INSECTS
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| Clockwise from top left: dancefly (Empis livida), long-nosed weevil (Rhinotia hemistictus), mole cricket (Gryllotalpa brachyptera), German wasp (Vespula germanica), emperor gum moth (Opodiphthera eucalypti), assassin bug (Harpactorinae) | |
Scientific classification | |
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| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Clade: | Pancrustacea |
| Subphylum: | Hexapoda |
| (unranked): | Ectognatha |
| Class: | Insecta Linnaeus, 1758 |
| Subclasses and orders | |
Insects (from Latin insectum,
a calque of Greek ἔντομον [éntomon],
"cut into sections") are a class of invertebrates within the arthropodphylum that have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of jointed legs, compound eyes and one pair
of antennae.
They are among the most diverse groups of animals on the planet, including more than a
million described speciesand representing more
than half of all known living organisms The number of extant species is
estimated at between six and ten million, and potentially represent over
90% of the differing animal life forms on Earth.Insects may be found in nearly
allenvironments,
although only a small number of species reside in the oceans, a habitat
dominated by another arthropod group, crustaceans.
The
life cycles of insects vary but most insects hatch from eggs. Insect growth is
constrained by the inelastic exoskeleton and development
involves a series of molts. The immature stages can
differ from the adults in structure, habit and habitat, and can include a
passive pupalstage in those groups
that undergo complete
metamorphosis.
Insects that undergo incomplete metamorphosis lack
a pupal stage and adults develop through a series of nymphal stages.The higher
level relationship of the hexapoda is unclear. Fossilized insects of
enormous size have been found from the Paleozoic Era, including giant dragonflies with
wingspans of 55 to 70 cm (22–28 in). The most diverse insect groups
appear to have coevolved with
flowering plants.
Adult
insects typically move about by walking, flying,
or sometimes swimming. As it allows for rapid yet stable movement, many insects
adopt a tripedal gait in which they walk with their legs touching the ground in
alternating triangles. Insects are the only invertebrates to have evolved
flight. Many insects spend at least part of their lives under water, with larval adaptations that include gills, and some adult insects are aquatic and
have adaptations for swimming. Some species, such as water striders, are capable of walking on the surface of
water. Insects are mostly solitary, but some, such as certain bees, ants and termites, are social and live in large,
well-organized colonies. Some insects, such as earwigs, show maternal care, guarding their eggs
and young. Insects can communicate with each other in a variety of ways.
Male moths can sense the pheromones of female moths over great distances.
Other species communicate with sounds: crickets stridulate, or rub their wings
together, to attract a mate and repel other males. Lampyridae in the beetle order Coleoptera communicate with
light.
Humans
regard certain insects as pests, and attempt to control
them using insecticides and a host of other
techniques. Some insects damage crops by feeding on sap, leaves or fruits. A few parasitic species are pathogenic. Some insects perform complex ecological
roles;blow-flies, for example, help consume carrion but also spread diseases.
lnsect pollinators are essential to
the life-cycle of many flowering plant species on which
most organisms, including humans, are at least partly dependent; without them,
the terrestrial portion of the biosphere (including humans) would be
devastated.Many other insects are considered ecologically beneficial as
predators and a few provide direct economic benefit. Silkworms and bees have been used extensively
by humans for the production of silk and honey, respectively. In some cultures, the
larvae or adults of certain insects are a food-source for humans.
Contents
Etymology
The
word "insect" comes from the Latin word insectum, meaning "with a
notched or divided body", or literally "cut into", from the
neuter singular past participle of insectare, "to cut into, to
cut up", from in- "into" and secare "to
cut";because insects appear "cut into" three sections. Pliny the Elder introduced the
Latin designation as a loan-translation of theGreek word ἔντομος (éntomos) or
"insect" (as in entomology), which was Aristotle's term for this class of life, also in
reference to their "notched" bodies. "Insect" first appears
documented in English in 1601 in Holland's translation of Pliny.
Translations of Aristotle's term also form the usual word for
"insect" in Welsh (trychfil, from trychu "cut"
and mil, "animal"), Serbo-Croatian (zareznik,
from rezati, "cut"), Russian (насекомое nasekomoe,
from sekat', "cut"), etc.
