCUTICLE - 10-200 um thick. - The endocuticle is composed of lamellae made up of masses of microfibrils arranged in succession of planes, all fibers in one plane being parallel to one another. - Extensive hydrogen bonding holds it together. Covalent bonding holds chitin together with protein in the endocuticle. 4. SCHMIDT'S LAYER = subcuticle; very thin. Located above the epidermis, below the endocuticle, originally thought to be an adhesive layer, now believed to represent a newly secrete cuticle that is less well-stabilized endocuticle. The chitin fibers in this region are not ordered EPIDERMIS - is the outermost cellular layer of the insect. It is sometimes called the hypodermis but this isn't a very good term as the insect has no dermis. - It is one-cell thick. - When inactive, its cell boundaries are indistinct.  - When active, as during the deposition of a new cuticle, the previously flattened cell bodies become more or less cuboidal and their plasma membranes are very distinct. At this time one or more nucleoli are evident and RNA synthesis is evident by the presence of much endoplasmic reticulum. - Epidermal cell densities range from about 3000/mm2 to 11000/mm2. - In the cyclorrhaphan Diptera, epidermal cells do not divide but grow in size as the larva grows. In Calliphora erythrocephala, they grow to 11 times their 1st instar size by the time they are in their 3rd insta PLASMA MEMBRANE PLAQUE - In contrast, in the Nematocera, epidermal cells divide as the larva grows. They also increase in size. - The shape of both outer and inner borders changes with secretory activity and within the molting cycle. Folding and the appearance of many microvilli on the plasma membrane is evident during cuticle deposition and resorption of the endocuticle via the molting fluid. - Pigments occur in the epidermis, usually orange and red. Oenocytes - cells produced by epidermal cells. Synthesize hydrocarbons and possibly other lipids that contribute to the epicuticle. BASEMENT MEMBRANE (BASAL LAMINA) - Contains connective tissue, collagen, and mucopolysaccharide. - Defined as the continuous layer ca. 0.5 mu thick beneath the epidermis. - Just before molting, it thickens by the deposition of more mucopolysaccharide from the hemocytes which can often be seen adhering closely to its surface. - Secreted in part by the hemocytes but the epidermis may also be involved in its formation. - Tracheoles, nerves and various sensory structures pass to or through it. - Where muscles are attached it is continuous with the sarcolemma. It is attached to the epidermis by hemidesmosomes. - Function isn't clear but may act as sort of a base for support while the outer layer of the cuticle (epicuticle) is being laid down. May also act as a molecular sieve. MOLTING AND ECDYSIS - Snodgrass equates molting and ecdysis. This is not the case however, they are independent processes. The process leading up to the shedding of the old cuticle and distinct from the final act of shedding is called MOLTING. The process of casting off the old cuticle is ECDYSIS. - One of the main problems insects encounter is the fact that muscles have to attach to an exoskeleton which is renewed at the molt. Therefore the attachments also have to be renewed. During this period of time the insect is vulnerable. - Muscles pass through the procuticle to the base of the cuticulin where they are anchored by some unknown mechanism. - The base of the epidermis and the muscle interdigitate which also helps in anchoring the muscles to the cuticle. MICROTUBULES (TONOFIBRILLAE) extend outward through the epidermis to the base of the cuticle. From here, MUSCLE ATTACHMENT FIBERS pass  through the cuticle to the base of the cuticulin layer through pore canals. These fibers are not contractile and continue to grow with the increase in thickness of the procuticle. As the cuticulin layer is one of the earliest formed during molting, the muscle attachments are maintained almost always. CUTICULAR PROTUBERANCES - These have been variously classified on the basis of: Movability - Spines are non-movable; spurs are movable. Relationship to underlying epidermal cell - setae and scales are unicellular; microtrichia are subcellular; and spines and spurs are multicellular. 4 basic types of cuticular protuberances MULTICELLULAR with cells similar in appearance to cells of the remainder of the epidermis. These are SPINES and SPURS. In the broad sense, legs and antennae conform to this definition. Also eversible glands, the endophallus, eversible pouches of butterflies and moths, the ptilinum of the cyclorrhapha Diptera, tracheae, tendons, large "teeth" in the proventriculus of acridids. The most basic type is the trichoid sensillum which originates from three specific cell types: Trichogen cell - hair-forming cell. The TRICHOGEN CELL actually makes the MULTICELLULAR with protuberance and specifically differentiated cells. secretes the cuticle on it. These are SETAE. Tormogen cell - socket-forming cell. The TORMOGEN CELL is considered to make the socket and secrete its cuticle. Sense cell - nerve cell  or neuron. The SENSE CELL differentiates into a bipolar sense cell with dendrites extending peripherally and an axon growing into the CNS. There are 2 types: TYPE A ACANTHAE - the process is nearly as large in cross-section as the underlying cell that UNICELLULAR - produced it. ACANTHAE - unicellular projections with no nerve cell; TYPE B ACANTHAE one projection. - the process is small relative to the cell. [examples: proventriculus of Mecoptera; tenent hairs on feet of Diptera]. SUBCELLULAR - more than one projection per cell. These are called MICROTRICHIA. Projections that are many per cell. Common on abdomen, taenidia of trachea, on setae, on wings. INSECT HEAD POSITIONS OF THE HEAD OR MOUTHPARTS RELATIVE TO THE BODY Hypognathous - this is the primitive condition. This is where the head is more or less vertical and the mouthparts are directed ventrally. (pointing downwards)  Example: grasshopper and cockroach Prognathous - This is where the mouthparts are directed anteriorly. This is common in insects that burrow and in predatory insects. Example: ground beetle and soldier termite Sometimes divided into 2 distinct types: Opisthognathous (opisthorhynchus in sucking Auchenorrhynchous - insects) - This is where the The beak arises from mouthparts are directed the back of the head or posteriorly. This is common in near the "neck". the Hemiptera and Homoptera. Example: cicadas Example: stink bug Sternorrhynchous - The beak arises from the front of the head. Example: aphid GENERAL EXTERNAL STRUCTURES OF THE INSECT HEAD - The head can be viewed as an ovoid envelope of sclerotized integument enclosing the brain centers, certain glands, and muscle systems for the operation of the head appendages. - The head capsule is open at its posterior junction with the thorax to permit a passageway for certain connectives such as the ingestive tube which connects the mouth with the digestive system. This opening is referred to as the occipital foramen. The thin, flexible cylinder of integument connecting the margins of the occipital foramen with the thorax is the neck or cervix. GENERAL EXTERNAL STRUCTURES OF THE INSECT HEAD The conspicuous photoreceptors or compound eyes occupy the dorso-lateral aspects of the head. Antennal sockets are situated on the frontal surface between the eyes. The ocular suture (CH: circumocular sulcus) encloses the ocular sclerite. There may also be a vertical sulcus from the eye to the subgenus sulcus called the subocular sulcus. The subocular sulcus acts as a brace against the pull of the muscles associated with feeding. The anterior surface of the head lying between the compound eyes is called the frons. Although the frons is usually easily identified as the broad frontal area between the eyes, an accurate identification of facial areas is best made with reference to the sutures lining the integument of the head. Ventrad of the frons is a short suture bearing at its lateral ends the anterior tentorial pits. This is the frontoclypeal or epistomal suture (CH: epistomal sulcus). In most insects, the epistomal suture is continuous across the face and is probably the most constant frontal suture to use for the identification of facial areas. When anterior tentorial pits are present they will always be on the epistomal suture. The facial area above the epistomal suture is the frons; the area below the suture is the clypeus. Sometimes the distal portion of the clypeus is membranous. In this case, the proximal sclerotized portion of the clypeus is called the postclypeus and the distal, membranous portion as the anteclypeus. Muscles attaching to the inner surface of the frons lead to the pharynx, labrum, and the hypopharynx; Those attached to the inner surface