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Insect

2007 Schools Wikipedia Selection. Related subjects: Insects, Reptiles and
Fish

   iInsects

                    Fossil range: Carboniferous - Recent

   Honeybee (Order Hymenoptera)
   Honeybee (Order Hymenoptera)
                Scientific classification

   Kingdom:   Animalia
   Phylum:    Arthropoda
   Subphylum: Hexapoda
   Class:     Insecta
              Linnaeus, 1758

                                   Orders

   See taxonomy

   Insects are invertebrates that are taxonomically referred to as the
   class Insecta. They are the most numerous and most widespread
   terrestrial taxon within the phylum Arthropoda, and indeed the most
   diverse group of animals on the earth, with around 925,000 species
   described—more than all other animal groups combined. Insects may be
   found in nearly all environments on the planet, although only a small
   number of species have adapted to life in the oceans where crustaceans
   tend to predominate instead.

   There are approximately 5,000 dragonfly species, 2,000 praying mantis,
   20,000 grasshopper, 170,000 butterfly and moth, 120,000 fly, 82,000
   true bug, 350,000 beetle, and 110,000 bee and ant species described to
   date. Estimates of the total number of current species, including those
   not yet known to science, range from two million to fifty million, with
   newer studies favouring a lower figure of about six to ten
   million.Adult modern insects range in size from 0.139 mm (see
   Dicopomorpha echmepterygis) to 555 mm (see Phobaeticus serratipes).

   The study of insects (from Latin insectus, meaning "cut into sections")
   is called entomology, from the Greek εντομος, also meaning "cut into
   sections" .
   Green bottle fly
   Enlarge
   Green bottle fly

Relationship to other arthropods

   Chinese mantis
   Enlarge
   Chinese mantis

   Other terrestrial arthropods, such as centipedes, millipedes, scorpions
   and spiders, are sometimes confused with insects since their body plans
   can appear similar, sharing (as do all arthropods) a jointed
   exoskeleton. However upon closer examination their features differ
   significantly; most noticeably they do not have six legs characteristic
   of insects.

   Within the subphylum Hexapoda, a few groups such as springtails (
   Collembola), are often treated as insects; however some authors treat
   them as distinct from the insects in having a different evolutionary
   origin. This may also be the case for the rest of the members of the
   Entognatha; Protura and Diplura.

   The true insects, those of the Class Insecta, are distinguished from
   all other arthropods in part by having ectognathous, or exposed,
   mouthparts and eleven (11) abdominal segments. The true insects are
   therefore sometimes also referred to as the Ectognatha. Many insect
   groups are winged as adults. The exopterygote part of the Neoptera are
   sometimes divided into Orthopteroida (cerci present) and Hemipteroida
   (cerci absent), also called lower and higher Exopterygota.

Morphology and development

   Insect anatomy A- Head   B- Thorax   C- Abdomen 1. antenna 2. ocelli
   (lower) 3. ocelli (upper) 4. compound eye 5. brain (cerebral ganglia)
   6. prothorax 7. dorsal artery 8. tracheal tubes (trunk with spiracle)
   9. mesothorax 10. metathorax 11. forewing 12. hindwing 13. mid-gut
   (stomach) 14. heart 15. ovary 16. hind-gut (intestine, rectum & anus)
   17. anus 18. vagina 19. nerve chord (abdominal ganglia) 20. Malpighian
   tubes 21. pillow 22. claws 23. tarsus 24. tibia 25. femur 26.
   trochanter 27. fore-gut (crop, gizzard) 28. thoracic ganglion 29. coxa
   30. salivary gland 31. subesophageal ganglion 32. mouthparts
   Enlarge
   Insect anatomy
   A- Head   B- Thorax   C- Abdomen
   1. antenna
   2. ocelli (lower)
   3. ocelli (upper)
   4. compound eye
   5. brain (cerebral ganglia)
   6. prothorax
   7. dorsal artery
   8. tracheal tubes (trunk with spiracle)
   9. mesothorax
   10. metathorax
   11. forewing
   12. hindwing
   13. mid-gut (stomach)
   14. heart
   15. ovary
   16. hind-gut (intestine, rectum & anus)
   17. anus
   18. vagina
   19. nerve chord (abdominal ganglia)
   20. Malpighian tubes
   21. pillow
   22. claws
   23. tarsus
   24. tibia
   25. femur
   26. trochanter
   27. fore-gut (crop, gizzard)
   28. thoracic ganglion
   29. coxa
   30. salivary gland
   31. subesophageal ganglion
   32. mouthparts

