Insect physiology includes thephysiology andbiochemistry ofinsectorgan systems.[1]
Although diverse, insects are quite similar in overall design, internally and externally. Theinsect is made up of three main body regions (tagmata), the head, thorax and abdomen.The head comprises six fused segments withcompound 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 thorax, each supporting a pair of legs which may also differ, depending on function, e.g. jumping, digging, swimming and running. Usually the middle and the last segment of the thorax have paired wings. The abdomen generally comprises eleven segments and contains the digestive and reproductive organs.[2]A general overview of the internal structure andphysiology of the insect is presented, including digestive, circulatory, respiratory, muscular, endocrine and nervous systems, as well assensory organs, temperature control, flight andmolting.
An insect usesits digestive system to extract nutrients and other substances from the food it consumes.[3]
Most of this food is ingested in the form ofmacromolecules and other complex substances (such asproteins,polysaccharides,fats, andnucleic acids) which must be broken down bycatabolic reactions into smaller molecules (i.e.amino acids,simple sugars, etc.) before being used by cells of the body for energy, growth, or reproduction. This break-down process is known asdigestion.
The insect's digestive system is a closed system, with one long enclosed coiled tube called thealimentary canal which runs lengthwise through the body. The alimentary canal only allows food to enter the mouth, and then gets processed as it travels toward theanus. The alimentary canal has specific sections for grinding and food storage,enzyme production, andnutrient absorption.[2]
[4]Sphincters control the food and fluid movement between three regions. The three regions include the foregut (stomatodeum)(27,) the midgut (mesenteron)(13), and the hindgut (proctodeum)(16).
In addition to the alimentary canal, insects also have pairedsalivary glands and salivary reservoirs. These structures usually reside in the thorax (adjacent to the fore-gut). The salivary glands (30) produce saliva; the salivary ducts lead from the glands to the reservoirs and then forward through the head to an opening called thesalivarium behind thehypopharynx; which movements of the mouthparts help mix saliva with food in the buccal cavity. Saliva mixes with food, which travels through salivary tubes into the mouth, beginning the process of breaking it down.[3][5]
The stomatodeum and proctodeum are invaginations of theepidermis and are lined withcuticle (intima). The mesenteron is not lined with cuticle but with rapidly dividing and therefore constantly replaced,epithelial cells.[2][4] The cuticle sheds with everymoult along with theexoskeleton.[4] Food is moved down the gut by muscular contractions calledperistalsis.[6]
The main function of insect blood, hemolymph, 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 transportshormones, nutrients and wastes and has a role in osmoregulation, temperature control,immunity, storage (water,carbohydrates and fats) and skeletal function. It also plays an essential part in the moulting process.[2][4] An additional role of the hemolymph in some orders, can be that of predatory defence. It can contain unpalatable and malodourous chemicals that will act as a deterrent to predators.[7]
Hemolymph contains molecules, ions and cells.[7] Regulating chemical exchanges betweentissues, hemolymph is encased in the insect body cavity orhaemocoel.[6][7] It is transported around the body by combined heart (posterior) andaorta (anterior) pulsations which are located dorsally just under the surface of the body.[2][4][7] It differs fromvertebrate blood in that it doesn't contain any red blood cells and therefore is without high oxygen carrying capacity, and is more similar tolymph found in vertebrates.[6][7]
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 hemolymph occurs by waves of peristaltic contraction, originating at the body's posterior end, pumping forwards into the dorsal vessel, out via the aorta and then into the head where it flows out into the haemocoel.[6][7] The hemolymph is circulated to the appendages unidirectionally with the aid of muscular pumps or accessory pulsatile organs which are usually found at the base of theantennae or wings and sometimes in the legs.[7] Pumping rate accelerates due to periods of increased activity.[4] Movement of hemolymph is particularly important for thermoregulation in orders such asOdonata,Lepidoptera,Hymenoptera andDiptera.[7]
Insect respiration is accomplished withoutlungs using a system of internal tubes and sacs through which gases either diffuse or are actively pumped, delivering oxygen directly to tissues that need oxygen and eliminatecarbon dioxide via theircells.[7] Since oxygen is delivered directly, the circulatory system is not used to carry oxygen, and is therefore greatly reduced; it has no closed vessels (i.e., noveins orarteries), consisting of little more than a single, perforated dorsal tube which pulsesperistaltically, and in doing so helps circulate thehemolymph inside the body cavity.[7]
