What Is a Bug?
Insects (Class Insecta) are arthropods, having a hard exoskeleton, a three-part body (head, thorax, and abdomen), three pairs of jointed legs, compound eyes, and two antennae. They are the most diverse group of animals on the planet and include approximately 30 Notoptera, 35 Zoraptera, 150 snakefly, 200 silverfish, 300 alderfly, 300 webspinner, 350 jumping bristletail, 550 scorpionfly, 600 Strepsiptera, 1,200 caddisfly, 1,700 stonefly, 1,800 earwig, 2,000 flea, 2,200 mantis, 2,500 mayfly, 3,000 louse, 3,000 walking stick, 4,000 cockroach, 4,000 lacewing, 4,000 termite, 5,000 dragonfly, 5,000 thrips, 5,500 booklouse, 20,000 cricket, grasshopper, and locust, 82,000 true bug, 110,000 ant, bee, sawfly, and wasp, 120,000 true fly, 170,000 butterfly and moth, and 360,000 beetle species described to date. The number of extant species is estimated at between six and ten million, with over a million species already described. Insects represent more than half of all known living organisms and potentially represent over 90% of the differing life forms on Earth. Insects may be found in nearly all environments, although only a small number of species occur in the oceans, a habitat dominated by another arthropod group, the crustaceans.
Adult modern insects range in size from a 0.139 mm (0.00547 in) fairyfly (Dicopomorpha echmepterygis) to a 56.7-centimetre (22.3 in) long stick insect (Phobaeticus chani). The heaviest documented present-day insect was 70 g (2? oz) Giant Weta, though the Goliath beetles Goliathus goliatus, Goliathus regius and Cerambycid beetles such as Titanus giganteus hold the title for some of the largest species in general.
The largest known extinct insect is an ancient dragonfly, Meganeura.
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 all the body ganglia fused into a single large thoracic ganglion.
Until very recently, no one had ever documented the presence of nociceptors (the cells that detect and transmit sensations of pain) in insects, though recent findings of nociception in larval fruit flies challenges this and raises the possibility that some insects may be capable of feeling pain.
An insect uses its digestive system to extract nutrients and other substances from the food it consumes. Most of this food is ingested in the form of macromolecules and other complex substances (such as proteins, polysaccharides, fats, and nucleic acids) which must be broken down by catabolic 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 as digestion.
The insect's digestive system is a closed system, with one long enclosed 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. Each of the three sections of the alimentary canal performs a different process of digestion. 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.
The first section of the alimentary canal is the fore-gut (27) or stomodaeum. In the fore-gut, initial breakdown of large food particles occurs, mostly by saliva. The fore-gut includes the Buccal cavity, the esophagus, and the crop, which stores food before it passes to the mid-gut.
Once food leaves the crop, it passes to the mid-gut (13) or mesenteron. The mid-gut is where digestion really happens, through enzymatic action. Microscopic projections from the mid-gut wall, called microvilli, increase surface area and allow for maximum absorption of nutrients.
In the hind-gut (16) or proctodaeum, is where undigested food particles join uric acid from Malphigian tubules to form fecal pellets. The rectum absorbs most of the water in this waste matter, and the dry pellet is then eliminated through the anus (17), there by completing the process of digestion.
Respiration and circulation
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 rigid and sclerotized. The exocuticle is greatly reduced in many soft-bodied insects, especially the larval stages (e.g., caterpillars).
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.
Senses and communication
A few such insects also have a well-developed number sense, such as the solitary wasps that provision with a single species of prey. 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 larva. The male solitary wasp in the genus Eumenes is smaller than the female, so the mother of one species 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.
Light production and vision
Most insects except some species of cave dwelling crickets are able to perceive light and dark. Many species have acute vision capable of detecting minute movements. The eyes include simple eyes or ocelli as well as compound eyes of varying sizes. Many species are able to detect light in the infrared, ultraviolet as well as the visible light wavelengths. Colour vision has been demonstrated in many species and phylogenetic analysis suggests that the basic bauplan of UV-green-blue trichromacy existed from at least the Devonian period.
Sound production and hearing
Very low sounds are also produced in various species of Neuroptera, Lepidoptera (butterflies and moths), Coleoptera and Hymenoptera produced by the mechanical actions of movement often aided by special microscopic stridulatory structures. Most sound-making insects also have tympanal organs that can perceive airborne sounds. Most insects are also able to sense vibrations transmitted by the substrate. Communication using substrate-borne vibrational signals is more widespread among insects because of the size constraints in producing air-borne sounds. Insects cannot effectively produce low-frequency sounds, and high-frequency sounds tend to disperse more in a dense environment (such as foliage), so insects living in such environments communicate primarily using substrate-borne vibrations. The mechanisms of production of vibrational signals are just as diverse as those for producing sound in insects.