   Insects range in size from less than a millimeter to over 18
   centimeters (some walkingsticks) in length. Insects possess segmented
   bodies supported by an exoskeleton, a hard outer covering made mostly
   of chitin. The body is divided into a head, a thorax, and an abdomen.
   The head supports a pair of sensory antennae, a pair of compound eyes,
   and mouth parts. The thorax has six legs (one pair per segment) and
   wings (if present in the species). The abdomen (made up of eleven
   segments some of which may be reduced or fused) has respiratory,
   excretory and reproductive structures.

   Their nervous system can be divided into a brain and a ventral nerve
   cord. The head capsule (made up of six fused segments) has six pairs of
   ganglia. The first three pairs are fused into the brain, while the
   three following pairs are fused into a structure called the
   subesophageal ganglion.

   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.
   Some cockroaches have just six ganglia in the abdomen, whereas the wasp
   Vespa crabro has only two in the thorax and three in the abdomen. And
   some like the house fly Musca domestica have fused all the body ganglia
   into a single large thoracic ganglion.

   Insects have a complete digestive system. That is, their digestive
   system consists basically of a tube that runs from mouth to anus,
   contrasting with the incomplete digestive systems found in many simpler
   invertebrates. The excretory system consists of Malpighian tubules for
   the removal of nitrogenous wastes and the hindgut for osmoregulation.
   At the end of the hindgut, insects are able to reabsorb water along
   with potassium and sodium ions. Therefore, insects don't usually
   excrete water with their feces, allowing storage of water in the body.
   This process of reabsorption enables them to withstand hot, dry
   environments.

   Most insects have two pairs of wings located on the second and third
   thoracic segments. Insects are the only invertebrates to have developed
   flight, and this has played an important part in their success. The
   winged insects, and their wingless relatives, make up the subclass
   Pterygota. Insect flight is not very well understood, relying heavily
   on turbulent aerodynamic effects. The primitive insect groups use
   muscles that act directly on the wing structure. The more advanced
   groups making up the Neoptera have foldable wings and their muscles act
   on the thorax wall and power the wings indirectly. These muscles are
   able to contract without nerve impulses allowing them to beat faster
   (see Insect flight).

   Their outer skeleton, the cuticle, is made up of two layers; the
   epicuticle which is a thin and waxy water resistant outer layer and
   contains no chitin, and another layer under it called the procuticle.
   This is chitinous and much thicker than the epicuticle and has two
   layers. The outer being the exocuticle while the inner is the
   endocuticle. The tough and flexible endocuticle is built from numerous
   layers of fibrous chitin and proteins, criss-crossing each others in a
   sandwich pattern, while the exocuticle is sclerotized.

   Insects use tracheal respiration with openings on the sides of the
   thorax and abdomen called spiracles leading to the tubular tracheal
   system. Air reaches internal tissues via a network of branches from the
   tracheal system. There is usually one pair of spiracles per segment.
   There can be up to 8 abdominal segments with spiracles and upto 2
   thoracic segments (restricted to the mesothorax and metathorax). Some
   groups have reduced numbers of spiracles with the hoverflies having
   none on their abdomen. There is a physical limit to the pressure that
   the walls of the tracheal tubes can withstand without collapsing, even
   though they are stiffened by bands of chitin, and this is one of the
   reasons why insects are relatively small. The spiracles have muscle
   controlled valves, enabling the insects to avoid drowning in water or
   to prevent desiccation. The spiracles often have hairs that help filter
   the air entering them.

   Some insect groups such as the Chironomidae or "blood worms" have true
   respiratory pigments such as hemoglobin in their blood during their
   larval stage. Here the trachea are often reduced as their body can
   absorb oxygen directly from the water, allowing them to live in bottom
   mud where the oxygen levels are low.

