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Honeybee (order Hymenoptera)
Honeybee (order Hymenoptera)
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Hexapoda
Class: Insecta
Linnaeus, 1758
Classes & Orders
See taxonomy
Insects are invertebrate animals of the Class Insecta, the largest and (on land) most widely-distributed taxon within the Phylum Arthropoda. Insects comprise the most diverse group of animals on the earth, with around 925,000 species described—more than all other animal groups combined: "Indeed, in no one of her works has Nature more fully displayed her exhaustless ingenuity," Pliny the Elder exclaimed. 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.
Chinese mantis
Chinese mantis
The exopterygote part of the Neoptera are sometimes divided into Orthopteroida (cerci present) and Hemipteroida (cerci absent), also called lower and higher Exopterygota. 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. Estimates of the total number of current species, including those not yet known to science, range from two to thirty million, with most authorities favoring a figure midway between these extremes. The study of insects is called entomology.
Green bottle fly
Green bottle fly

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Relationship to other arthropods
Morphology and development
Roles in the environment and human society

Relationship to other arthropods - Contents

A few smaller groups with similar body plans, such as springtails ( Collembola), are united with the insects in the Subphylum Hexapoda. But this Subphylum is proved to be artificial as springtails are no longer considered as relatives. They have most likely a different origin. This may also be that case for the rest of the members of the Entognatha; Protura and Diplura. The true insects (that is, species classified in the Class Insecta) are distinguished from all other arthropods in part by having ectognathous, or exposed, mouthparts and eleven (11) abdominal segments. Their mouthparts are also the reason why they are called Ectognatha. Most species, but by no means all, have wings as adults. Terrestrial arthropods, such as centipedes, millipedes, scorpions and spiders, are sometimes confused with insects due to the fact that both have similar body plans, sharing (as do all arthropods) a jointed exoskeleton.

Morphology and development - Contents

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. first wing   12. second wing   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
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. first wing
12. second wing
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 a mouth. The thorax has six legs (one pair per segment) and wings (if present in the species). The abdomen, originally made up of eleven segments, has excretory and reproductive structures.Their nervous system can be divided into a brain and a ventral nerve cord. As the head capsule are made up of six anterior body segments, the brain reflects this in its anatomy in containing 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 thorax pairs have one ganglion on each side, which are connected into a pair, one pair of ganglia in each thoraic segment. This arrangement is also found in the abdomen, but here there is one pair of ganglia in the first eight segments only, that is, three thoraic and eight abdominal paired ganglia. Many species of insects have since then reduced this number by losing or fusing some of the abdominal ganglions and/or fusing those in the thorax. Some cockroach have just six ganglia in the abdomen, whereas the wasp Vespa crabro have reduced the number further with only two in the thorax and three abdominal. And then finally insects like the well known housefly have fused all the body ganglions into on big thoraic 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 invertebrate group 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 atmospheric effects. In more primitive insects, flight tends to rely heavily on direct flight muscles, which act upon the wing structure. More advanced flyers, which make up the Neoptera, in general, have wings that can be folded over their back, keeping them out of the way when not in use. In these insects, the wings are powered mainly by indirect-flight muscles that move the wings by stressing the thorax wall. These muscles are able to contract when stretched without nervous impulses, allowing the wings to beat much faster than would be otherwise possible.Their outer skeleton called 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 procuticle. This is chitinous and much thicker than the first one, and who can be divided into two new layers. The first one is named the exocuticle and the second, last and deepest one is the endocuticle. The very tough and flexible endocuticle is built like numerous layers, made of fibres of chitin and proteins, crossing each others in a sandwich pattern.Insects use tracheal respiration in order to transport oxygen through their bodies. Openings on the surface of the body called spiracles lead to the tubular tracheal system. Air reaches internal tissues via this system of branching trachea. There are never more than one pair of spiracles per segment. And never more than two pairs of spiracles on thorax (mesothorax and metathorax), or more than eight pairs on the amdomen (the first eight segments). Many higher insects have reduced the number of spiracles; the hoverflies have lost all the spiracles on their abdomen. There is a limit to the pressure that the walls of the tracheal tubes can withstand without collapsing, even if they are stiffened with bands of chitin, which is one of the reasons why insects are relatively small. The spiracles are equipped with muscle controlled valves, enabling the insects to open and close them. By closing them, they can avoid drowning in water, or prevent moisture from escaping their body by opening them only when new air is needed. With little activity, the spiracles are often partially closed. To stop dust and other unwelcome small particles from entering their trachea system when inhaling, the spiracles are blocked with hair that filters the particles away. There are some species of insects, like members of Chironomidae, commonly called "blood worms," that contain true respiratory pigments such as hemoglobin in 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. Three pairs of the spiracles in water bugs are covered by a pressure-sensitive membrane. These work in much the same way as the human inner ear, and make it possible to know their position in the water. The last abdominal spiracle and associated trachea of caterpillars in the Lepidoptera have also been modified; the trachea of the eighth segment are modified into what can be called a trachea lung, as it has adapted to 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 expels air from certain spiracles to create a loud hissing sound.A diffuse tissue of cells found through out the hemocoel of insects, most of all in the abdomen, is called the fat body. Energy storage and metabolic processes are among its main functions. It is also the closest insects comes to a liver.The circulatory system of insects, like that of other arthropods, is open: The heart pumps the hemolymph through arteries 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 a very few exceptions, they also depends on long-chain fatty acids in their diet, especially 18-carbon chains. A lack of these fatty acids will affect ther development in a negative way, causing such things as longer time to mature and deformed adults.We can also find polyembryony in some insects. A single fertilized egg from polyembryonic parastic wasps can actually divide into literally thousands of separate embryos.
A butterfly is the adult stage of an insect with complete metamorphosis. This species is Anartia amathea.
A butterfly is the adult stage of an insect with complete metamorphosis. This species is Anartia amathea.
Most insects hatch from eggs, 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 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 outer covering. In most types of insects, the young, called nymphs, are basically similar in form to the adults (an example is the grasshopper), though wings are not developed until the adult stage. This is called incomplete metamorphosis. Complete metamorphosis distinguishes the Endopterygota, which 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, exarate and coarctate. 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. Other development traits are haplodiploidy, polymorphism, paedomorphosis (metathetely and prothetely), sexual dimorphism, parthenogenesis and more rarely hermaphroditism.

