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Introduction
Although diverse, insects are quite similar in overall design, internally and externally. The insect is made up of three main body regions (tagmata), the head, thorax and abdomen. The head comprises six fused segments with compound eyes, ocelli, antennae and mouthparts, which differ according to the insect’s particular diet, e.g. grinding, sucking, lapping and chewing. The thorax is made up of three segments the pro, meso and meta, each supporting a pair of legs and these can also differ depending on function, e.g. jumping, digging, swimming and running. Often the middle and the last segment of the thorax have wings. The abdomen generally comprises eleven segments and contains the digestive and reproductive organs. A general overview of the internal structure and physiology of the insect is presented including digestive, circulatory, respiratory, muscular, endocrine, nervous systems, including sensory organs, temperature control, and flight and moulting (McGavin, 2001).

Digestive System
Insects possess a coiled tube or an alimentary canal extending from the mouth to the anus that has specific sections for grinding and food storage, enzyme production and nutrient absorption (McGavin, 2001; Triplehorn & Johnson, 2005). Sphincters control the food and fluid movement between three regions. The three regions include the foregut (stomatodeum), the midgut (mesenteron), and the hindgut (proctodeum).

The stomatedeum and proctodeum are invaginations of the epidermis and are lined with cuticle (intima). The mesenteron is not lined with cuticle but with rapidly dividing and therefore constantly replaced, epithelial cells (McGavin, 2001; Triplehorn & Johnson, 2005). The cuticle sheds with every moult along with the exoskeleton (Triplehorn & Johnson, 2005). Food is moved down the gut by muscular contractions called peristalsis (Elzinga, 2004).

1. Stomatodeum this region stores, grinds and transports food to the next region (Gullan & Cranston, 2005). Included in this are the pharynx, the oesophagus, the crop (stores food), and proventriculus or gizzard (grinds food) (Triplehorn & Johnson, 2005). Salivary secretions from the labial glands dilute the ingested food. In mosquitoes (Diptera), which are blood-feeding insects, anticoagulants and blood thinners are also released here.

2. Mesenteron digestive enzymes in this region are produced and secreted into the lumen and here nutrients are absorbed into the insect’s body. Food is enveloped by this part of the gut as it arrives from the foregut by the peritrophic membrane which is a mucopolysaccharide layer secreted from the midgut’s epithelial cells (McGavin, 2001). It is thought that this membrane prevents food pathogens from contacting the epithelium and attacking the insects’ body (McGavin, 2001). It also acts as a filter allowing small molecules through, but preventing large molecules and particles of food from reaching the midgut cells (Gullan & Cranston, 2005). After the large substances are broken down into smaller ones, digestion and consequent nutrient absorption takes place at the surface the epithelium (McGavin, 2001).

3. Proctodeum this is divided into three sections; the anterior is the ileum, the middle portion, the colon, and the wider, posterior section is the rectum (Gullan & Cranston, 2005). This extends from the pyloric valve which is located between the mid and the hindgut to the anus (Triplehorn & Johnson, 2005). Here absorption of water, salts and other beneficial substances take place before excretion (Gullan & Cranston, 2005). Like other animals, the removal of toxic metabolic waste requires water. However, for very small animals like insects, water conservation is a priority. Because of this, blind-ended ducts called Malpighian tubules come into play. These ducts emerge as evaginations at the anterior end of the hindgut and are the main organs of osmoregulation and excretion (Triplehorn & Johnson, 2005; Gullan & Cranston, 2005). These extract the waste products from the haemolymph, in which all the internal organs are bathed (McGavin, 2001). Effectively, they continually produce the insect’s urine, which is transported to the hindgut. The excretory organs and osmoregulation in insects are not orchestrated by the Malpighian tubules alone, but require a joint function of the ileum and/or rectum, (McGavin, 2001).

Interestingly, only aphids lack these tubules. The hindgut and rectum reabsorb important salts and water, and when the excrement or frass is voided, it is in the form of uric acid granules, which are non-toxic.