Some species use vibrations for communicating within members of the same species, such as to attract mates as in the songs of the shield bug Nezara viridula while it can also be used to communicate between entirely different species, such as between ants and myrmecophilous lycaenid caterpillars.
The Madagascar hissing cockroach has the ability to press air through the spiracles to make a hissing noise, and the Death's-head Hawkmoth makes a squeaking noise by forcing air out of their pharynx.
Only those insects which live in nests or colonies demonstrate any true capacity for fine-scale spatial orientation or homing - this can be quite sophisticated, however, and allow an insect to return unerringly to a single hole a few millimetres in diameter among a mass of thousands of apparently identical holes all clustered together, after a trip of up to several kilometres' distance, and (in cases where an insect hibernates) as long as a year after last viewing the area (a phenomenon known as philopatry). A few insects migrate, but this is a larger-scale form of navigation, and often involves only large, general regions (e.g., the overwintering areas of the Monarch butterfly).
Care of young
Insects are the only group of invertebrates to have developed flight. The evolution of insect wings has been a subject of debate. Some proponents suggest that the wings are para-notal in origin while others have suggested they are modified gills. In the Carboniferous age, some of the Meganeura dragonflies had as much as a 50 cm (20 in) wide wingspan. The appearance of gigantic insects has been found to be consistent with high atmospheric oxygen. The percentage of oxygen in the atmosphere found from ice core-samples was as high as 35% compared to the current 21%. The respiratory system of insects constrains their size, however the high oxygen in the atmosphere allowed larger sizes. The largest flying insects today are much smaller and include several moth species such as the Atlas moth and the White Witch (Thysania agrippina). Insect flight has been a topic of great interest in aerodynamics due partly to the inability of steady-state theories to explain the lift generated by the tiny wings of insects.
Unlike birds, insects are swept along by the prevailing winds. This includes aphids, which are often transported long distances by low-level jet streams. As such, fine line patterns associated with converging winds within weather radar imagery, like the WSR-88D, are dominated by insect returns.
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. A few insects have evolved to walk on the surface of the water, especially the bugs of the family, Gerridae, also known as water striders. A few species of ocean-skaters in the genus Halobates even live on the surface of open oceans, a habitat that has few insect species.
Insect walking is of particular interest as an alternative form of locomotion to the use of wheels for robots (Robot locomotion).
A large number of insects live either parts or the whole of their lives underwater. In many of the more primitive orders the immature stages are aquatic while some other groups have aquatic adults as well.
Many of these species have adaptations to help in locomotion under water. The water beetles and water bugs have legs adapted into paddle like structures. Dragonfly naiads use jet propulsion, forcibly expelling water out of the rectal chamber. Some species like the water striders are capable of walking on the surface of water. They can do this because their claws are not at the tips of the legs as in most insects, but recessed in a special groove further up the leg; this prevents the claws from piercing the water's surface film. Other insects such as the Rove beetle Stenus are known to emit salivary secretions that reduce surface tension making it possible for them to move on the surface of water by Marangoni propulsion (also known by the German term Entspannungsschwimmen). Species that are submerged also have adaptations to aid in respiration. Many larval forms have gills that can extract oxygen dissolved in water, while others need to rise to the water surface to replenish air supplies which may be held or trapped in special structures.
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 the six legs characteristic of adult insects.
Hexapoda (Insecta, collembola, diplura, protura)
The higher level phylogeny of the arthropods continues to be a matter of debate and research. In 2008, a Tufts University student and a faculty lecturer uncovered what they believe is the world's oldest known full-body impression of a primitive flying insect, a 300 million-year-old specimen from the Carboniferous Period. The scientists presented the fossil at the Second International Congress on Ichnology, in Krakow, Poland in September 2008. The oldest definitive insect fossil is the Devonian Rhyniognatha hirsti, from the 396 million year old Rhynie chert. This species already possessed dicondylic mandibles, a feature associated with winged insects, suggesting that wings may already have evolved at this time. Thus, the first insects probably appeared earlier, in the Silurian period.
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 no evidence 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, (22-28 in) making them far larger than any living insect. 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 extinct 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.
Simplified Cladogram of insect groups and very simplified. Note that Apterygota, Palaeoptera and Exopterygota are possibly paraphyletic groups.