   In certain types of water bugs the three pairs of the spiracles are
   covered by a pressure-sensitive membrane that help them sense their
   position in water. The last abdominal spiracle and associated trachea
   of certain Lepidopteran caterpillars are modified into a tracheal lung
   adapted for hemocyte gas exchange. Short tracheoles from this trachea
   ends in knots within the tracheole cell basement membrane. Since they
   do not supply any cellular tissue, it seems most likely that they are
   supplying the hemocytes with oxygen.

   The Madagascar hissing cockroach uses certain spiracles to forcibly
   expel air to create a loud hissing sound when threatened.

   A diffuse tissue called a fat body is found in the abdominal haemocoel
   of some insects. This is believed to help in energy storage and
   metabolic processes and acts like a liver for the insects.

   The circulatory system of insects, like that of other arthropods, is
   open: the heart, which is little more than a perforated muscular tube
   along the dorsal midline, pumps the hemolymph to open spaces
   surrounding the internal organs; when the heart relaxes, the hemolymph
   seeps back into the heart.

   Like some other invertebrates, insects cannot synthesise cholesterol
   and must receive it from the diet. With very few exceptions, they also
   require long-chain fatty acids in their diet. Lack of these fatty acids
   affects their development leading to delayed maturity or deformations.

Development

   Hoverflies mating in midair flight
   Enlarge
   Hoverflies mating in midair flight

   Most insects hatch from eggs, but others are ovoviviparous or
   viviparous, and all undergo a series of moults as they develop and grow
   in size. This manner of growth is necessitated by the inelastic
   exoskeleton. Moulting is a process by which the individual escapes the
   confines of the exoskeleton in order to increase in size, then grows a
   new and larger outer covering. In some insects, the young are called
   nymphs and are similar in form to the adults except that the wings are
   not developed until the adult stage. This is called incomplete
   metamorphosis and insects showing this are termed as Hemimetabolous.
   Holometabolous insects show Complete metamorphosis, which distinguishes
   the Endopterygota and includes many of the most successful insect
   groups. In these species, an egg hatches to produce a larva, which is
   generally worm-like in form, and can be divided into five different
   forms; eruciform (caterpillar-like), scarabaeiform (grublike),
   campodeiform (elongated, flattened, and active), elateriform
   (wireworm-like) and vermiform (maggot-like). The larva grows and
   eventually becomes a pupa, a stage sealed within a cocoon or chrysalis
   in some species. There are three types of pupae; obtect (the pupa is
   compact with the legs and other appendages enclosed), exarate (where
   the pupa has the legs and other appendages free and extended) and
   coarctate (where the pupa develops inside the larval skin). In the
   pupal stage, the insect undergoes considerable change in form to emerge
   as an adult, or imago. Butterflies are an example of an insect that
   undergoes complete metamorphosis. Some insects have even evolved
   hypermetamorphosis.

   Some insects (parastic wasps) show polyembryony where a single
   fertilized egg can divide into many and in some cases thousands of
   separate embryos. Other developmental and reproductive variations
   include haplodiploidy, polymorphism, paedomorphosis (metathetely and
   prothetely), sexual dimorphism, parthenogenesis and more rarely
   hermaphroditism.
   A butterfly is the adult stage of an insect with complete
   metamorphosis. This species is Anartia amathea.
   Enlarge
   A butterfly is the adult stage of an insect with complete
   metamorphosis. This species is Anartia amathea.

Behaviour

   Flies attracted to an incandescent light bulb
   Enlarge
   Flies attracted to an incandescent light bulb

   Many insects possess very sensitive organs of perception. Some insects
   such as bees can see in the ultraviolet spectrum while male moths can
   detect the pheromones of female moths over distances of many
   kilometers.

   Many insects also have a well-developed number sense, especially among
   the solitary wasps. The mother wasp lays her eggs in individual cells
   and provides each egg with a number of live caterpillars on which the
   young feed when hatched. Some species of wasp always provide five,
   others twelve, and others as high as twenty-four caterpillars per cell.
   The number of caterpillars is different among species, but it is always
   the same for each sex of larvae. The male solitary wasp in the genus
   Eumenes is smaller than the female, so the mother supplies him with
   only five caterpillars; the larger female receives ten caterpillars in
   her cell. She can in other words distinguish between both the numbers
   five and ten in the caterpillars she is providing and which cell
   contains a male or a female.