Circulatory System
The insect blood or haemolymph’s main function is that of transport and it bathes the insect’s body organs. Making up usually less than 25 % of an insect’s body weight, it transports hormones, nutrients and wastes and has a role in, osmoregulation, temperature control, immunity, storage (water, carbohydrates and fats) and skeletal function where it has a role in moulting (McGavin, 2001; Triplehorn & Johnson, 2005). An additional role of the haemolymph in some orders, can be that of predatory defence. It can contain unpalatable and malodourous chemicals that will act as a deterrent to predators (Gullan & Cranston, 2005).

Haemolymph contains molecules, ions and cells (Gullan & Cranston, 2005). Regulating chemical exchanges between tissues, haemolymph is encased in the insect body cavity or haemocoel (Elzinga, 2004; Gullan & Cranston, 2005). It is transported around the body by combined heart (posterior) and aorta (anterior) pulsations which are located dorsally just under the surface of the body (McGavin, 2001; Gullan & Cranston, 2005; Triplehorn & Johnson, 2005). It differs from vertebrate blood in that it doesn’t contain any red blood cells and therefore without high oxygen carrying capacity, it is more similar to lymph (Elzinga, 2004; Gullan & Cranston, 2005).

Body fluids enter through one way valved ostia which are openings situated along the length of the combined aorta and heart organ. Pumping of the haemolymph occurs by peristaltic contraction which moves through the aorta to underneath the brain where it flows out into the haemocoel (Elzinga, 2004; Gullan & Cranston, 2005). This originates at the body’s posterior end, pumping forwards into the dorsal vessel, out via the aorta and then into the head (Gullan & Cranston, 2005). Movement of haemolymph is particularly important for thermoregulation in orders such as Odonata, Lepidoptera, Hymenoptera and Diptera (Gullan & Cranston, 2005). Pumping rate increases due to periods of increased activity (Triplehorn & Johnson, 2005). The haemolymph is circulated to the appendages unidirectionally with the aid of muscular pumps or accessory pulsatile organs (Gullan & Cranston, 2005). These occur at the base of wings and antennae and are sometimes found in the legs (Gullan & Cranston, 2005).

Respiratory system
Insects obtain oxygen from the atmosphere and eliminate carbon dioxide via their cells (Gullan & Cranston, 2005). The respiratory system is a network of branched tubes which deliver gases directly to the body’s cells which are necessary for metabolism (McGavin, 2001; Elzinga, 2004). Oxygen enters the body via openings called spiracles. These openings are positioned laterally in the pleural wall, usually a pair on the anterior margin of the meso and meta thorax, and pairs on each of the eight or less abdominal segments, Numbers of spiracles vary from 1 to 10 pairs (McGavin 2001; Elzinga, 2004; Gullan & Cranston, 2005; Triplehorn & Johnson, 2005). The oxygen passes through the tracheae to the trachioles, and enters the body by the process of diffusion. Carbon dioxide leaves the body by the same process (Triplehorn & Johnson, 2005).

The major tracheae are thickened spirally like a flexible vacuum hose to prevent them from collapsing and often swell into air sacs. Larger insects can augment the flow of air through their tracheal system, with body movement and rhythmic flattening of the tracheal air sacs (Triplehorn & Johnson, 2005). Spiracles are closed and opened by means of valves and can remain partly or completely closed for extended periods in some insects, which minimises water loss (McGavin, 2001; Triplehorn & Johnson, 2005,).

Terrestrial and a large proportion of aquatic insects perform gaseous exchange as previously mentioned under an open system. Other smaller numbers of aquatic insects have a closed tracheal system, for example, Odonata, Tricoptera, Ephemoptera, which have tracheal gills and no functional spiracles. Endoparasitic larvae are without spiracles and also operate under a closed system. Here the tracheae separate peripherally, covering the general body surface which results in a cutaneous form of gaseous exchange. This peripheral tracheal division may also lie within the tracheal gills where gaseous exchange may also take place (Gullan & Cranston, 2005).