of the clypeus lead to the cibarium, and the frontal ganglion is always situated between these two sets of muscles. An oblong sclerite freely articulating at its proximal margin with the clypeus, is the labrum. This sclerite serves as an upper lip for the mouth cavity. Although the labrum is generally considered as a part of the organs of ingestion, it is a true sclerite of the head and was not evolved from an appendicular structure The gena or cheek is a poorly defined area in most insects, but usually lies below and immediately behind the compound eyes. Sometimes this area may be set off by a subocular groove. An area immediately above the articulations of the mandibles may be heavily sclerotized to support the powerful jaws. This area, margined by a subgenal suture (CH: subgenal sulcus), is called the subgena. Actually, the subgenal sulcus above the mandibles is often called the pleurostomal sulcus and the portion more posterior, behind the mandibles is often referred to as the hypostomal sulcus. The subgenal suture is usually continuous with the epistomal suture. The subgenal area above the mandibles is called the pleurostoma and the area above and posterior to the mandibles is the hypostoma. A frontal suture in the form of an inverted Y is common in immature insects and sometimes faintly visible in adult insects. It is often called the epicranial suture. The stem of the Y is referred to as the coronal suture and the arms as the frontal sutures. This is actually an ecdysial suture or a point of rupture in the integument during the molting process. When this suture is developed the area enclosed by it is called the frons. BACK OF THE HEAD The top of the head is another poorly defined area called the vertex. When the epicranial suture is developed the vertex is the area directly on either side of the coronal suture.  An additional suture may occur anterior to the post occiput and margins the posterior aspect of the head. It is referred to as the occipital suture (CH: occipital sulcus) and the area it encloses as the occiput. Generally, the term occiput is used only to describe the posterior area immediately behind the vertex. The lateral, ventral portion of this sclerite is then referred to as the postgena. INTERNAL STRUCTURE OF THE INSECT HEAD - The main internal structure of the head is the tentorium. It supports the cranial wall, it supports the stomadeum, and it serves as the origin of many muscles of the mouthparts and the antennae. It basically is made up of two pairs of apodemal arms (sometimes 3) and a body. - The anterior and posterior arms are often united in a tent-like manner, hence the name tentorium. The tentorium may be strengthened by a central plate called the corpotentorium. The anterior tentorial arms have their origin in the epistomal sulcus (frontoclypeal sulcus). The external evidence is the anterior tentorial pits. The posterior tentorial arms take their origin in the postoccipital suture and have as their external evidence the posterior tentorial pits.  Dorsal tentorial arms are sometimes present. If they are, they are considered outgrowths of the anterior arms. Pits on the frons called the tentorial maculae, are not true envaginations and are not homologous with the anterior tentorial pits. INSECT ABDOMEN Basic structures - Segmentation is more evident in abdomen - The basic number of abdomen segments in insect is eleven plus a telson which bears anus - Abdominal segments are called uromeres - On 8th and 9th segment of female and 9th segment if male, the appendages are modified as external organs of reproduction or genitalia. These segments are known as genital segments. - Usually 8 pairs of small lateral openings (spiracles) are present on the first eight abdominal segments. - In grasshoppers, a pair of tympanum is found on either side of the first abdominal segment. - It is an auditory organ. - It is obliquely placed and connected to the metathoracic ganglia through auditory nerve. BASIC STRUCTURE OF INSECT ABDOMEN ABDOMINAL MODIFICATIONS IN INSECTS Reduction in number of abdominal segments has taken place in many insects. In springtail only six segments are present. In house fly only segments 2 to 5 are visible and segments 6 to 9 are telescoped within others. In ants, bees and wasps, the first abdominal segment is fused with the metathorax and is called propodeum. Often the second segment forms a narrow petiole. The rest of the abdomen is called gaster. In queen termite after mating the abdomen becomes gradually swollen due to the enlargement of ovaries. The abdomen becomes bloated and as a result sclerites are eventually isolated as small islands. Obesity of abdomen of queen termite is called physogastry. ABDOMINAL APPENDAGES I. Pregenital abdominal appendages in wingless insects: 1. Styli (Stylus : Singular) - Varying number of paired tube-like outgrowths are found on the ventral side of the abdomen of silverfish. These are reduced abdominal legs which help in locomotion. 2. Collophore or ventral tube or glue peg - It is located on the ventral side of the first abdominal segment of springtail. It is protruded out by the hydrostatic pressure of haemolymph. It might serve as an organ of adhesion. It aids in water absorption from the substratum and also in respiration. 3. Retinaculum or tenaculum or catch - It is present on the ventral side of the third abdominal segment. It is useful to hold the springing organ when not in use. 4. Furcula or Furca - This is a 'Y' shaped organ. It is present in the center of the fourth abdominal segment. When it is released from the catch, it exerts a force against the substratum and the insect is propelled in the air. II. Abdominal appendages in immature insects: 1. Tracheal gills - Gills are lateral outgrowths of body wall which are richly supplied with tracheae to obtain oxygen from water in naiads (aquatic immature stages of hemimetabolous insects). Seven pairs of filamentous gills are present in the first seven abdominal segments of naiads of mayfly and are called lateral gills. Three or two leaf like gills (lamellate) are found at the end of the abdomen of naiad of damselfly and are called as caudal gills. In dragonfly the gills are retained within the abdomen in a pouch like rectum and are called as rectal gills. 2. Anal papillae - A group of four papillae surrounds the anus in mosquito larvae. These papillae are concerned with salt regulation. 3. Dolichasters - These structures are found on the abdomen of antlion grub. Each dolichaster is a segmental protuberance fringed with setae. 4. Prologs - These are present in the larvae of moth, butterfly and sawfly. Two to five pairs are normally present. They are unsegmented, thick and fleshy. The tip of the proleg is called planta upon which are borne heavily sclerotised hooks called crochets. They aid in crawling and clinging to surface. III. Abdominal appendages in winged adults : 1. Cornicles - Aphids have a pair of short tubes known as cornicles or siphonculi projecting from dorsum of fifth or sixth abdominal segment. They permit the escape of waxy fluid which perhaps serves for protection against predators. 2. Caudal breathing tube - It consists of two grooved filaments closely applied to each other forming a hollow tube at the apex of abdomen. e.g. water scorpion. 3. Cerci: (Cercus - Singular) - They are the most conspicuous appendages associated normally with the 11th abdominal segment. They are sensory in function. They exhibit wide diversity and form. Long and many segmented. e.g. Mayfly Long and unsegmented. e.g. Cricket Short and many segmented. e.g. Cockroach Short and unsegmented. e.g. Grasshopper  Sclerotised and forceps like. e.g. Earwig. Cerci are useful in defense, prey capture, unfolding wings and courtship. Asymmetrical cerci. Male embiid. Left cercus is longer than the right and functions as a clasping organ during copulation. 4. Median caudal filament - In mayfly (and also in a wingless insect silverfish) the epiproct is elongated into cercus like median caudal filament. 5. Pygostyles - A pair of unsegmented cerci like structures are found in the last abdominal segment of scoliid wasp. 6. Anal styli - A pair of short unsegmented structure found at the end of the abdomen of male cockroach. They are used to hold the female during copulation. 