   Social insects, such as the ant and the bee, are the most familiar
   species of eusocial animal. They live together in large well-organized
   colonies that are so tightly integrated and genetically similar that
   the colonies are sometimes considered superorganisms.

Locomotion

Walking

   Ladybird animation showing tripedal gait

   Many adult insects use six legs for walking and have adopted a tripedal
   gait. The tripedal gait allows for rapid walking whilst always having a
   stable stance and has been studied extensively in cockroaches. The legs
   are used in alternate triangles touching the ground. For the first step
   the middle right leg and the front and rear left legs are in contact
   with the ground and move the insect forward, whilst the front and rear
   right leg and the middle left leg are lifted and moved forward to a new
   position. When they touch the ground to form a new stable triangle the
   other legs can be lifted and brought forward in turn and so on.

   The purest form of the tripedal gait is seen in insects moving at speed
   and is illustrated in the gif animation of a 7 spot Ladybird
   (Coccinellidae, Coccinella septempunctata). However, this type of
   locomotion is not rigid and insects can adapt a variety of gaits; for
   example, when moving slowly, turning, or avoiding obstacles, four or
   more feet may be touching the ground. Insects can also adapt their gait
   to cope with the loss of one or more limbs.

   Cockroaches are amongst the fastest insect runners and at full speed
   actually adopt a bipedal run to reach a high velocity in proportion to
   their body size. As Cockroaches move extremely rapidly, they need
   recording at several hundred frames per second to reveal their gait.
   More sedate locomotion is also studied by scientists in stick insects
   Phasmatodea.

   Insect walking is of particular interest as an alternative form of
   locomotion to the use of wheels for robots ( Robot locomotion).

Roles in the environment and human society

   Aedes aegypti, a parasite, and vector of dengue fever and yellow fever
   Enlarge
   Aedes aegypti, a parasite, and vector of dengue fever and yellow fever

   Many insects are considered pests by humans. Insects commonly regarded
   as pests include those that are parasitic ( mosquitoes, lice, bedbugs),
   transmit diseases ( mosquitos, flies), damage structures ( termites),
   or destroy agricultural goods ( locusts, weevils). Many entomologists
   are involved in various forms of pest control, often using
   insecticides, but more and more relying on methods of biocontrol.

   Although pest insects attract the most attention, many insects are
   beneficial to the environment and to humans. Some pollinate flowering
   plants (for example wasps, bees, butterflies, ants). Pollination is a
   trade between plants that need to reproduce, and pollinators that
   receive rewards of nectar and pollen. A serious environmental problem
   today is the decline of populations of pollinator insects, and a number
   of species of insects are now cultured primarily for pollination
   management in order to have sufficient pollinators in the field,
   orchard or greenhouse at bloom time.

   Insects also produce useful substances such as honey, wax, lacquer and
   silk. Honeybees have been cultured by humans for thousands of years for
   honey, although contracting for crop pollination is becoming more
   significant for beekeepers. The silkworm has greatly affected human
   history, as silk-driven trade established relationships between China
   and the rest of the world. Fly larvae ( maggots) were formerly used to
   treat wounds to prevent or stop gangrene, as they would only consume
   dead flesh. This treatment is finding modern usage in some hospitals.
   Adult insects such as crickets, and insect larvae of various kinds are
   also commonly used as fishing bait.

   In some parts of the world, insects are used for human food ("
   Entomophagy"), while being a taboo in other places. There are
   proponents of developing this use to provide a major source of protein
   in human nutrition. Since it is impossible to entirely eliminate pest
   insects from the human food chain, insects already are present in many
   foods, especially grains. Most people do not realize that food laws in
   many countries do not prohibit insect parts in food, but rather limit
   the quantity. According to cultural materialist anthropologist Marvin
   Harris, the eating of insects is taboo in cultures that have protein
   sources that require less work, like farm birds or cattle.