Air is taken in throughspiracles, openings which are positioned laterally in thepleural wall, usually a pair on the anterior margin of the meso and metathorax, and pairs on each of the eight or less abdominal segments, Numbers of spiracles vary from 1 to 10 pairs.[2][4][6][7] The oxygen passes through thetracheae to thetracheoles, and enters the body by the process of diffusion. Carbon dioxide leaves the body by the same process.[4]
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 trachealair sacs.[4] Spiracles are closed and opened by means ofvalves and can remain partly or completely closed for extended periods in some insects, which minimises water loss.[2][4]
There are many different patterns ofgas exchange demonstrated by different groups of insects. Gas exchange patterns in insects can range from continuous,diffusive ventilation, todiscontinuous gas exchange.[7]
Terrestrial and a large proportion ofaquatic 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,Trichoptera,Ephemeroptera, which have trachealgills and no functional spiracles. Endoparasiticlarvae are without spiracles and also operate under a closed system. Here the tracheae separate peripherally, covering the general body surface which results in acutaneous form ofgaseous exchange. This peripheral tracheal division may also lie within the tracheal gills where gaseous exchange may also take place.[7]
Many insects, such as therhinoceros beetle, are able to lift many times their own body weight and may jump distances that are many times greater than their own length. This is because their energy output is high in relation to their body mass.[4][6]
The muscular system of insects ranges from a few hundred muscles to a few thousand.[4] Unlike vertebrates that have both smooth and striated muscles, insects have only striated muscles. Muscle cells are amassed intomuscle fibers and then into the functional unit, the muscle.[6] Muscles are attached to the body wall, with attachment fibers running through the cuticle and to the epicuticle, where they can move different parts of the body including appendages such aswings.[4][7]The muscle fiber has many cells with aplasma membrane and outer sheath orsarcolemma.[7] The sarcolemma is invaginated and can make contact with the tracheole carrying oxygen to the muscle fiber. Arranged in sheets or cylindrically, contractilemyofibrils run the length of the muscle fiber. Myofibrils comprising a fineactin filament enclosed between a thick pair ofmyosin filaments slide past each other instigated bynerve impulses.[7]
Muscles can be divided into four categories:
Flight has allowed the insect to disperse, escape from enemies and environmental harm, and colonise newhabitats.[2] One of the insect's keyadaptations is flight, the mechanics of which differ from those of other flying animals because their wings are not modified appendages.[2][6] Fully developed and functional wings occur only in adult insects.[7] To fly,gravity and drag (air resistance to movement) have to be overcome.[7] 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-likethorax.[7]
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 thetergum andsternum. 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 upward. 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.[7]
Hormones are the chemical substances that are transported in the insect's body fluids (haemolymph) that carry messages away from their point ofsynthesis to sites where physiological processes are influenced. These hormones are produced byglandular, neuroglandular andneuronal centres.[7] Insects have several organs that produce hormones, controllingreproduction,metamorphosis andmoulting.[4] It has been suggested that abrain hormone is responsible forcaste determination intermites anddiapause interruption in some insects.[4]
Fourendocrine centers have been identified:
Insects have a complexnervous system which incorporates a variety of internal physiological information as well as external sensory information.[7] As in the case of vertebrates, the basic component is theneuron or nerve cell. This is made up of a dendrite with two projections that receive stimuli and anaxon, which transmits information to another neuron or organ, like amuscle. As with vertebrates, chemicals (neurotransmitters such asacetylcholine anddopamine) are released atsynapses.[7]
An insect's sensory,motor and physiological processes are controlled by thecentral nervous system along with theendocrine system.[7] Being the principal division of the nervous system, it consists of abrain, aventral nerve cord and asubesophageal ganglion which is connected to the brain by two nerves, extending around each side of theoesophagus.
The brain has three lobes:
The ventral nerve cord extends from the suboesophageal ganglion posteriorly.[4] A layer of connective tissue called theneurolemma covers the brain,ganglia, major peripheral nerves and ventral nerve cords.