7. Ovipositor - The egg laying organ found in female insect is called ovipositor. It is suited to lay eggs in precise microhabitats. It exhibits wide diversity and form. Short and horny: e.g. Short horned grasshopper Long and sword like : e.g. Katydid, long horned grasshopper Needle like : e.g. Cricket Ovipositor modified into sting: e.g. Worker honey bee. Pseudoovipositor - An appendicular ovipositor is lacking in fruit flies and house flies. In fruit flies, the elongated abdomen terminates into a sharp point with which the fly pierces the rind of the fruit before depositing the eggs. In the house fly the terminal abdominal segments are telescopic and these telescopic segments aid in oviposition. The ovipositor of a housefly is called a pseudo ovipositor or ovitubus or oviscapt. Male genitalia - External sexual organs of male insects are confined to ninth abdominal segment. In damselfly, the functional copulatory organ is present on the center of second abdominal segment PHOTORECEPTOR ORGANS OF INSECTS Compound eyes - Most adult insects have a pair of compound eyes, one on either side of the head, which bulge out to a greater or lesser extent so that they give a wide field of vision in all directions. The compound eyes may, however, be reduced or even absent in some parasitic insects.  - Each compound eye is an aggregation of similar units called ommatidia, the number of which varies from one in the worker of the ant Ponera punctatissima to over 10,000 in the eyes of dragonflies. When there is only a few ommatidia they tend to be round in shape, whereas when there is a large number they are hexagonal in shape. Usually the two compound eyes are separated (called dichoptic), but in some insects, they meet on top of the head (called holoptic). - In ommatidia in which the Semper cells have produced a crystalline cone are called eucone eyes. In some Hemiptera, Coleoptera, and Diptera the Semper cells do not form a crystalline cone, but become transparent and undergo only a little modification. Ommatidia of this type are described as acone ommatidia. The acone type may be primitive as many Apterygota also have this type. The Semper cells of most Diptera and some Odonata produce cones which are liquid-filled or gelatinous rather than crystalline. These are called pseudacone ommatidia. Finally, in some Coleoptera, the lens is formed from an inward extension of the cornea, not from the Semper cells which form a refractile structure between the cuticle and the retinula cells. This is an exocone ommatidia. - In many insects the rhabdom is very long and extends from the back of the lens almost to the basement membrane. This is called apposition eyes. In some insects (some Coleoptera and nocturnal Lepidoptera), the rhabdom is short and separated from the lenses by a space which is crossed by processes of the retinula cells. This is called superposition eyes. OTHER PHOTORECEPTORS - Dorsal ocelli are found in most adult insects and the immatures of hemimetabolous insects. Typically there are three, forming an inverted triangle anterodorsally on the head. Frequently one or more of the ocelli are absent. These are not concerned with form vision, but probably are important in detection of light. They can also detect changes in light intensity. - In the larval holometabolous insects the only visual organs are the stemmata. These are often called ocelli, but since they are somewhat different in structure they should not be called ocelli. These are also probably primarily concerned with light reception. - In some insects the individuals will still react to light even when all of the known visual receptors have been occluded. Thus it appears that there must be light receptors in the general body surface, but none have been located.  MECHANORECEPTION A trichoid sensillum is a hair-like projection of the cuticle articulated with the body wall by a membranous socket so that it is free to move. The hair is produced by a cell, the trichogen cell, and the socket by another, the tormagen cell. Associated with each hair is one or more nerve cells. Hairs concerned only with mechanoreception have only one neuron, but chemosensory hairs with a number of neurons may also have mechanoreceptor function. The Trichoid Sensilla receptor potentials produced may be of 2 types. In most cases a potential develops only during the movement of the hair, that is a bending or straightening of the hair. This is called a phasic response. But in others the potential is maintained all the time the hair is bent, adapting only very slowly. This is known as a tonic response. Hairs that show phasic responses function as tactile receptors found primarily on the antennae, tarsi, and wherever the insect touches the substratum. They may also respond to vibrations and so may be involved in sound reception. Hairs that have tonic response are usually concerned with proprioception and are often associated with joints of legs and between body segments. Chordotonal, or scolopophorus, organs consist of single units or groups of similar units called scolopidia. They are usually Chordotonal subcuticular and have no external evidence. Organs They generally consist of 3 cells arranged in a linear manner: the neuron, the scolopale cell, and the attachment, or cap cell. These may function as proprioceptors, but often they may function as sound receptors  It is a chordotonal organ lying in the 2nd segment of the antenna with its distal Johnston’s Organ insertion in the articulation between the 2nd and 3rd segments. It occurs in all adult insects except Collembola and Diplura. This organ perceives movements of the antennal flagellum. Tympanal organs are specialized chordotonal organs. Each consists of a thin area of cuticle, the tympanal membrane, backed by an air-sac so that it is free to vibrate. Attached to the inside of the membrane or adjacent to is a Tympanal Organ chordotonal organ. Tympanal organs occur on the prothoracic legs in some Orthoptera, on the mesothorax in many aquatic insects, on the metathorax in some Lepidoptera, and on the abdomen in some grasshoppers, Homoptera, and some Lepidoptera. These function in sound reception. It is a chordotonal organ usually containing between 10 and 40 scolopidia in the Subgenal Organ proximal part of the tibia. These are concerned with sound reception and the reception of vibrations from the substratum Are areas of thin cuticle, domed and usually oval in shape. These sensilla often occur in groups which probably function as Capaniform a unit. They occur on all parts of the body Sensible subject to stress and are concentrated near the joints as at the base of the wing or the haltere in Diptera. They function as proprioceptors. Stretch receptors differ from other insect sensilla in consisting of a multipolar neuron with free nerve endings (called Type II Stretch Receptor neurons), while all others contain a bipolar neuron with a dendrite associated with the cuticle (called Type I neurons). Stretch receptors occur in connective tissue or associated with muscles. Olfactory Reception - The identification of olfaction receptors is often uncertain, but it is reasonably certain that thin-walled basiconic pegs and coeloconic pegs are the olfactory receptor. Contact Chemoreception - This type of reception is usually accomplished by trichoid sensilla, which we have already discussed. Those trichoid sensilla associated with chemoreception usually have 4-6 neurons associated with the hair, one of which is usually involved with mechanoreception. Other receptors: There is some slight evidence that a few insects possess temperature receptors. Some insects also have humidity receptors. In some insects it is believed that humidity reception is by the basiconic pegs, while in others it is probably by coeloconic pegs. SOUND PRODUCTION Sounds produced as a byproduct of some other activity - Many sounds are produced by insects when they are feeding, cleaning, or copulating, but there is no evidence that any of these sounds have any particular significance. Sounds produced in flight, however, may have some significance. The wingbeat frequency is relatively constant within a species but may vary with temperature, age, and sex. Sounds produced by the impact of part of the body against the substratum - Some insects produce sound by striking the substratum, mostly without any related structural modifications. Some female Psocoptera have a small knob on the ventral surface of the abdomen with which they tap the ground. The death watch beetle produces tapping sounds by bending its head down and banging it against the floor of its burrow. Some grasshoppers drum on the ground with its hind tibia. Some termites bang different parts of their bodies against the substratum Sounds produced by frictional mechanisms - Many insects produce sounds by rubbing a roughened part of the body against another part. Often it is possible to distinguish a long ridged or roughened file, called the strigil, from the single scraper, called the plectrum. Frictional types of sound are produced by many orders of insects but are particularly associated with Orthoptera, Heteroptera, and Coleoptera. The Orthoptera 2 main methods are used: elytral stridulation found in Grylloidea and Tettigonioidea, and femoro-elytral stridulation found in