Phylogeny and
evolution
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.
| Evolution of insects |
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A phylogenetic tree of the arthropods and related groups
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The evolutionary relationship of insects to other animal groups remains unclear. Although more traditionally grouped with millipedes andcentipedes, evidence has emerged favoring closer evolutionary ties with crustaceans. In the Pancrustacea theory, insects, together withRemipedia and Malacostraca, make up a natural clade. Other terrestrial arthropods, such as centipedes, millipedes, scorpions andspiders, are sometimes confused with insects since their body plans can appear similar, sharing (as do all arthropods) a jointeThe higher-level phylogeny of the arthropods continues to be a matter of debate and research. In 2008, researchers at Tufts University 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 oldest definitive insect fossil is the DevonianRhyniognatha hirsti, from the 396-million-year-old Rhynie chert. It may have superficially resembled a modern-day silverfish insect. This species already possessed dicondylic mandibles (two articulations in the mandible), 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.
There
have been four super radiations of insects: beetles (evolved ~300 million years ago),flies (evolved
~250 million years ago), moths and wasps (evolved ~150 million years ago)
These four groups account for the majority of described species. The flies and
moths along with the fleas evolved from the Mecoptera.
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. As of 2009, there
is no evidence that suggests that the insects were a particularly successful
group of animals before they evolved to have wings.
Late Carboniferous and Early Permian insect orders include both extant groups and a number
of Paleozoic species, now extinct. During this era, some giant dragonfly-like
forms reached wingspans of 55 to 70 cm (22 to 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 period 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 as long as
146 million years ago in the Cretaceous period, but achieved their wide
diversity more recently in the Cenozoicera, which began 66 million years ago. A
number of highly successful insect groups evolved in conjunction with flowering plants, a powerful illustration
of coevolution.
Many
modern insect genera developed during
the Cenozoic. Insects from this period on are often found preserved in amber, often in perfect condition. The body
plan, ormorphology,
of such specimens is thus easily compared with modern species. The study of
fossilized insects is called paleoentomology.
Evolutionary relationships
Insects
are prey for a variety of organisms, including terrestrial vertebrates. The
earliest vertebrates on land existed 400 million years ago and were large amphibious piscivores. Through gradual evolutionary
change, insectivory was the next diet
type to evolve.
Insects
were among the earliest terrestrial herbivores and acted as major selection agents
on plants. Plants evolved chemical defenses
against this herbivory and the insects in turn evolved
mechanisms to deal with plant toxins. Many insects make use of these toxins to
protect themselves from their predators. Such insects often advertise their
toxicity using warning colors. This successful evolutionary pattern has
also been utilized by mimics. Over time, this has led
to complex groups of coevolved species. Conversely, some interactions between
plants and insects, like pollination, are beneficial to both
organisms. Coevolution has led to the development of very specific mutualisms in
such systems.
| Taxonomy |
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Cladogram of living insect groups, with
numbers of species in each group. Note that Apterygota, Palaeoptera andExopterygota are possibly paraphyleticgroups.
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Traditional morphology-based or appearance-based systematics has usually given Hexapoda the rank of superclass, and identified four groups within it: insects (Ectognatha), springtails (Collembola), Protura and Diplura, the latter three being grouped together as Entognatha on the basis of internalized mouth parts. Supraordinal relationships have undergone numerous changes with the advent of methods based on evolutionary history and genetic data. A recent theory is that Hexapoda is polyphyletic (where the last common ancestor was not a member of the group), with the entognath classes having separate evolutionary histories from Insecta Many of the traditional appearance-based taxa have been shown to be paraphyletic, so rather than using ranks likesubclass, superorder and infraorder, it has proved better to use monophyletic groupings (in which the last common ancestor is a member of the group). The following represents the best supported monophyletic groupings for the Insecta.
Insects
can be divided into two groups historically treated as subclasses: wingless
insects, known as Apterygota, and winged insects, known as Pterygota. The Apterygota
consist of the primitively wingless order of the silverfish (Thysanura).