Muscular system
Many insects are able to lift 20 times their own body weight and may jump distances that are many times greater than their own length. This is not because they are strong but because they are so small. Muscle power varies with the size of its cross- sectional area or what the muscle moves, i.e the insect’s body mass varies with the cube of its dimensions. Simply, the smaller the body, the more powerful the muscle. (Elzinga, 2004; Triplehorn & Johnson, 2005).

The muscular system of insects ranges from a few hundred muscles to a few thousand (Triplehorn & Johnson, 2005). Unlike vertebrates that have both smooth and striated muscles, insects have only striated muscles. Muscle cells are amassed into muscle fibres and then into the functional unit, the muscle (Elzinga, 2004). Muscles are attached to the body wall, with attachment fibres running through the cuticle and to the epicuticle, where they can move different parts of the body including appendages such as wings (Gullan & Cranston, 2005; Triplehorn & Johnson, 2005). The muscle fibre has many cells with a plasma membrane and outer sheath or sarcolemma (Gullan & Cranston, 2005). The sarcolemma is invaginated and can make contact with the tracheole carrying oxygen to the muscle fibre. Arranged in sheets or cylindrically, contractile myofibrils run the length of the muscle fibre. Myofibrils comprising a fine actin filament enclosed between a thick pair of myosin filaments slide past each other instigated by nerve impulses (Gullan & Cranston, 2005).

Muscles can be divided into four categories:

1. Visceral these muscles surround the tubes and ducts and produce peristalsis as demonstrated in the digestive system.

2. Segmental causing telescoping of muscle segments required for moulting, increase in body pressure and locomotion in legless larvae.

3. Appendicular originating from either the sternum or the tergum and inserted on the coxae these muscles move appendages as one unit. (Elzinga, 2004) These are arranged segmentally and usually in antagonistic pairs (Triplehorn & Johnson, 2005). Appendage parts of some insects, e.g the galea and the lacinia of the maxillae, only have flexor muscles. Extension of these structures is by haemolymph pressure and cuticle elasticity (Triplehorn & Johnson, 2005).

4. Flight flight muscles are the most specialised category of muscle and are capable of rapid contractions. Nerve impulses are required to initiate muscle contractions and therefore flight. These muscles are also known as neurogenic or synchronous muscles. This is because there is a one to one correspondence between action potentials and muscle contractions. In insects with higher wing stroke frequencies the muscles contract more frequently than at the rate that the nerve impulse reaches them and are known as asynchronous muscles (McGavin, 2001; Gullan & Cranston, 2005).

Flight has allowed the insect to disperse, escape from enemies, environmental harm, and colonise new habitats (McGavin, 2001). One of the insect’s key adaptations, the mechanics of flight differ from other flying animals because their wings are not modified appendages (McGavin, 2001; Elzinga, 2004). Fully developed and functional wings occur only in adult insects (Gullan & Cranston, 2005). To fly, gravity and drag (air resistance to movement) has to be overcome (Gullan & Cranston, 2005). Most insects fly by beating their wings and to power their flight they have either direct flight muscles attached to the wings, or an indirect system where there is no muscle to wing connection and instead they are attached to a highly flexible box like thorax (Gullan & Cranston, 2005).

Direct flight muscles generate the upward stroke by the contraction of the muscles attached to the base of the wing inside the pivotal point. Outside the pivotal point the downward stroke is generated through contraction of muscles that extend from the sternum to the wing. Indirect flight muscles are attached to the tergum and sternum. Contraction makes the tergum and base of the wing pull down. In turn this movement lever the outer or main part of the wing in strokes upwards. Contraction of the second set of muscles, which run from the back to the front of the thorax, powers the downbeat. This deforms the box and lifts the tergum (Gullan & Cranston, 2005).

Endocrine system
Hormones are the chemical substances that are transported in the insect’s body fluids (haemolymph) which carry messages away from their point of synthesis to sites that where physiological processes are influenced. These hormones are produced by glandular, neuroglandular and neuronal centres (Gullan & Cranston, 2005). Insects have several organs that produce hormones, controlling reproduction, metamorphosis and moulting (Triplehorn & Johnson, 2005). It has been suggested that a brain hormone is responsible for caste dermination in termites and diapause interruption in some insects (Triplehorn & Johnson, 2005).