Acridoidea. In the Heteroptera stridulation is most common in the Pentatomoid groups where 15 different methods have been recorded. The most common mechanisms involve a file on the ventral surface rubbed by a scraper on the leg, or a file on the wing rubbed by a scraper on the dorsal surface. In the Reduvioidea the file is between the front coxae which is rasped by the tip of the rostrum. In the Coleoptera many different parts of the body are used to stridulate, but most commonly the elytra are involved. In some Lepidoptera sound is produced by rubbing veins on the wings together Sounds produced by a vibrating membrane - Sounds produced by the vibration of a membrane driven directly by muscles are common amongst Homoptera and also occurs in some Heteroptera (Pentatomidae) and some Lepidoptera  (Arctiidae). This is most studied in the male Cicada. Sound is produced when the tymbal muscle contracts, pulling on the tymbal so that it buckles inward producing a click as it does. On relaxation of the tymbal muscle the tymbal returns to its normal position due to elasiticity of the surrounding cuticle and so produces a second click. There are often large air sacs below the membrane which help amplify the sound. Sounds produced by a pulsed stream of air - The only well documented case of a sound produced by a pulsed air stream is the stridulation of Acherontia (Lepidoptera). Air is sucked through the proboscis by dilation of the pharynx causing the epipharynx to vibrate and create a pulsed stream of air. INSECT CIRCULATORY SYSTEM Circulatory system - There are two types of circulatory systems in the animal kingdom. In many animals , the blood travels through vessels like arteries, capillaries, and veins. This is known as closed type of circulatory system. In insects the blood flows through the body cavity (i.e., heamocoel) irrigating various tissues and organs. It is known as open type of circulatory system. - Haemocoel of the insects is divided into 3 sinuses (or) regions due to the presence of two fibro muscular septa (or) diaphragms composed of connective tissues. Dorsal or Pericardial Sinus - The area lying in between the tergum and dorsal diaphragm. It contains the heart. Ventral or Perineural Sinus - The area lying in between the sternum and ventral diaphragm. It contains a nerve cord. Visceral Sinus - The area in between dorsal and ventral diaphragms. It harbour the visceral organs like alimentary canal and gonads. ORGANS ASSOCIATED WITH BLOOD CIRCULATION It is the main organ of circulation and consists of anterior aorta and posterior heart. Dorsal blood vessel The dorsal vessel is a simple tube closed at its posterior end and bears a number of vulvular openings called as ostia (prevents back flow of  haemolymph) The number of ostia are - 3 pairs - thorax - 9 pairs - abdomen Insects consists of sac like structures called accessory pulsatile organs, which are present at the base of the Accessory pulsatile organs appendages such as wings, legs and antenna. They pulsate independently and supply adequate blood to the appendages. Composition of Haemolymph - contains a fluid portion called plasma and cellular fractions called haemocytes. Plasma - is an aqueous solution of inorganic ions, lipids, sugars (mainly trehalose), amino acids, proteins, organic acids and other compounds. pH is usually acidic (6-7). Density is 1.01 to 1.06. Water content is 84-92 per cent. Inorganic ions present are `Na' in predators and parasites, `Mg' and `K‘ in phytophagous insects. Blood lacks vitamin ‘K’ Carbohydrate is in the form of trehalose sugar. Major proteins are lipoproteins, glycoproteins and enzymes. Lipids in the form of fat particles or lipoproteins. Glycerol is present which acts as a anti freezing compound Haemocytes - The blood cells or haemocytes are of several types and all are nucleate. DIFFERENT TYPES OF HAEMOCYTES Prohaemocyte Smallest of all cells with largest nucleus. Plasmatocyte (Phagocyte) It aids in phagocytocis. Granular hemocytes Contains large number of cytoplasmic inclusions. Spherule cell Cytoplasmic inclusions obscure the nucleus. Role in blood coagulation and Cystocyte (Coagulocyte) plasma precipitation. Oenocytoids Large cells with ecentric nucleus Adipo haemocytes Round or avoid with distinct fat droplets.  Large flattened cells with Podocyte number of protoplasmic projections. Vermiform cells Rare type, long thread like. Process of blood circulation: Heart mainly functions as a pulsatile organ whose expansion and contraction leads to blood circulation. It takes place generally in an anti clock manner starting from posterior end to the anterior end in a forward direction. Circulation of blood takes place in two phases due to the action of the alary muscles as well as the muscles of the walls of the heart. The two phases are 1. Diastole - During which heart expansion of heart takes place. - It results in increase of volume of heart and decrease in the area of pericardial sinus. This creates a pressure on the blood in the pericardial sinus forcing the blood to enter into the heart through the incurrent ostia. These incurrent ostia allow only the entry of blood from the sinus into the heart and prevent its backflow from the heart to the sinus. 2. Systole - Contraction of heart takes place - This creates pressure on the blood within the heart leading to its forward movement in to the aorta. From the aorta blood enters into the head and flows back bathing the visceral organs in the visceral sinus and neural cord in the perineural sinus. In between diastole and systole there will be a short period of rest which is known as diastasis. Functions of haemolymph 1. Lubricant - Haemolymph keeps the internal cells moist and the movement of internal organs is also made easy. 2. Hydraulic medium - Hydrostatic pressure developed due to blood pumping is useful in the following processes. Ecdysis (moulting) Wing expansion in adults Ecolosion in diptera (adult emergence from the puparium using ptilinum) Eversion of penis in male insects Eversion of osmeteria in papilionid larvae Eversion of mask in naiad of dragonfly. Maintenance of body shape in soft bodied caterpillars 3. Transport and storage  - Digested nutrients, hormones and gases (chironomid larva) were transported with the help of haemolymph. It also removes the waste materials to the excretory organs. Water and raw materials required for histogenesis is stored in haemolymph. 4. Protection - It helps in phagocytocis, encapsulation, detoxification, coagulation, and wound healing. Non cellular components like lysozymes also kill the invading bacteria. 5. Heat transfer - Haemolymph through its movement in the circulatory system regulate the body heat (Thermoregulation) 6. Maintenance of osmotic pressure - Ions, amino acids and organic acids present in the haemolymph helps in maintaining osmotic pressure required for normal physiological functions. 7. Reflex bleeding - Exudation of heamolymph through slit, pore etc. repels natural enemies. e.g. Aphids. 8. Metabolic medium - Haemolymph serves as a medium for on going metabolic reactions (trehalose is converted into glucose) TRACHEAL SYSTEM IN INSECTS Introduction - Gaseous exchange in insects is carried on through a system of internal tubes, the tracheae, the finer branches of which extend to all parts of the body and may become functionally intracellular in muscle fibers. Thus oxygen is carried directly to its sites of utilization and the blood is not concerned with gas transport. The tracheae open to the outside of the body through segmental pores called spiracles, which generally have some closing mechanism which keeps water loss at a minimum. The spiracles open in response to low internal concentration of oxygen or a high concentration of carbon dioxide in the tissues. - Diffusion alone can account for the gaseous requirements of most insects at rest, but larger insects and active insects require a better system - the tracheal system THE TRACHEAL SYSTEM The tracheae are the larger tubes of the tracheal system, running inwards from the spiracles and Tracheae usually breaking into finer branches the smallest of which are about 2 microns in diameter. Tracheae are ectodermal in origin and as such have a  cuticular lining which is shed during each molt. A spiral thickening of the intima runs along each tube, each ring of the spiral being called a taenidium. The taenidia prevent the collapse of the trachea if the pressure within is reduced. The intima consists of a layer of cuticulin, forming the surface lining the lumen, and a protein/chitin layer on the outside. In places tracheae are expanded to form thin-walled air-sacs in which the taenidia are absent or poorly developed. Consequently Air-sacs the air-sacs will collapse under pressure and they play a very important part in ventilation of the tracheal system as well as having other functions. Air-sacs occur in many insects. With the exception of some Diplura, the largest number of spiracles found in insects is 10 pairs, 2 thoracic and 8 abdominal, and the respiratory system can be classified on the basis of the number and Number and distribution of distribution of the functional spiracles spiracles: 1. Polypneustic - at least 8 functional spiracles on each side: Holopneustic - 10 spiracles - 1 mesothoracic, 1 metathoracic, 8 abdominal - as in bibionid larvae. Peripneustic - 9 spiracles - 1 mesothoracic, 8 abdominal - as in cecidomyid larvae. Hemipneustic - 8 spiracles - 1 mesothoracic, 7 abdominal - as in mycetophilid larvae.  