Archaeognatha make up the Monocondylia based on the shape of their mandibles,
while Thysanura and Pterygota are grouped together as Dicondylia. It is
possible that the Thysanura themselves are not monophyletic, with the family Lepidotrichidae being a sister group to the Dicondylia
(Pterygota and the remaining Thysanura).
Paleoptera
and Neoptera are the winged orders of insects differentiated by the presence of
hardened body parts called sclerites; also, in Neoptera, muscles that allow
their wings to fold flatly over the abdomen. Neoptera can further be divided
into incomplete metamorphosis-based (Polyneoptera and Paraneoptera) and complete
metamorphosis-based groups. It has proved difficult to clarify the
relationships between the orders in Polyneoptera because of constant new
findings calling for revision of the taxa. For example, 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 Phasmatodea and Embiidina have been suggested to form
Eukinolabia. Mantodea, Blattodea and Isoptera are thought to form a
monophyletic group termed Dictyoptera
It
is likely that Exopterygota is paraphyletic in regard to Endopterygota. Matters
that have had a lot of controversy 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 systematic entomology. If one works with a
more specific order or even a family, the term may also be made specific to
that order or family, for example systematic dipterology.
Distribution
and diversity
Though the true dimensions of species
diversity remain uncertain, estimates range from 1.4 to 1.8 million species.
This probably represents less than 20% of all species on Earth, and with only
about 20,000 new species of all organisms being described each year, most species
likely will remain undescribed for many years unless species descriptions
increase in rate. About 850,000–1,000,000 of all described species are insects.
Of the 24 orders of insects, four dominate in terms of numbers of described
species, with an estimated 600,000–795,000 species included in Coleoptera, Diptera, Hymenoptera and Lepidoptera. Almost as many species
of beetles have been named as all other insects added together, or all other
noninsects (plants and animals
Comparison of the estimated number of species in the four most speciose insect orders
Comparison of the estimated number of species in the four most speciose insect orders
Comparison of the estimated number of species
in the four most speciose insect orders
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Described species
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Average description rate (species per year)
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Publication effort
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Coleoptera
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300,000–400,000
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2308
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0.01
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Lepidoptera
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110,000–120,000
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642
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0.03
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Diptera
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90,000–150,000
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1048
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0.04
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Hymenoptera
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100,000–125,000
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1196
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0.02
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Morphology and physiology
External
Insect morphology
A- Head B- Thorax C- Abdomen
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 blood vessel
8. tracheal tubes (trunk with spiracle)
9. mesothorax
10. metathorax
11. forewing
12. hindwing
13. mid-gut (stomach)
14. dorsal tube (Heart)
15. ovary
16. hind-gut (intestine, rectum & anus)
17. anus
18. oviduct
19. nerve chord (abdominal ganglia)
20. Malpighian tubes
21. tarsal pads
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
2. ocelli (lower)
3. ocelli (upper)
4. compound eye
5. brain (cerebral ganglia)
6. prothorax
7. dorsal blood vessel
8. tracheal tubes (trunk with spiracle)
9. mesothorax
10. metathorax
11. forewing
12. hindwing
13. mid-gut (stomach)
14. dorsal tube (Heart)
15. ovary
16. hind-gut (intestine, rectum & anus)
17. anus
18. oviduct
19. nerve chord (abdominal ganglia)
20. Malpighian tubes
21. tarsal pads
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
have segmented bodies
supported by exoskeletons, the hard outer covering
made mostly of chitin. The segments of the
body are organized into three distinctive but interconnected units, or tagmata: a head, a thorax and
an abdomen. The head supports
a pair of sensory antennae, a pair of compound eyes, and, if present, one to
three simple eyes (or ocelli) and three sets of
variously modified appendages that form the mouthparts. The thorax has six
segmented legs—one pair each for the
prothorax, mesothorax and the metathorax segments making up the thorax—and,
none, two or four wings. The abdomen consists of
eleven segments, though in a few species of insects, these segments may be
fused together or reduced in size. The abdomen also contains most of the digestive, respiratory, excretory and reproductive internal
structures.Considerable variation and many adaptations in the body parts of
insects occur, especially wings, legs, antenna and mouthparts.