   Many insects, especially beetles, are scavengers, feeding on dead
   animals and fallen trees, recycling the biological materials into forms
   found useful by other organisms. The ancient Egyptian religion adored
   beetles and represented them as scarabeums.

   Although mostly unnoticed by most humans, the most useful of all
   insects are insectivores, those that feed on other insects. Many
   insects, such as grasshoppers, can potentially reproduce so quickly
   that they could literally bury the earth in a single season. However,
   there are hundreds of other insect species that feed on grasshopper
   eggs, and some that feed on grasshopper adults. This role in ecology is
   usually assumed to be primarily one of birds, but insects, though less
   glamorous, are much more significant. For any pest insect one can name,
   there is a species of wasp that is either a parasitoid or predator upon
   that pest, and plays a significant role in controlling it.

   Human attempts to control pests by insecticides can backfire, because
   important but unrecognized insects already helping to control pest
   populations are also killed by the poison, leading eventually to
   population explosions of the pest species.

Taxonomy

   Dragonfly
   Enlarge
   Dragonfly

   Subclass: Apterygota

          Orders

          + Archaeognatha (bristletails)
          + Thysanura (silverfish)
          + Monura - extinct

   Subclass: Pterygota

          + Infraclass: " Paleoptera" (may be paraphyletic)

                Orders

               o Ephemeroptera (mayflies)
               o Palaeodictyoptera - extinct
               o Megasecoptera - extinct
               o Archodonata - extinct
               o Diaphanopterodea - extinct
               o Protodonata - extinct
               o Odonata ( dragonflies and damselflies)

          + Infraclass: Neoptera

               o Superorder: Exopterygota

                      Orders

               o Caloneurodea - extinct
               o Titanoptera - extinct
               o Protorthoptera - extinct

                      Polyneoptera

               o Grylloblattodea (ice-crawlers)
               o Mantophasmatodea (gladiators)
               o Plecoptera (stoneflies)
               o Embioptera (webspinners)
               o Zoraptera (angel insects)
               o Dermaptera (earwigs)

                      Orthopteroidea

               o Orthoptera ( grasshoppers, etc)
               o Phasmatodea (stick insects)

                      Dictyoptera

               o Blattodea (cockroaches)
               o Isoptera (termites)
               o Mantodea (mantids)

                      Paraneoptera

               o Psocoptera (booklice, barklice)
               o Thysanoptera (thrips)
               o Phthiraptera ( lice)
               o Hemiptera (true bugs)

               o Superorder: Endopterygota

                      Orders

               o Hymenoptera (ants, bees, etc.)
               o Coleoptera (beetles)
               o Strepsiptera (twisted-winged parasites)

                      Neuropteroidea

               o Raphidioptera (snakeflies)
               o Megaloptera ( alderflies, etc.)
               o Neuroptera (net-veined insects)

                      Mecopteroidea

               o Mecoptera (scorpionflies, etc.)
               o Siphonaptera ( fleas)
               o Diptera (true flies)
               o Protodiptera extinct

                      Amphiesmenoptera

               o Trichoptera ( caddisflies)
               o Lepidoptera ( butterflies, moths)

                Incertae sedis

               o Glosselytrodea extinct
               o Miomoptera - extinct

   As seen above, insects are divided into two subclasses; Apterygota and
   Pterygota (flying insects), but this could relatively soon change.
   Apterygota is made up of two orders; Archaeognatha (bristletails) and
   Thysanura (silverfish). In the suggested classification, the
   Archaeognatha makes up the Monocondylia while Thysanura and Pterygota
   are grouped together as Dicondylia. It is even possible that the
   Thysanura itself are not monophyletic, making the family
   Lepidotrichidae a sister group to the Dicondylia (Pterygota + the rest
   of the Thysanura).

   Also within the infraclass Neoptera we will probably see some
   re-organization in not too long. Today Neoptera is divided into the
   superorders Exopterygota and Endopterygota. But even if the
   Endopterygota are monophyletic, the Exopterygota seems to be
   paraphyletic, and can be separated into smaller groups; Paraneoptera,
   Dictyoptera, Orthopteroidea and to other groups (Grylloblattodea +
   Mantophasmatodea and Plecoptera + Zoraptera + Dermaptera). Phasmatodea
   and Embioptera has been suggested to form Eukinolabia, while
   Strepsiptera and Diptera are sometimes grouped together in Halteria.
   Paraneoptera has turned out to be more closeley related to
   Endopterygota than to the rest of the Exopterygota. It is not still
   clear how closley related the remaining Exopterygote groups are and if
   they belongs together in a larger unit. Only more research will give
   the answer.