2. Oligopneustic - 1 or 2 functional spiracles on each side: Amphipneustic - 2 spiracles - 1 mesothoracic, 1 post-abdominal - as in psychodid larvae. Metapneustic - 1 spiracle - 1 post-abdominal - as in culicid larvae. Propneustic - 1 spiracle - 1 mesothoracic - as in dipterous pupae. 3. Apneustic - no functional spiracles - as in chironomid larvae. Apneustic does not mean that the insect has no tracheal system, but rather that the tracheal system does not open externally These are the smaller branches of the tracheal system. There is no clear distinction between the tracheae and the tracheoles but the tracheoles are always intracellular and retain their Tracheoles cuticular lining at molting. Proximally the tracheoles are about 1 micron in diameter, tapering to about 0.1 micron distally. They are formed in cells called tracheoblasts which are derived from the epidermal cells lining the tracheae. The tracheoles are very intimately associated with the tissues and in fibrillar muscle they may indent the muscle plasma membrane and penetrate deep into the fiber, but it is probable that they never truly become  intercellular. Distally the tracheoles end blindly or they may anastomose. The tracheal system arises externally at the spiracles. In many of the Apterygota (except Lepismatidae) the tracheae from each spiracle form a series of unconnected tufts. But in most insects the tracheae from neighboring spiracles anastomose to form longitudinal trunks running the length of the Distribution of the tracheal body. Usually there is a lateral system trunk on either side of the body and these are often the largest tracheae. Also there is often a dorsal and a ventral longitudinal trunk. The longitudinal tracheae are often connected to those of the other side of the body by transverse commissures, while smaller branches extend to the various tissues which give rise to the tracheoles which run to the cells. The arrangement of the tracheal system varies in different insects, but in general the heart and dorsal muscles are supplied by branches from the dorsal trunks; the alimentary canal, gonads, legs and wings from the lateral trunks; and the central nervous system from the ventral trunks or transverse commissures. The head is supplied from spiracle 1 through 2 main tracheal branches on each side, a dorsal branch to the antennae, eyes, and brain, and a ventral branch to the mouthparts and their muscles. In some insects the connecting tubes are constricted which somewhat isolates the tracheal system of the head from the rest of the body ensuring a good supply of oxygen to the brain and major sense organs. Accordingly, the pterothorax in some insects also has its tracheal system somewhat isolated from the rest. Spiracles - The spiracles are the external openings of the tracheal system. They are lateral in position, usually on the pleura, and, except in Japyx (Diplura) which has 2 pairs of spiracles on the metathorax, there are never more than one pair per segment. Often the spiracle is contained in a small, distinct sclerite, the peritreme. Structure of the spiracles  - In its simplest form, in the Apterygota, the spiracle is a direct opening from the outside to the tracheae, but generally the visible opening leads into a cavity, the atrium, from which the tracheae arise. In this case the opening and the atrium collectively are called the spiracle. Often the walls of the atrium are lined with hairs which filter out the dust. In other insects there may be a sieve plate with small pores which filters out the dust. - The spiracles of most terrestrial insects have a closing mechanism which is important in the conservation of water. The closing mechanism may consist of 1 or 2 movable valves in the spiracular opening itself or it may be internal, closing off the atrium from the tracheae by means of a constriction. Control of spiracle opening - The spiracles are normally open for the shortest time necessary for efficient respiration in order to keep water loss from the tracheal system to a minimum. Spiracle closure results from the sustained contraction of the closer muscle, while opening commonly results from the elasticity of the surrounding cuticle when the closer muscle is relaxed. The muscle is controlled by the central nervous system, but may also respond to local chemical stimuli which interact with the central control Cutaneous respiration - Some gaseous exchange takes place through the cuticle of most insects, but this does not amount to very much of the total respiration. On the other hand, Protura and most Collembola have no tracheal system and must depend on cutaneous respiration together with transport from the body surface to the tissues by the haemolymph. Cutaneous respiration is also important in eggs, aquatic insects, and endoparasitic insects cicadas that functions as a resonating chamber to help amplify its call Respiration in Aquatic insect - Aquatic insects obtain oxygen from the air or from air dissolved in the water. Most get their oxygen directly from the air. This usually requires periodic visits to the surface. A few insects, however, maintain a semi- permanent connection with the air via a long respiratory siphon or through the aerenchyma of certain aquatic plants. Insects returning to the surface face 2 main problems. First they must be able to break the surface film of the water, and secondly they must be able to keep the water from entering the spiracles once they re-enter the water. Evaginated trachea or tracheoles. Found in immatures Tracheal gills Example: Stoneflies, Mayflies, some Odonata  Found in Odonata. Inside the rectum there are 5 tracheoles, water is taken into the rectum, Rectal gills the oxygen is removed, and then the water is expelled. Example: dragonfly naiad Found in aquatic Diptera. Often are insects that are associated with streams that periodically dry up. When in water they function as gills. If the stream Spiracular gills dries up part of the gills break off leaving a hole so air can enter directly. Example: Spiracular gills in pupae dipteran Found in Nepidae (water Respiratory tubes scorpions). Simply is a siphon or respiratory tube. Found in aquatic Hemiptera and Coleoptera. The insect captures a bubble of air at the surface and Air bubble carries it below the surface and uses it to breathe. Sometimes the oxygen diffuses from the water directly into the air bubble. This is called a hydrofuge and consists of special hairs which Plaston respiration trap a bubble. The hairs prevent the bubble from collapsing. The insect can stay underwater with 1 bubble for up to 4 months Respiration in Endoparasitic Insects - The majority of endoparasites obtain some oxygen by diffusion through the cuticle from the host tissues. Other insects, and particularly older, actively growing larvae, communicate with the outside air either through the body wall of the host or via its respiratory system. The majority of these insects are metapneustic or amphipneustic, using the posterior spiracles to obtain their oxygen. Hemogoblin  - Most insects have no respiratory pigments, but a few have haemoglobin in solution in the blood. The best known examples are the aquatic larvae of Chironomus and related insects, the aquatic bug Anisops, and the endoparasitic larvae of Gasterophilus (Diptera). The molecular weight of the haemoglobin in insects is about half that found in vertebrates indicating that insect haemoglobin probably consists of only 2 haem groups. INSECT DIGESTIVE AND EXCRETORY SYSTEM An insect uses its digestive system to extract nutrients and other substances from the food it consumes. Most of this food is ingested in the form of macromolecules and other complex substances (such as proteins, polysaccharides, fats, nucleic acids, etc.) which must be broken down by catabolic reactions into smaller molecules (i.e. aminoacids, simple sugars, etc.) before being used by cells of the body for energy, growth, or reproduction. This break-down process is known as digestion All insects have a complete digestive system. This means that food processing occurs within a