Traditional morphology-based systematics has included in the subphylum Hexapoda four groups - Insects (Ectognatha), springtails (Collembola), Protura and Diplura, the latter three being grouped together as Entognatha on the basis of internalized mouthparts. Supraordinal relationships have undergone numerous changes with the advent of cladistic methods and genetic data. A recent hypothesis is that Hexapoda is polyphyletic, with the entognath classes having separate evolutionary histories from Insecta.
As many of the traditional morphology-based taxa have been shown to be paraphyletic, it is best to avoid using terms such as subclass, superorder and infraorder and instead focus on monophyletic groupings. The following list represents the best supported monophyletic groupings for the Insecta.
signifies an extinct taxon.
* Archaeognatha=Microcoryphia (bristletails)
* Thysanura=Zygentoma (silverfish)
* Ephemeroptera (mayflies)
* Isoptera (termites - included in Blattaria)
* Mantodea (mantids)
* Psocodea (booklice, barklice)
* Mallophaga (chewing lice - in Psocodea)
* Thysanoptera (thrips)
* Raphidioptera (snakeflies)
* Mecoptera (scorpionflies, hangingflies)
* Siphonaptera (fleas - in Mecoptera)
* Diptera (true flies)
* Trichoptera (caddisflies)
Insects can be divided into two groups, historically treated as subclasses: Apterygota (wingless) and Pterygota (winged). The Apterygota consists of two primitively wingless orders - Archaeognatha (bristletails) and Thysanura (silverfish). Archaeognatha makes up the Monocondylia (based on mandibular morphology) while Thysanura and Pterygota are grouped together as Dicondylia. It is possible that the Thysanura itself is not monophyletic, with the family Lepidotrichidae a sister group to the Dicondylia (Pterygota and the remaining Thysanura).
Paleoptera and Neoptera are the winged orders of insects, separated by the presence of sclerites and musculature that allow for folding of the wings flat over the abdomen in the latter group. Neoptera can further be divided into hemimetabolous (Polyneoptera & Paraneoptera) and Holometabolous groups. It has proven particularly difficult to elucidate interordinal relationships within Polyneoptera. Phasmatodea and Embiidina have been suggested to form Eukinolabia. Mantodea, Blattodea & Isoptera are thought to form a monophyletic group termed Dictyoptera. Paraneoptera has turned out to be more closely related to Endopterygota than to the rest of the Exopterygota. The recent molecular finding that the traditional louse orders Mallophaga and Anoplura are derived from within Psocoptera has led to the new taxon Psocodea.
It is quite likely that Exopterygota is paraphyletic in regards to Endopterygota. Contentious matters include Strepsiptera and Diptera grouped together as Halteria based on a reduction of one of the wing pairs - a position not well-supported in the entomological community. The Neuropterida are often lumped or split on the whims of the taxonomist. Fleas are now thought to be closely related to boreid mecopterans. Many questions remain to be answered when it comes to basal relationships amongst endopterygote orders, particularly Hymenoptera.
The study of the classification or taxonomy of any insect is called systemic entomology. Normally, if one chooses to work with a more specific order or even a family, the sytemics would be added to the study of that order or family, an example would be systemic dipterology
Relationship to humans
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, and 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. Honey bees 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. In some cultures, insects, especially deep-fried cicadas, are considered to be delicacies, and, in fact, have a high protein content for their mass. In most first-world countries, however, the consumption of insects is eschewed, although peoples in these cultures tend to accidentally consume between 50 and 90 insects in a given year, though this number varies based on region. 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, 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 safety 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, and insects are responsible for much of the process by which topsoil is created. The ancient Egyptian religion adored dung beetles and represented them as beetle-shaped amulets, or scarabs. Dung beetles have been used in countries including Australia as an agent of biological control to reduce the populations of pestilent flies and parasitic worms. The Australian Dung Beetle Project (1965-1985), led by Dr. George Bornemissza of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) successfully introduced 23 species of dung beetle, including Onthophagus gazella and Euoniticellus intermedius from South Africa and Europe and found a reduction in bush flies of 90% as well as improved soil fertility and quality.
The most useful of all insects are insectivores, those that feed on other insects. Many insects can potentially reproduce so quickly that if all of their offspring were to survive, they could literally bury the earth in a single season. However, for any given insect, there will be plenty of species of insects that are either parasitoids or predators that play a significant role in controlling it. This role in ecology is usually assumed to be primarily one of birds, but insects, though less glamorous, are much more significant.
Human attempts to control pests by insecticides can backfire, because important but unrecognised insects already helping to control pest populations are also killed by the poison, leading eventually to population explosions of the pest species.
This webpage was updated on 27th October 2010
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