Behavior - Contents

Flies attracted to a light in summer
Flies attracted to a light in summer
Many insects possess very refined organs of perception. In some cases, their senses can be more capable than humans. For example, bees can see in the ultraviolet spectrum, and male moths have a specialized sense of smell that enables them to 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 eggs. The male solitary wasp in the genus Eumenus 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.

Roles in the environment and human society - Contents

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, (pictured above) 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. 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.
Lubber grasshopper
Lubber grasshopper
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 fast 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 - Contents

Subclass: ApterygotaSubclass: Pterygota
  • Infraclass: " Paleoptera" (paraphyletic)
  • Ephemeroptera (mayflies)
  • Palaeodictyoptera - extinct
  • Megasecoptera - extinct
  • Archodonata - extinct
  • Diaphanopterodea - extinct
  • Protodonata - extinct
  • Odonata (dragonflies and damselflies)
  • Infraclass: Neoptera
  • Superorder: Exopterygota
  • Caloneurodea - extinct
  • Titanoptera - extinct
  • Protorthoptera - extinct
  • Grylloblattodea (ice-crawlers)
  • Mantophasmatodea (gladiators)
  • Plecoptera (stoneflies)
  • Embioptera (webspinners)
  • Zoraptera (angel insects)
  • Dermaptera (earwigs)
  • Orthoptera (grasshoppers, etc)
  • Phasmatodea (walking sticks)
  • Blattodea (cockroaches)
  • Isoptera (termites)
  • Mantodea (mantids)
  • Psocoptera (booklice, barklice)
  • Thysanoptera (thrips)
  • Phthiraptera ( lice)
  • Hemiptera (true bugs)
  • Superorder: Endopterygota
  • Raphidioptera (snakeflies)
  • Megaloptera ( alderflies, etc.)
  • Neuroptera (net-veined insects)
  • Mecoptera (scorpionflies, etc.)
  • Siphonaptera (fleas)
  • Diptera (true flies)
  • Protodiptera extinct
Incertae sedis
  • Glosselytrodea extinct
  • 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). Paraneoptera are today believed 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 answear.

Evolution - Contents

Evolution has produced astonishing variety in insects. Pictured are some of the possible shapes of antennae.
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 origin of insect flight remains 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 - Contents

  • "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)
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