tube-like enclosure, the alimentary canal, running lengthwise through the body from mouth to anus. Ingested food usually travels in only one direction. Most biologists regard a complete digestive system as an evolutionary improvement over an incomplete digestive system because it permits functional specialization — different parts of the system may be specially adapted for various functions of food digestion, nutrient absorption, and waste excretion. In most insects, the alimentary canal is subdivided into three functional regions: foregut (stomodeum), midgut (mesenteron), and hindgut (proctodeum) Stomodaeum An insect’s mouth, located centrally at the base of the mouthparts, is a muscular valve (sphincter) that marks the “front” of the foregut. Food in the buccal cavity is sucked through the mouth opening and into the pharynx by the action of cibarial muscles. These muscles are located between the head capsule and the anterior wall of the pharynx. When they contract, they create suction by enlarging the volume of the pharynx (like opening a bellows). This “suction pump” mechanism is called the cibarial pump. It is especially well-developed in insects with piercing/sucking mouthparts. From the pharynx, food passes into the esophagus by means of peristalsis (rhythmic muscular contractions of the gut wall). The esophagus is just a simple tube that connects the pharynx to the crop, a food-storage organ.  Food remains in the crop until it can be processed through the remaining sections of the alimentary canal. While in the crop, some digestion may occur as a result of salivary enzymes that were added in the buccal cavity and/or other enzymes regurgitated from the midgut. In some insects, the crop opens posteriorly into a muscular proventriculus. This organ contains tooth-like denticles that grind and pulverize food particles. The proventriculus serves much the same function as a gizzard in birds. The stomodeal valve, a sphincter muscle located just behind the proventriculus, regulates the flow of food from the stomodeum to the mesenteron In a developing embryo, the foregut arises as a simple invagination of the anterior body wall: this means that all of its tissues and organs are derived from embryonic ectoderm. In effect, the inside of the stomodeum is continuous with the outside of the insect’s body. Since exoskeleton is secreted to protect the insect externally, it is not surprising to find that cells lining the foregut produce a similar structure (known as the intima) to protect themselves from abrasion by food particles. The hard denticles inside the proventriculus are made from this same material Mesenteron The midgut begins just past the stomodeal valve. Near its anterior end, finger-like projections (usually from 2 to 10) diverge from the walls of the midgut. These structures, the gastric caecae, provide extra surface area for secretion of enzymes or absorption of water (and other substances) from the alimentary canal. The rest of the midgut is called the ventriculus — it is the primary site for enzymatic digestion of food and absorption of nutrients. Digestive cells lining the walls of the ventriculus have microscopic projections (microvilli) that increase surface area for nutrient absorption.  The midgut is derived from embryonic endoderm so it is not protected by an intima. Instead, the midgut is lined with a semipermeable membrane secreted by a cluster of cells (the cardial epithelium) that lie just behind the stomodeal valve. This peritrophic membrane consists of chitin fibrils embedded in a protein-carbohydrate matrix. It protects the delicate digestive cells without inhibiting absorption of nutrient molecules. The posterior end of the midgut is marked by another sphincter muscle, the pyloric valve. It regulates the flow of material from the mesenteron into the proctodaeum Proctodaeum The pyloric valve serves as a point of origin for dozens to hundreds of Malpighian tubules. These long, spaghetti-like structures extend throughout most of the abdominal cavity where they serve as excretory organs, removing nitrogenous wastes (principally ammonium ions, NH4+) from the hemolymph. The toxic NH4+ is quickly converted to urea and then to uric acid by a series of chemical reactions within the Malpighian tubules. The uric acid, a semi-solid, accumulates inside each tubule and is eventually emptied into the hindgut for elimination as part of the fecal pellet The rest of the hindgut plays a major role in homeostasis by regulating the absorption of water and salts from waste products in the alimentary canal. In some insects, the hindgut is visibly subdivided into an ileum, a colon, and a rectum. Efficient recovery of water is facilitated by six rectal pads that are embedded in the walls of the rectum. These organs remove more than 90% of the water from a fecal pellet before it passes out of the body through the anus. Embryonically, the hindgut develops as an invagination of the body wall (from ectodermal tissue). Just like the foregut, it is lined with a thin, protective layer of cuticle (intima) that is secreted by the endothelial cells of the gut wall. When an insect molts, it sheds and replaces the intima in both the foregut and the hindgut. INSECT ENDOCRINE SYSTEM - Nervous system regulates all physiological requirements of an insect including growth, reproduction, and protein formation through the endocrine system via hormones. - Hormones complement the nervous system, which provides short term coordination, and the activities of both systems are closely linked. Primary Function of Hormones & Homeostasis Growth and Development Reproduction Energy Metabolism Behavior Hormones - Chemical substances that are transported in the insect's body fluids (haemolymph) that carry messages away from their point of synthesis to sites that where physiological, behavioral and developmental processes are influenced The endocrine organs of insects are of two types (most of which are within the central nervous system) the specialized and neurosecretory cells. 1. Specialized endocrine glands Glands producing ecdysteroids - In the immature stages of all insects, molting hormones are produced by the prothoracic glands. - In females, where the same hormones are produced to regulate embryonic development, the follicle cells in the ovary are the principal source. - It may also produced in the abdomen of some insects Prothoracic glands - Diffuse, paired glands located at the back of the head or in the thorax. - These glands secrete an ecdysteroid called ecdysone, or the molting hormone, which initiates the epidermal molting process. - Additionally it plays a role in accessory reproductive glands in the female, differentiation of ovarioles and in the process of egg production. Corpora allata - Small, paired glandular bodies are usually one on either side of the esophagus. - They produce the juvenile hormone, which regulates metamorphosis and yolk synthesis and deposition in the oocytes of adults. Corpora cardiaca - A pair of neuroglandular bodies that are found behind the brain and on either side of the aorta. - The corpora cardiaca store and release hormones from the neurosecretory cells of the brain, to which they are connected by one or two pairs of nerves  Endocrine cells of the midgut - These are isolated cells scattered amongst the principal midgut cells. - They secrete some peptides which have a hormonal function relating to digestion and absorption, perhaps regulating the synthesis of digestive enzymes and post-feeding dieresis. - Other cells may have different functions, perhaps including the regulation of gut motility 2. Neurosecretory cell - Occur in the ganglia of the central nervous system. - These cells secrete hormones that may affect growth, reproduction, homeostasis and metamorphosis. - In the brain, two main groups of neurosecretory cells on each side. - The secretions of neurosecretory cells are usually neuropeptides The most common hormones that are secreted by these cells are: Ecdysiotropin - Protocerebrum secretes ecdysiotropin or prothoracicotropic hormone (PTTH) or brain hormone (BH) that acts on ecdysial glands Bursicon (Tanning hormone) - Triggers the tanning or darkening of adult cuticle Eclosion hormone - It is stored in the corpora cardiaca and is released into the blood at the time of switchover from pupil to adult stage initiating the pre-eclosion behavior. Endocrine control of growth and metamorphosis Upon emergence from the egg, the immature insects gradually increase in size to reach adults through some mechanisms called moulting. Moulting involves the periodic digestion of old cuticle, secretion of new cuticle (usually with larger surface area than the older one) and shedding of undigested old cuticle Shedding of undigested old cuticle is commonly referred to