Four endocrine centres have been identified:-

1. Neurosecretory cells in the brain can produce one or more hormones that affect growth, reproduction, homeostasis and metamorphosis (Gullan & Cranston, 2005; Triplehorn & Johnson, 2005).

2. Corpora cardiaca are a pair of neuroglandular bodies that are found behind the brain and on either sides of the aorta. These not only produce their own neurohormones but they store and release other neurohormones including PTTH prothoracicotropic hormone (brain hormone), which stimulates the secretory activity of the prothoracic glands, playing an integral role in moulting.

3. Prothoracic glands are diffuse, paired glands located at the back of the head or in the thorax. These glands secrete an ecdysteroid called ecdysone, or the moulting hormone, which initiates the epidermal moulting process (Gullan & Cranston, 2005). Additionally it plays a role in accessory reproductive glands in the female, differentiation of ovarioles and in the process of egg production.

4. Corpora allata are small-paired glandular bodies originating from the epithelium located on either sides of the foregut. They secrete the juvenile hormone, which regulate reproduction and metamorphosis (Gullan & Cranston, 2005; Triplehorn & Johnson, 2005).

The Nervous System
Insects have a complex nervous system which incorporates a variety of internal physiological information as well as external sensory information (Gullan & Cranston, 2005) Like invertebrates, the basic component is the neuron or nerve cell. This is made up of a dendrite with two projections that receive stimuli and an axon, which transmits information to another neuron or organ, like a muscle. As for vertebrates, chemicals (neurotransmitters such as acetylcholine and dopamine) are released at synapses (Gullan & Cranston, 2005).

Central nervous system An insect’s sensory, motor and physiological processes are controlled by the central nervous system along with the endocrine system (Gullan & Cranston, 2005). Being the principal division of the nervous system, it consists of a brain and a suboesophageal ganglion. This is connected to the brain by two nerves, extending around each side of the oesophagus. The ventral nerve cord extends from the suboesophageal ganglion posteriorly (Triplehorn & Johnson, 2005). A layer of connective tissue called the neurolemma covers the brain, ganglia, major peripheral nerves and ventral nerve cords.

The brain has three lobes:

1. Procerebrum innervating the compound eyes and the ocelli

2. Deutcerebrum innervating the antennae

3. Tritocerebrum innervating the foregut and the labrum (Gullan & Cranston, 2005; Triplehorn & Johnson, 2005).

The ganglia of the central nervous system act as the coordinating centres with their own specific autonomy where each may coordinate impulses in specified regions of the insect’s body (Triplehorn & Johnson, 2005).

Peripheral nervous system This consists of motor neuron axons that branch out to the muscles from the ganglia of the central nervous system, parts of the sympathetic nervous system and the sensory neurons of the cuticular sense organs that receive chemical, thermal, mechanical or visual stimuli from the insects environment (Gullan & Cranston, 2005). The sympathetic nervous system includes nerves and the ganglia that innervate the gut both posteriorly and anteriorly, some endocrine organs, the spiracles of the tracheal system and the reproductive organs (Gullan & Cranston, 2005).

Sense Organs Chemical senses include the use of chemoreceptors, related to taste and smell, affecting mating, habitat selection, feeding and parasite-host relationships. Taste is usually located on the mouthparts of the insect but in some insects, such as bees, wasps and ants, taste organs can also be found on the antennae. Taste organs can also be found on the tarsi of moths, butterflies and flies. Olfactory sensilla enable insects to smell and are usually found in the antennae (McGavin, 2001). Chemoreceptor sensitivity related to smell in some substances, is very high and some insects can detect particular odours that are at low concentrations miles from their original source (Triplehorn & Johnson, 2005).

Mechanical senses, provide the insect with information that may direct orientation, general movement, flight from enemies, reproduction and feeding and are elicited from the sense organs that are sensitive to mechanical stimuli such as pressure, touch and vibration (Triplehorn & Johnson, 2005). Hairs on the cuticle are responsible for this as they are sensitive to vibration touch and sound (McGavin, 2001).

