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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" •.
Relationship to other arthropods
Morphology and development
Development 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. Behavior 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. Walking
Roles in the environment and human society 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 Subclass: Apterygota Orders
Subclass: Pterygota
Orders
Orders
Polyneoptera
Orthopteroidea
Dictyoptera
Paraneoptera
Orders
Neuropteroidea
Mecopteroidea
Amphiesmenoptera Incertae sedis
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 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 —Maurice Maeterlinck (1862–1949) See also For further reading Gallery Image:Prachtlibelle-2.jpg|Beautiful Demoiselle (Calopteryx virgo) Image:Forficula auricularia.jpg|Common earwig (Forficula auricularia) Image:Patanga_japonica_larva.jpg|A juvenile Patanga japonica Image:Ctenomorpha chronus02.jpg|A stick insect (Ctenomorpha chronus) Image:Termite_Cathedral_DSC03570.jpg|Cathedral termite mound Image:Wasserläufer bei der Paarung crop.jpg|Water strider (Gerris najas) Image:Aelia acuminata.JPG|Bishop's mitre shield bug (Aelia acuminata) Image:Ant on mosshill02 crop.jpg|Ant Image:Osmia rufa couple (aka).jpg|Red mason bee (Osmia rufa) Image:Guepe.jpg|A wasp drinking Image:Aleiodes indiscretus wasp parasitizing gypsy moth caterpillar.jpg|Aleiodes indiscretus parasitising a gypsy moth (Lymantria dispar) larva Image:Adult citrus root weevil, Diaprepes abbreviatus.jpg|Adult citrus root weevil (Diaprepes abbreviatus) Image:Schwebfliege.jpg|A flower fly, Episyrphus balteatus Image:Iphiclides podalirius.jpg|Scarce swallowtail (Iphiclides podalirius) Image:GiantLeopardMoth.jpg|Giant Leopard Moth (Ecpantheria scribonia) Image:Rosy Maple Moth.png|Rosy maple moth (Dryocampa rubicunda) | |||||||||||||||||||||||||||||||||||
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