as ecdysis. Each developmental stage of the insect itself is called an instar, and the interval of time passed in that instar is referred to as stadium The whole developmental process by which the first instar immature stage of an insect is transformed into the adult insect is called metamorphosis Hormones required: Brain hormone, Ecdysone, Juvenile hormone Brain hormone (prothoracicotropic hormone (PTTH)) : - Protocerebrum secretes brain hormone (BH) or prothoracicotropic hormone (PTTH) or ecdysiotropin which accumulate in the carpora allata and subsequently released into the haemolymph (except in Lepidoptera, in other insects BH is stored in corpora cardiaca). Through the haemolymph, PTTH reach to prothoracic gland and stimulate its secretory activity - The prothoracic glands secrete a-ecdysone or moulting hormone (MH) which through haemolymph reach the target (epidermis) which initiates the growth and moulting activities of the cells. - Ecdysone favors the development of adult structures and favours the moulting processes that terminate into successive larval instars  - The corpora allata secrete juvenile hormone (JH), which promote larval development and inhibit development of adult characteristics - In fact, JH interacts with MH to stimulate larval maturation during each stage of development. The concentration of JH evidently decreases toward the end of a larval instar, allowing the ecdysone to cause moulting. - The total picture here should be one of balanced interaction-synergism-between these two hormones to induce normal growth and differentiation, rather than a simple antagonism. - During the last immature instar, two separate and distinct peaks of ecdysone are present in both the holometabola and hemimetabola. The first one is low and in absence of JH, the epidermal cells are reprogrammed from larval to pupal commitment in holometabolous insects, and from nymphal to adult stage in hemimetabolous insects Eclosion - Eclosion hormone or EH is released from the brain by a circadian clock and declining ecdysteroid titers. If ecdysone titer is artificially kept high, the release of eclosion and its activity are inhibited. - This hormone influences many aspects of pupal-adult ecdysis, including the behavior associated with ecdysis and subsequent degeneration of abdominal inter-segmental muscles used in the act of ecdysis. Ecdysis triggering hormone - It is the most recent hormone discovered that plays an important role in ecdysis. This 26 amino acid peptide hormone is synthesized by the epitracheal glands that are located segmentally in larvae, pupae and adults of Manduca sexta. According to Zitnan (1996), this hormone may act upstream from the eclosion hormone in a series of cascade events leading to ecdysis. Bursicon (Tanning Hormone) - Bursicon, commonly found in neurohaemal organs associated with the ventral chain ganglia is suggested to stimulate tanning and sclerotisation of the cuticle following ecdysis Hormonal control of reproduction - Like other higher multicellular organisms, reproduction in insects is a complex process. - Different stages of reproduction, starting from the production of male and female gametes to oviposition, are seem to be influenced by several hormones. Spermatogenesis - Ecdysone controls the permeability of the testis walls to the humoral factor differentiating the spermatocytes. - Juvenile hormone is shown to have some inhibitory effects on spermatogenesis in many insects Oogenesis - Hormones from corpora allata help in egg maturation through the incorporation of yolk into the oocyte.  - In addition to secretions from brain cells and corpora allata, ecdysone has been found to be involved in control of oogenesis in female mosquitoes. Following a blood meal, lateral neurosecretory cells secrete egg development neurosecretory hormone, which in turn, induces the ovary to secrete ecdysone. Ecdysone, in turn triggers the synthesis of yolk protein vitellogenin in the fat bodies. - Juvenile hormones secreted by corpora allata also activate fat body and ovaries Fertilization - In many insects studied, ovulation (the passage of egg from the ovary into the oviduct) and oviposition, (passage of fertilized eggs to the outside, are closely linked. Both these events are affected by some peptides secreted by female accessory glands and neurosecretory products of the brain. - The process of reproduction involves both the nervous and endocrine systems. The major centers are the neurosecretory cells of brain and the major events are the secretion of juvenile hormone by corpora allata, and either ecdysone production by ecdysial gland in immature insects or ecdysone biosynthesis by the ovary in adult insects. Both hormones act either independently or together in association with nervous system to make reproduction success Vitellogenesis - Vitellogenesis or egg yolk synthesis is also known to depend on JH from the corpora allata. In mosquitoes, juvenile hormone is required for egg development only during the early previtellogenic stages of development of the follicles. INSECT NERVOUS SYSTEM - The nervous system is the primary mechanism of conduction and control in the body. - In insects it serves as an elaborate (complex) connecting link between the sense organs and the effectors' organs (muscles and sometimes glands) Structures (Anatomy) - Cells  - Anatomy Functions - Signal transduction - Signal transmission Cells in the nerve system 1. Nerve cells (Neurons) - Conducting cells that transduce, transmit or process nerve impulses. 2. Glial cells - Non-conducting supporting cells that surround neurons and help to protect neurons and maintain stable ionic environment Neuron - The basic unit of the nervous system is the neuron - It consists of a nucleated cell b o d y (neurocyte) giving off slender cell extension (axon) - The neuron may have several dendrites, but only one axon. Axon - A slender cell extension arises from the cell body of the neuron which transmits nerve impulses from one cell to the next. Collaterals Lateral branches arising from the axon generally near its origin They are fibrils arising directly from the nerve cell body. They Dendrites are specialized for the reception of the stimuli and transmitting impulses towards the central cell body. Both axon and collateral end i n Terminal arborisation fine branching fibrils, these are the terminal arborizations. Ganglion  - The greater part of the neuron and their processes do not occur singly but are aggregated in a series of segmental ganglia. - The ganglia are united by longitudinal connectives which constitute the central nervous system. Neuron is similar to other cells, but: 1. Have specialized extensions called dendrites and axons. Dendrites bring information to the soma and axons take information away from the soma. 2. Neurons communicate with each other through specialized structures called synapses and chemicals (e.g. neurotransmitters). Neuron-neuron junction: synapse NEURON CLASSIFICATION Have one projection extending Unipolar from the soma (Most insect are of this type). Have two projection extending Bipolar from the soma (the peripheral sense cells are of this type). Have many projections Multipolar extending from the soma. However, each has only one axon. Types of neuron: two ways of classification  1. By the number of extensions Unipolar neurons - Have one projection extending from the soma. Bipolar neurons - Have two projection extending from the soma. Multipolar neurons - Have many projections extending from the soma. However, each has only one axon. 2. By the direction of information that they send (function) Afferent (sensory) neurons - Bipolar or multipolar cells have dendrites that are associated with sense organs. They carry information TOWARD the central nervous system (CNS). Efferent (motor) neurons - Unipolar cells that conduct signals AWAY from CNs and stimulate responses in muscles and glands. Interneuron (association neuron) - Unipolar cells that form connections between afferent and efferent neurons and conduct signals WITHIN CNS. Functionally: 1. Sensory (afferent) - Sensory neuron convey impulses inwards from these sense organs 2. Motor (efferent) - Neurons convey the impulses mainly outwards to the effector organs mainly the muscles. 3. Association (internuncial) - Association neurons link the sensory and motor neurons together within the central nervous system. Synapsis - The site at which the axon of one neuron contacts the dendrite of another is called a synapsis. It is the point at which neurons receive information from or convey it to other cells. Nerve Sheath - The ganglia of the nervous system is clothed in a non-nervous sheath which are differentiated into a non cellular neural lemalla and cellular perineurium. - The neural lamella comprises a thin outer homogeneous layer containing narrow filaments, and a much thicker layer which consists of collagen like fibrils-like protein in a matrix and neutral muco- polysaccharide. - The neural lamellar is probably secreted by cells in the perineurium and other cells may be contributed too.  - Neural lamella provides mechanical support for the central nervous system, holding the cells and axon together while permitting the flexibility necessitated by the movements of insect. They offer no resistance to diffusion of material from the hemolymph into the nerve cord. - The perineurium extends over the whole of the central system and the larger peripheral nerves, but it is absent from the fine branches. It forms the blood-brain barrier allowing the passage only of specific substances into the neural environment. Glial cells - The Glial cells form an insulating protective sheath almost wholly investing round each neuron. - They serve to insulate the axons from each other. - They also pass nutrient materials to the neuron. Extra - cellular spaces - These are spaces between the glial cells. - The fluid in the extra - cellular spaces bathes the nervous elements directly and is therefore of great importance in nervous conduction. - It differs in composition of ions from the hemolymph. The nerve term is commonly applied to bundles of sheathed neuronal extensions leading from ganglia to various parts of the body. A nerve generally includes both motor and sensory extensions. A nerve trunk is a larger nerve. The central nerve cords : are two longitudinal nerve trunks running side by side connecting the ventral chain of segmented ganglia. Where are motor neurons and interneurons? Ganglia - The somata of interneurons and motor neurons are aggregated to form the ganglia of the central nervous system. - The somata are grouped peripherally, and the center is occupied by the terminal arborizations of sensory axons, by the dentritic arborization of motor neurons, and by axons and arborizations of interneurons. - Primitively, a pair of ganglia is present ventrally in each postoral segment. - Some segmental ganglia fused to form the brain. - So the central nervous system consists of the brain followed by a series of segmental ganglia.  - Adjacent ganglia are joined by a pair of interganglionic connectives that contain only axons and glia; there are no soma or synapses. The first ganglion is the subesophageal ganglion There are three thoracic ganglia, but in some insects they fuse to form a single ganglion. The largest number of ganglia are the abdominal ganglia which are present in the abdomen. Sometime most or all of the ventral ganglia are fused to form a single compound ganglion as in the blood Sucking bug. INSIDE GANGLION Cell bodies cluster on outside ring Center region: axons and dendrites of interneurons a n d motor neurons AND axon arborizations of sensory neurons Center region = Neuropil INSIDE GANGLION: GLIAL CELLS Surround ganglion Neural lamella - Mechanical support for NS, secreted by perineurium Perineurium=brain-blood barrier - A layer of glial cells that maintain stable ionic environment Nerve sheath - lamella + perinerium  Surrounding individual nerve (axon) glial cells Protect, insulate and repair neuron Pass nutrients to nerve and control ionic environment More glial cells than neurons Nutrition of the ganglia - The ganglia are almost the only solid organs in the insect body - The respiratory needs are provided by the tracheae and tracheoles which penetrate with the ganglia. - There is no circulation of body fluids inside the ganglia. - Nutrient and excretion must be provided by diffusion through the sheath of the ganglia Anatomically, the insect nervous system is divided into 1. Central Nervous System (C.N.S.). 2. Visceral (Sympathetic) Nervous System (S.N. S.) 3. Peripheral Nervous System. - All the three parts are connected with each other Insect Nerve system: Anatomy Central nerve system (CNS) - Most ganglia included: brain + ventral nerve cord Stomatogastric nerve system (SNS) - Frontal ganglion + hypocerebral ganglion + ventricular ganglion; innervate muscles of the mouth cavity, foregut, midgut; and regulate food uptake and food transport Peripheral Nerve system (PNS) - all sensory neurons; not bundled in ganglia; located in integument. PNS: All sensory neurons In epidermis Sensory neurons Outside ganglion  SNS: FG, HG, and VG FG : Frontal ganglion HG : Hypocerebral ganglion VG : Ventricular ganglion or cluster of neurons FG: connect with tritocerebrum and HG, send axons to pharynx and esophagus. control of food passage through the gut and crop emptying HG: sends axons to C C and VG VG: associate with foregut and midgut CNS: Brain + ventral nerve cord Brain: a compound ganglion, major association center Ventral nerve cord: SG + TG + AG: local association center SG: a compound ganglion (mandible maxillae, and labium Brain: proto-, deuto- and tritocerebrum Brain: another perspective Mushroom body : Center for higher-order sensory integration and learning. - Receive information mainly from olfactory (antennal lobe). Hymenoptera also from optic lobe - Olfactory learning and memory, place memory, associative memory, and roles in motor control. Ventral Nerve Cord - It consists of series of ganglia lying on the floor of the thorax and abdomen. - They are united by a pair of connectives. - Thoracic ganglia : The first 3 - ganglia which are situated one in each of the thoracic segment. Function : Control the locomotor organs; it gives off 1. A pair of nerves supply the general muscles. 2. A pair innervates muscles of legs. Meso - and Meta thoracic ganglion: They have the above (1), (2), and a 3rd pair controls movements of wings. - Abdominal ganglia : They are variable in number. Cerebral development 1. The volume of the brain is changed in different insects. 2. The optic lobes : developed in proportional to the size of the eyes. 3. The antennary lobes : related to the development of senses and organs connected with them. 4. Mushroom bodies : Attain their greatest size in Hymenoptera with elaborate behavior, as there exists structural differences in the brain of drone, worker and queen bees correlated with the degree of activities Ventral Nerve Cord 1. Primitive; Thysanura & Dictyoptera: - Suboesophageal g. - Thoracic g. - Abdominal g. Separate and visible (all the above ganglia are separate) - The most posterior being a compound centre 2. Orthoptera & Hymenoptera : - Suboesophageal, Pro - & Meso- thoracic g. (all are separate & visible) - Metathoracic + 1 st (1- 3) abdominal are joined. - The 7th & the subsequent form a compound centre. 3. Heteroptera : - Suboesophageal & prothoracic ganglia (are separate) - All the others are fused. 4. Higher Diptera, Homoptera: - Suboesophageal, Single compound thoracic - abdominal ganglion. 5. Coleoptera larvae - Suboesophageal & all the ventral ganglia are fused Roles of central complex Control of locomotor activity, particularly flight and walking center for direction perception and spatial navigation Coordinates L and R brain.  1. Specialized endocrine glands - Producing ecdysteroids - Prothoracic glands - Corpora allata - produce juvenile hormones - Corpora cardiaca - Endocrine cells of the midgut - function for gut motility 2. Neurosecretory glands - Occur in the ganglia of CNS - Affect growth, reproductions, homeostasis, and metamorphosis - Neuropeptides - Ecdysiotropin (Brain hormone) (Protoracicotrophic hormone / PTTH) - Bursicon (tanning hormone) - color of the insect. Sclerotasition of cuticle - Eclosion hormone (EH)- stored in corpora cardiaca, switch from pupa to adult (pupa-adult ecdysis) Insect head Position of the head or mouth parts relative to the body 1. Hypognathous - head is vertical and mouthparts are ventral pointing downwards (primitive type) eg. grasshoppers, cockroach 2. Prognathous - head is horizontal and mouthparts are anterior in position. Eg. soldiers of termites and larvae of endopterygota 3. Opisthognathous - head is deflexed backwards so that mouthparts/proboscis slopes backwards between the front legs. Eg. homoptera and hemiptera Thorax 1. Neck region or cervix - narrow passageway between the head and the thorax. Allow for the passage of all internal organ systems. 2. Prothorax 3. Pterothorax Abdomen - Segmentation is evident - 11 segments (8 and 9 reproductive segment female; 9 reproductive segment male) - In grasshoppers, a pair of tympanum is going one on either side of the the first abdominal segment 1. Styli - paired tube like outgrowths 2. Collophore 3. Retinaculum 4. Cerci - 11th abdominal segment Haemocytes 1. Prohaemocyte - smallest of all cells with largest nucleus  2. Plasmotocyte Tracheal system - Gaseous exchange - Tracheae (internal tubes) - Spiracles (segmental pores) Different parts of insect’s body