Encyclopedia
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. Insects are 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.
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 thirty million, with most authorities favoring a figure midway between these extremes. Adult modern insects range in size from 555 mm to 0.139 mm
The study of insects is called
entomology, from the Greek e?t?µ??, also meaning "cut into sections" .
Relationship to other arthropods
Other terrestrial arthropods, such as
centipedes,
millipedes,
scorpions and
spiders, are sometimes confused with insects since their body plans can appear similar, sharing 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 , 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 that 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 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 and Hemipteroida , also called lower and higher Exopterygota.
Morphology and development
Insects range in size from less than a millimeter to over 18 centimeters 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 and
wings . The abdomen has respiratory, excretory and reproductive structures.
Their nervous system can be divided into a brain and a ventral nerve cord. The head capsule 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 .
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 . 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
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 , scarabaeiform , campodeiform , elateriform and vermiform . 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.
Some insects 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 ,
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.
Locomotion
Walking
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 . 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 .
Roles in the environment and human society
Many insects are considered pests by humans. Insects commonly regarded as pests include those that are parasitic , transmit diseases , damage structures , or destroy agricultural goods . 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 . 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 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 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
- Ephemeroptera
- Palaeodictyoptera - extinct
- Megasecoptera - extinct
- Archodonata - extinct
- Diaphanopterodea - extinct
- Protodonata - extinct
- Odonata
Orders
- Caloneurodea - extinct
- Titanoptera - extinct
- Protorthoptera - extinct
Polyneoptera
Orthopteroidea
Dictyoptera
Paraneoptera
Orders
Neuropteroidea
Mecopteroidea
Amphiesmenoptera
Incertae sedis
- Glosselytrodea extinct
- Miomoptera - extinct
As seen above, insects are divided into two subclasses; Apterygota and Pterygota , but this could relatively soon change. Apterygota is made up of two orders; Archaeognatha and Thysanura . 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 .
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 . 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.
References
- — a classic textbook in North America
- — an up to date review of the evolutionary history of the insects
- — detail coverage of various aspects of the evolutionary history of the insects
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
- 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
See also
- - 8MB XviD in Ogg container showing a flesh fly using its front and back pairs of legs to clean wings and head. The film runs at half speed to enable the viewer to appreciate the fast movements of the animal.
External links
- A free to read book courtesy the Carnegie Museum of Natural History Library
- A collection of 6000+ insect pictures . Brazilian collection .
- A library of 4,000+ reference quality large format insect pictures. Creative Commons licensed
- A reference library of digitized insect sounds.
- Insect appreciation.
- by Gene DeFoliart. Information about insects as a food resource.
- Website about insect evolution and fossil record.
- Website of the International Palaeoentomological Society.
- – Insecta
- , documenting "insect champions" in different categories
- Photographs, life history information, and identification of North American arthropods, especially insects
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- Insect information and User-submitted Insect pictures
- Fred Delcomyn, Research into Insect Locomotion at the University of Illinois
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- Insect research portal
For further reading
- Davidson, E. 1981. Pathogenesis of Invertebrate Micorobial Diseases. Allanheld, Osmun & Co. Publishers, Inc., Totowa, New Jersey, USA. 562 pages.
- Davidson, E. 2006. Big Fleas Have Little Fleas: How Discoveries of Invertebrate Diseases Are Advancing Modern Science University of Arizona Press, Tucson, 208 pages, ISBN 0-8165-2544-7
- Davidson, RH and William F. Lyon. 1979 Insect Pests of Farm, Garden, and Orchard. John Wiley & Sons., New York. 596 pages, ISBN 0-471-86314-9.
- Triplehorn, Charles A. and Norman F. Johnson . Borror and DeLong's Introduction to the Study of Insects, 7th edition, Thomas Brooks/Cole. ISBN 0-03-096835-6. — a classic textbook in North America
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