Segmentation
The head is enclosed in a hard, heavily
sclerotized, unsegmented, exoskeletal head capsule, or epicranium, which contains most of the sensing
organs, including the antennae, ocellus or eyes, and the mouthparts. Of all the
insect orders, Orthoptera displays the most features found in other insects,
including the sutures and sclerites Here, the vertex, or the apex (dorsal
region), is situated between the compound eyes for insects with a hypognathous and opisthognathous head. In
prognathous insects, the vertex is not found between the compound eyes, but
rather, where the ocelli are normally. This
is because the primary axis of the head is rotated 90° to become parallel to
the primary axis of the body. In some species, this region is modified and
assumes a different name.
The thorax is a tagma composed of three
sections, the prothorax, mesothorax and the metathorax. The anterior segment, closest to the
head, is the prothorax, with the major features being the first pair of legs
and the pronotum. The middle segment is the mesothorax, with the major features
being the second pair of legs and the anterior wings. The third and most
posterior segment, abutting the abdomen, is the metathorax, which features the
third pair of legs and the posterior wings. Each segment is dilineated by an
intersegmental suture. Each segment has four basic regions. The dorsal surface
is called the tergum (ornotum) to distinguish it from the abdominal
terga. The two lateral regions are called the pleura (singular: pleuron)
and the ventral aspect is called the sternum. In turn, the notum of the
prothorax is called the pronotum, the notum for the mesothorax is called the
mesonotum and the notum for the metathorax is called the metanotum. Continuing
with this logic, the mesopleura and metapleura, as well as the mesosternum and
metasternum, are used.
The abdomen: is the largest tagma of the insect, which
typically consists of 11–12 segments and is less strongly sclerotized than the
head or thorax. Each segment of the abdomen is represented by a sclerotized
tergum and sternum. Terga are separated from each other and from the adjacent
sterna or pleura by membranes. Spiracles are located in the pleural area.
Variation of this ground plan includes the fusion of terga or terga and sterna
to form continuous dorsal or ventral shields or a conical tube. Some insects
bear a sclerite in the pleural area called a laterotergite. Ventral sclerites
are sometimes called laterosternites. During the embryonic stage of many insects and the
postembryonic stage of primitive insects, 11 abdominal segments are present. In
modern insects there is a tendency toward reduction in the number of the
abdominal segments, but the primitive number of 11 is maintained during
embryogenesis. Variation in abdominal segment number is considerable. If the
Apterygota are considered to be indicative of the ground plan for pterygotes,
confusion reigns: adult Protura have 12 segments, Collembola have 6. The
orthopteran family Acrididae has 11 segments, and a fossil specimen of
Zoraptera has a 10-segmented abdomen.
Exoskeleton
The
insect 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 a lower layer called the procuticle. The procuticle is chitinous and much
thicker than the epicuticle and has two layers: an outer layer known as the
exocuticle and an inner layer known as the endocuticle. The tough and flexible
endocuticle is built from numerous layers of fibrous chitin and proteins,
criss-crossing each other in a sandwich pattern, while the exocuticle is rigid
and hardened. The exocuticle is
greatly reduced in many soft-bodied insects (e.g., caterpillars), especially during
their larval stages.
Insects
are the only invertebrates to have developed
active flight capability, and this has played an important role in their success. Their
muscles are able to contract multiple times for each single nerve impulse,
allowing the wings to beat faster than would ordinarily be possible. Having
their muscles attached to their exoskeletons is more efficient and allows more
muscle connections; crustaceans also use the same method, though
all spiders use hydraulic pressure to extend their legs, a
system inherited from their pre-arthropod ancestors. Unlike insects, though,
most aquatic crustaceans are biomineralized with calcium carbonate extracted from the
water.
Internal
Nervous system
The nervous system of an insect can be
divided into a brain and a ventral nerve cord.
