Insect physiology

Source: Wikipedia, the free encyclopedia.

Insect physiology includes the physiology and biochemistry of insect organ systems.[1]

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

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 and
molting
.

Digestive system

An insect uses its digestive system to extract nutrients and other substances from the food it consumes. [3]

Most of this food is ingested in the form of

simple sugars, etc.) before being used by cells of the body for energy, growth, or reproduction. This break-down process is known as digestion
.

The insect's digestive system is a closed system, with one long enclosed coiled tube called the

alimentary 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 the anus. The alimentary canal has specific sections for grinding and food storage, enzyme production, and nutrient
absorption. [2]

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 paired

salivary 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 the salivarium behind the hypopharynx; 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 the

moult along with the exoskeleton.[4] Food is moved down the gut by muscular contractions called peristalsis.[6]

Orthopteran
type)
  1. Stomatodeum (foregut): This region stores, grinds and transports food to the next region.
    anticoagulants
    and blood thinners are also released here.
  2. Mesenteron (midgut): Digestive enzymes in this region are produced and secreted into the
    microvilli
    , increase surface area and allow for maximum absorption of nutrients.
  3. Proctodeum (hindgut): This is divided into three sections; the anterior is the
    haemolymph, in which all the internal organs are bathed.[2] These tubules continually produce the insect's uric acid, which is transported to the hindgut, where important salts and water are re-absorbed by both the hindgut and rectum. Excrement is then voided as insoluble and non-toxic uric acid granules.[2] Excretion and osmoregulation in insects are not orchestrated by the Malpighian tubules alone, but require a joint function of the ileum and/or rectum.[7]

Circulatory system

See also: Hemolymph

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 transports

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.

haemocoel.[6][7] 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.[2][4][7] It differs from vertebrate blood in that it doesn't contain any red blood cells and therefore is without high oxygen carrying capacity, and is more similar to lymph 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.

Diptera.[7]

Respiratory system

See also:
Invertebrate trachea and Respiratory system of insects

Insect respiration is accomplished without lungs 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 eliminate carbon dioxide via their cells.[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., no veins or arteries), consisting of little more than a single, perforated dorsal tube which pulses peristaltically, and in doing so helps circulate the hemolymph inside the body cavity.[7]

Air is taken in through

tracheae to the tracheoles, 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 tracheal

valves and can remain partly or completely closed for extended periods in some insects, which minimises water loss.[2][4]

There are many different patterns of gas exchange demonstrated by different groups of insects. Gas exchange patterns in insects can range from continuous, diffusive ventilation, to discontinuous gas exchange.[7]

gaseous exchange. This peripheral tracheal division may also lie within the tracheal gills where gaseous exchange may also take place.[7]

Muscular system

Many insects, such as the

rhinoceros 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.

muscle 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 as wings.[4][7]
The muscle fiber has many cells with a
myofibrils run the length of the muscle fiber. Myofibrils comprising a fine actin filament enclosed between a thick pair of myosin filaments slide past each other instigated by nerve impulses.[7]

Muscles can be divided into four categories:

  1. digestive system.[6]
  2. Segmental: causing telescoping of muscle segments required for moulting, increase in body pressure, and locomotion in legless larvae.[6]
  3. haemolymph pressure and cuticle elasticity.[4]
  4. Flight: Flight muscles are the most specialised category of muscle and are capable of rapid contractions.
    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.[2][7]

Flight has allowed the insect to disperse, escape from enemies and environmental harm, and colonise new

adaptations 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-like thorax.[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 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 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]

Endocrine system

termites and diapause interruption in some insects.[4]

Four

endocrine
centers have been identified:

  1. Neurosecretory cells in the brain can produce one or more hormones that affect growth, reproduction, homeostasis and metamorphosis.[4][7]
  2. Corpora cardiaca are a pair of neuroglandular bodies that are found behind the brain and on either sides of the
    PTTH prothoracicotropic hormone
    (brain hormone), which stimulates the secretory activity of the prothoracic glands, playing an integral role in moulting.
  3. Prothoracic
    ovarioles
    and in the process of egg production.
  4. Corpora allata are small, paired glandular bodies originating from the epithelium located on either side of the foregut. They secrete the juvenile hormone, which regulate reproduction and metamorphosis.[4][7]

Nervous system

Insects have a complex

synapses.[7]

Central nervous system

An insect's sensory,

oesophagus
.

The brain has three lobes:

The ventral nerve cord extends from the suboesophageal ganglion posteriorly.

ganglia
, major peripheral nerves and ventral nerve cords.

The head capsule (made up of six fused segments) has six pairs of

Musca 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]

Peripheral nervous system

This consists of

sensory 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, the spiracles of the tracheal system and the reproductive organs.[7]

Sensory organs

See also: Hygroreception

Chemical senses include the use of

sensilla 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.

setae) on the cuticle 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 have

digestive system stretching.[2][4]

The

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.[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

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.[4][6]

The body temperature of butterflies and

bumblebees, insulated by scales 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 of nociceptors (the cells that detect and transmit sensations of pain) in insects,[9] though recent findings of nociception in larval fruit flies challenges this[10] and proves that all insects are very likely to feel pain.

Reproductive system

Main article: Insect reproductive system

Most insects have a high reproductive rate. With a short

ovarian glands), with different insect groups.[7]

Female

The female insect's main reproductive function is to produce eggs, including the egg's protective coating, and to store the male

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.[7] 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 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 of oogenesis 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 female ovipositor.[4][6]

Male

The male's main reproductive function is to produce and store spermatozoa and provide transport to the reproductive tract of the female.

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.[7] The vas deferentia then unite posteriorally to form a central ejaculatory duct, this opens to the outside on an aedeagus or a penis.[4] Accessory glands secrete fluids that comprise the spermatophore. This becomes a package that surrounds and carries the spermatozoa, forming a sperm-containing capsule.[4][7]

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

scale insects.[6]

Life cycle

An insect's life-cycle can be divided into three types:

Moulting

Main article: Moulting

As an insect grows it needs to replace the rigid

tracheae, foregut, hindgut and endoskeletal structures.[2][4]

The stages of molting:

  1. exocuticle
    .
  2. Ecdysis—this begins with the splitting of the old cuticle, usually starting in the midline of the thorax's dorsal side. The rupturing force is mostly from haemolymph pressure that has been forced into thorax by abdominal muscle contractions caused by the insect swallowing air or water. After this the insect wriggles out of the old cuticle.
  3. Sclerotisation—after emergence the new cuticle is soft and 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 the wing
    veins.[2][4]

References

  1. ^ Nation, . L. (2002) Insect Physiology and Biochemistry. CRC Press.
  2. ^
    ISBN 9780198500025
    .
  3. ^ a b "General Entomology – Digestive and Excretory system". NC state University. Retrieved 2009-05-03.
  4. ^
    ISBN 9780030968358
    .
  5. ^ Duncan, Carl D. (1939). A Contribution to The Biology of North American Vespine Wasps (1 ed.). Stanford: Stanford University Press. pp. 24–29.
  6. ^
    ISBN 9780130480309
    .
  7. ^
    ISBN 1-4051-1113-5
    .
  8. JSTOR 1539265
    .
  9. S2CID 3071
    .
  10. PMID 12705873
    .

External links
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