Hearing structures or tympanal organs are located on different body parts such as, wings, abdomen, legs and antennae. These can respond to various frequencies ranging from 100 to 240 khz depending on insect species (Triplehorn & Johnson, 2005). Many of the joints of the insect have tactile setae that register movement. Hair beds and groups of small hair like sensilla, determine proprioreception or information about the position of a limb, and are found on the cuticle at the joints of segments and legs. Pressure on the body wall or strain gauges are detected by the campiniform sensilla and internal stretch receptors sense muscle distension and digestive system stretching (McGavin 2001; Triplehorn & Johnson, 2005,).

The compound eye and the ocelli supply insect vision. The compound eye consists of individual light receptive units called ommatidia. Some ants may have only one or two, however dragonflies may have over 10,000. The more ommatidia the greater the visual acuity. These units have a clear lens system and light sensitive retina cells. By day, the image flying insects receive is made up of a mosaic of specks of differing light intensity from all the different ommatidia. At night or dusk, visual acuity is sacrificed for light sensitivity (McGavin, 2001). The ocelli are unable to form focussed images but are sensitive mainly, to differences in light intensity (Triplehorn & Johnson, 2005) Colour vision occurs in all orders of insects. Generally insects see better at the blue end of the spectrum than at the red end. In some orders sensitivity ranges can include ultraviolet (McGavin, 2001).

A number of insects have temperature and humidity sensors (McGavin, 2001) and insects being small, cool more quickly than larger animals. Insects are generally considered cold-blooded or ectothermic, their body temperature rising and falling with the environment. However, flying insects raise their body temperature through the action of flight, above environmental temperatures (Elzinga, 2004; Triplehorn & Johnson, 2005).

The body temperature of butterflies and grasshoppers in flight may be 5oC or 10oC above environmental temperature, however moths and bumblebees, insulated by scales and hair, during flight, may raise flight muscle temperature 20-30oC above the environment temperature. Most flying insects have to maintain their flight muscles above a certain temperature to gain power enough to fly. Shivering, or vibrating the wing muscles allow larger insects to actively increase the temperature of their flight muscles, enabling flight (Triplehorn & Johnson, 2005).

Reproductive System
Most insects have a high reproductive rate. With a short generation time, they evolve faster and can adjust to environmental changes more rapidly than other slower breeding animals (McGavin, 2001). Although there are many forms of reproductive organs in insects, there remains a basic design and function of each reproductive part. These individual parts may vary in shape (gonads), position (accessory gland attachment), and number (testicular and ovarian glands), with different insect groups (Gullan & Cranston, 2005).

Female Reproductive System The female insect’s main reproductive function is to produce eggs, including the egg’s protective coating, and to store the male spermatozoa until egg fertilisation is ready. The female reproductive organs include, paired ovaries which empty their eggs (oocytes) via the calyces into lateral oviducts, joining to form the common oviduct. The opening (gonopore) of the common oviduct is concealed in a cavity called the genital chamber and this serves as a copulatory pouch (bursa copulatrix), when mating (Gullan & Cranston, 2005). The external opening to this is the vulva. Often in insects the vulva is narrow and the genital chamber becomes pouch or tube like and is called the vagina. Related to the vagina is a saclike structure, the spermatheca, where spermatozoa are stored ready for egg fertilisation.

A secretory gland (Gullan & Cranston, 2005; Triplehorn & Johnson, 2005) nourishes the contained spermatozoa here. Egg development is mostly completed by the insect’s adult stage and is controlled by hormones that control the initial stages of oogenesis and yolk deposition (Gullan & Cranston, 2005). Most insects are oviviparous, where the young hatch after the eggs have been laid (Triplehorn & Johnson, 2005).

In simple terms insect sexual reproduction starts with sperm entry that stimulates oogenesis, meiosis occurs , the egg moves down the genital tract. Accessory glands of the female secrete an adhesive substance to attach eggs to an object or supplies material that provides the eggs with a protective coating and oviposition takes place via the female ovipositor (Elzinga, 2004; Triplehorn & Johnson, 2005).