Evolution

   Evolution has produced astonishing variety in insects. Pictured are
   some of the possible shapes of antennae.
   Enlarge
   Evolution has produced astonishing variety in insects. Pictured are
   some of the possible shapes of antennae.

   The relationships of insects to other animal groups remain unclear.
   Although more traditionally grouped with millipedes and centipedes,
   evidence has emerged favoring closer evolutionary ties with the
   crustaceans. In the Pancrustacea theory insects, together with
   Remipedia and Malacostraca, make up a natural clade.

   Apart from some tantalizing Devonian fragments, insects first appear
   suddenly in the fossil record at the very beginning of the Late
   Carboniferous period, Early Bashkirian age, about 350 million years
   ago. Insect species were already diverse and highly specialized by this
   time, with fossil evidence reflecting the presence of more than half a
   dozen different orders. Thus, the first insects probably emerged
   earlier in the Carboniferous period, or even in the preceding Devonian.
   Research to discover these earliest insect ancestors in the fossil
   record continues.

   The origins of insect flight remain obscure, since the earliest winged
   insects currently known appear to have been capable fliers. Some
   extinct insects had an additional pair of winglets attaching to the
   first segment of the thorax, for a total of three pairs. So far, there
   is nothing that suggests that the insects were a particularly
   successful group of animals before they got their wings.

   Late Carboniferous and Early Permian insect orders include both several
   current very long-lived groups and a number of Paleozoic forms. During
   this era, some giant dragonfly-like forms reached wingspans of 55 to 70
   cm, making them far larger than any living insect. Also their nymphs
   must have had a very impressive size. This gigantism may have been due
   to higher atmospheric oxygen levels that allowed increased respiratory
   efficiency relative to today. The lack of flying vertebrates could have
   been another factor.

   Most extant orders of insects developed during the Permian era that
   began around 270 million years ago. Many of the early groups became
   extinct during the Permian-Triassic extinction event, the largest mass
   extinction in the history of the Earth, around 252 million years ago.

   The remarkably successful Hymenopterans appeared in the Cretaceous but
   achieved their diversity more recently, in the Cenozoic. A number of
   highly-successful insect groups evolved in conjunction with flowering
   plants, a powerful illustration of co-evolution.

   Many modern insect genera developed during the Cenozoic; insects from
   this period on are often found preserved in amber, often in perfect
   condition. Such specimens are easily compared with modern species. The
   study of fossilized insects is called paleoentomology.

Quotes

     * "Something in the insect seems to be alien to the habits, morals,
       and psychology of this world, as if it had come from some other
       planet: more monstrous, more energetic, more insensate, more
       atrocious, more infernal than our own."

                — Maurice Maeterlinck ( 1862– 1949)

     * When asked what can be learned about the Creator by examining his
       work, J.B.S. Haldane said "an inordinate fondness for beetles."

     * "To understand the success of insects is to appreciate our own
       shortcomings" — Thomas Eisner

Gallery

   Beautiful Demoiselle (Calopteryx virgo)

   Common earwig (Forficula auricularia)

   A juvenile Patanga japonica

   A stick insect (Ctenomorpha chronus)

   Cathedral termite mound

   Water strider (Gerris najas)

   Bishop's mitre shield bug (Aelia acuminata)

   Ant

   Red mason bee (Osmia rufa)

   A wasp drinking

   Aleiodes indiscretus parasitising a gypsy moth (Lymantria dispar) larva

   Adult citrus root weevil (Diaprepes abbreviatus)

   A flower fly, Episyrphus balteatus

   Scarce swallowtail (Iphiclides podalirius)

   Giant Leopard Moth (Ecpantheria scribonia)

   Rosy maple moth (Dryocampa rubicunda)

   Retrieved from " http://en.wikipedia.org/wiki/Insect"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