The head capsule (made up of six fused segments) has six pairs ofganglia. The first three pairs are fused into the brain, while the three following pairs are fused into the subesophageal ganglion.[7] The thoracic segments have one ganglion on each side, which are connected into a pair, one pair per segment. This arrangement is also seen in the abdomen but only in the first eight segments. Many species of insects have reduced numbers of ganglia due to fusion or reduction.[8] Some cockroaches have just six ganglia in the abdomen, whereas the waspVespa crabro has only two in the thorax and three in the abdomen. And some, like the house flyMusca domestica, have all the body ganglia fused into a single large thoracic ganglion. 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.[4]
This consists ofmotor neuronaxons that branch out to the muscles from the ganglia of the central nervous system, parts of thesympathetic nervous system and thesensory neurons of the cuticular sense organs that receive chemical, thermal, mechanical or visual stimuli from the insect's environment.[7] The sympathetic nervous system includes nerves and the ganglia that innervate the gut both posteriorly and anteriorly, some endocrine organs, thespiracles of the tracheal system and the reproductive organs.[7]
Chemical senses include the use ofchemoreceptors, 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 asbees,wasps andants, taste organs can also be found on the antennae. Taste organs can also be found on thetarsi ofmoths,butterflies andflies.Olfactorysensilla enable insects to smell and are usually found in the antennae.[2] 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.[4]
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.[4] Hairs (setae) on thecuticle are responsible for this as they are sensitive to vibration touch and sound.[2]
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 Hz to 240 kHz depending on insect species.[4]Many of the joints of the insect havetactile 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 internalstretch receptors sense muscle distension anddigestive system stretching.[2][4]
Thecompound eye and theocelli supply insect vision. The compound eye consists of individual light receptive units calledommatidia. 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 clearlens system and light sensitiveretina 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.[2] The ocelli are unable to form focused images but are sensitive mainly, to differences in light intensity.[4] 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.[2]
A number of insects have temperature and humidity sensors[2] and insects being small, cool more quickly than larger animals. Insects are generally considered cold-blooded orectothermic, their body temperature rising and falling with the environment. However, flying insects raise their body temperature through the action of flight, above environmental temperatures.[4][6]
The body temperature of butterflies andgrasshoppers in flight may be 5 °C or 10 °C above environmental temperature, however moths andbumblebees, insulated byscales and hair, during flight, may raise flight muscle temperature 20–30 °C 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.[4]
Until very recently, no one had ever documented the presence ofnociceptors (the cells that detect and transmit sensations ofpain) in insects,[9] though recent findings of nociception in larvalfruit flies challenges this[10] and proves that all insects are very likely to feel pain.
Most insects have a high reproductive rate. With a shortgeneration time, they evolve faster and can adjust to environmental changes more rapidly than other slower breeding animals.[2] Although there are many forms of reproductive organs in insects, there remains a basic design and function for each reproductive part. These individual parts may vary in shape (gonads), position (accessory gland attachment), and number (testicular andovarian glands), with different insect groups.[7]
The female insect's main reproductive function is to produce eggs, including the egg's protective coating, and to store the malespermatozoa until eggfertilisation is ready. The femalereproductive organs include pairedovaries 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.[7] The external opening to this is thevulva. Often in insects the vulva is narrow and the genital chamber becomes pouch or tube like and is called thevagina. Related to the vagina is a saclike structure, thespermatheca, where spermatozoa are stored ready for egg fertilisation. A secretory gland nourishes the contained spermatozoa in the vagina.[4]
Egg development is mostly completed by the insect's adult stage and is controlled by hormones that control the initial stages ofoogenesis and yolk deposition.[7] Most insects are oviparous, where the young hatch after the eggs have been laid.[4]
Insect sexual reproduction starts with sperm entry that stimulates oogenesis,meiosis occurs and the egg moves down the genital tract. Accessory glands of the female secrete an adhesive substance to attach eggs to an object and they also supply material that provides the eggs with a protective coating. Oviposition takes place via the femaleovipositor.[4][6]
The male's main reproductive function is to produce and store spermatozoa and provide transport to the reproductive tract of the female.[7]Sperm development is usually completed by the time the insect reaches adulthood.[4] The male has twotestes, which containfollicles in which the spermatozoa are produced. These open separately into the sperm duct orvas deferens and this stores the sperm.[7] The vas deferentia then unite posteriorally to form a centralejaculatory duct, this opens to the outside on anaedeagus or a penis.[4] Accessory glands secrete fluids that comprise thespermatophore. This becomes a package that surrounds and carries the spermatozoa, forming a sperm-containing capsule.[4][7]
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 precisemicrohabitat on or near the required food.[6] However, some adult females can reproduce without male input. This is known asparthenogenesis and in the most common type of parthenogenesis the offspring are essentially identical to the mother. This is most often seen inaphids andscale insects.[6]
An insect'slife-cycle can be divided into three types:
As an insect grows it needs to replace the rigidexoskeleton regularly.[2][4]Moulting may occur up to three or four times or, in some insects, fifty times or more during its life.[2] A complex process controlled byhormones, it includes thecuticle of the body wall, the cuticular lining of thetracheae,foregut,hindgut and endoskeletal structures.[2][4]
The stages of molting:
Media related toInsect physiology at Wikimedia CommonsPhysiology types | ||
|---|---|---|
| Animals |  | |
| Plants | ||
| Cells | ||
| Related topics | ||