Male reproductive system The male’s main reproductive function is to produce and store spermatozoa and provide transport to the reproductive tract of the female (Gullan & Cranston, 2005). Sperm development is usually completed by the time the insect reaches adulthood (Triplehorn & Johnson, 2005). The male has two testes, which contain follicles in which the spermatozoa are produced. These open separately into the sperm duct or vas deferens and this stores the sperm (Gullan & Cranston, 2005). The vas deferentia then unite posteriorally to form a central ejaculatory duct, this opens to the outside on an aedeagus or a penis (Triplehorn & Johnson, 2005). Accessory glands secrete fluids that comprise the spermatophore. This is the package that surrounds and carries the spermatozoa, forming a sperm-containing capsule (Gullan & Cranston, 2005; Triplehorn & Johnson, 2005,).

Sexual and asexual reproduction Most insects reproduce via sexual reproduction, i.e the egg is produced by the female, fertilised by the male and oviposited by the female. Eggs are usually deposited in a precise microhabitat on or near the required food (Elzinga, 2004). However, some adult females can reproduce without male input. This is known as parthenogenesis and in the most common type of parthenogenesis the offspring are essentially identical to the mother. This is most often seen in aphids and scale insects (Elzinga, 2004).

Metamorphosis and insect’s life cycle
An insect’s life-cycle can be divided into three types:

1. Ametabolous no metamorphosis, these insects are primitively wingless where the only difference between adult and nymph is size, e.g Order: Thysanura (Triplehorn & Johnson, 2005).

2. Hemimetabolous or incomplete metamorphosis. Here the young are called nymphs and are usually similar to the adult. Wings appear as buds on the nymphs or early instars. When the last moult is completed the wings expand to the full adult size, e.g. Order: Odonata.

3. Holometabolus, or complete metamorphosis. These insects usually have a different form in their immature and adult stages, have different behaviours and live in different habitats. The immature form is called larvae and remains similar in form but increases in size. They usually have chewing mouthparts even if the adult form mouth parts suck. At the last larval instar phase the insect forms into a pupa, it doesn’t feed and is inactive, and here wing development is completed and the adult emerges e.g Order: Lepidoptera, (Triplehorn & Johnson, 2005).

Moulting
As an insect grows it needs to replace the rigid exoskeleton regularly (McGavin 2001; Triplehorn & Johnson, 2005,). Moulting may occur up to 3 or 4 times or, in some insects 50 times or more during its life (McGavin, 2001). A complex process controlled by hormones, it includes the cuticle of the body wall, the cuticular lining of the tracheae, foregut, hindgut and endoskeletal structures (McGavin 2001; Triplehorn & Johnson, 2005).

The stages of moulting:

1. Apolysis moulting hormones are released into the haemolymph and the old cuticle separates from the underlying epidermal cells. The epidermis increases in size due to mitosis and then the new cuticle is produced. Enzymes secreted by the epidermal cells digest the old endocuticle, not affecting the old sclerotised exocuticle.

2. Ecdysis this begins with the splitting of the old cuticle, usually starting in the midline of the of the thorax’s dorsal side. The rupturing force is mostly from haemolymph pressure that has been forced into thorax by abdominal muscle contraction caused by the insect swallowing air or water. After this the insect wriggles out of the old cuticle.

3. Sclerotinisation after emergence the new cuticle is soft, this a particularly vulnerable time for the insect as its hard protective coating is missing. After an hour or two the exocuticle hardens and darkens. The wings expand by the force of haemolymph into wing veins (McGavin, 2001).

References:
Elzinga R J (2004) Fundamentals of Entomology (6th Ed) Pearson / Prentice Hall, New Jersey USA

Gullan D J and Cranston P S (2005) The Insects: An Outline of Entomology  Blackwell Publishing Ltd, U.K.

McGavin G C (2001) Essential Entomology; An order by order introduction. Oxford University Press, New York

Triplehorn, C A and Johnson N F (2005) Borror and DeLong’s Introduction to the Study of Insects (7th Ed). Brooks / Thomson Cole USA