The head capsule is made up of six fused segments, each with either a pair
of ganglia, or a cluster of nerve
cells outside of the brain. The first three pairs of ganglia are fused into the
brain, while the three following pairs are fused into a structure of three
pairs of ganglia under the insect's esophagus, 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. Some insects, like the house fly Musca domestica, have all the body
ganglia fused into a single large thoracic ganglion.
At
least a few insects have nociceptors, cells that detect and transmit sensations
of pain. This was
discovered in 2003 by studying the variation in reactions of larvae of the common fruitfly Drosophila to the touch of a heated probe and an
unheated one. The larvae reacted to the touch of the heated probe with a
stereotypical rolling behavior that was not exhibited when the larvae were
touched by the unheated probe Although nociception has been demonstrated
in insects, there is no consensus that insects feel pain consciously but
see Pain in
invertebrates.
Digestive system
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 like proteins, polysaccharides, fats and nucleic acids. These macromolecules
must be broken down by catabolic reactions into smaller
molecules like amino acids and simple sugars before being used
by cells of the body for energy, growth, or reproduction. This break-down
process is known as digestion.
The
main structure of an insect's digestive system is a long enclosed tube called
the alimentary canal,
which runs lengthwise through the body. The alimentary canal directs food
unidirectionally from the mouth to the anus. It has three sections, each of which
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 foregut
The salivary glands (element 30 in
numbered diagram) in an insect's mouth produce saliva. The salivary ducts lead
from the glands to the reservoirs and then forward through the head to an
opening called the salivarium, located behind the hypopharynx. By moving its
mouthparts (element 32 in numbered diagram) the insect can mix its food with
saliva. The mixture of saliva and food then travels through the salivary tubes
into the mouth, where it begins to break down. Some insects, like flies, have extra-oral digestion.
Insects using extra-oral digestion expel digestive enzymes onto their food to
break it down. This strategy allows insects to extract a significant proportion
of the available nutrients from the food source. The gut is where almost
all of insects' digestion takes place. It can be divided into the foregut, midgut and hindgut.
Foregut
Stylized
diagram of insect digestive tract showing malpighian
tubule,
from an insect of the order Orthoptera.
The
first section of the alimentary canal is the foregut (element 27 in numbered diagram), or
stomodaeum. The foregut is lined with a cuticular lining made of chitin and proteins as protection from tough food. The
foregut includes the buccal cavity (mouth), pharynx,esophagus and crop and proventriculus (any part may be
highly modified) which both store food and signify when to continue passing
onward to the midgut
Digestion
starts in buccal cavity (mouth) as
partially chewed food is broken down by saliva from the salivary glands. As the
salivary glands produce fluid and carbohydrate-digesting enzymes (mostly amylases), strong muscles in the pharynx pump fluid
into the buccal cavity, lubricating the food like the salivarium does, and
helping blood feeders, and xylem and phloem feeders.
From
there, the pharynx passes food to the esophagus, which could be just a simple
tube passing it on to the crop and proventriculus, and then onward to the
midgut, as in most insects. Alternately, the foregut may expand into a very
enlarged crop and proventriculus, or the crop could just be a diverticulum, or fluid-filled
structure, as in some Diptera species.
Once
food leaves the crop, it passes to the midgut (element 13 in numbered diagram),
also known as the mesenteron, where the majority of digestion takes place.
Microscopic projections from the midgut wall, called microvilli, increase the surface
area of the wall and allow more nutrients to be absorbed; they tend to be close
to the origin of the midgut. In some insects, the role of the microvilli and
where they are located may vary. For example, specialized microvilli producing
digestive enzymes may more likely be near the end of the midgut, and absorption
near the origin or beginning of the midgut.
Hindgut
In
the hindgut (element 16 in
numbered diagram), or proctodaeum, undigested food particles are joined
by uric acid to form fecal
pellets. The rectum absorbs 90% of the water in these fecal pellets, and the
dry pellet is then eliminated through the anus (element 17), completing the
process of digestion. The uric acid is formed using hemolymph waste products
diffused from the Malpighian
tubules (element
20). It is then emptied directly into the alimentary canal, at the junction
between the midgut and hindgut. The number of Malpighian tubules possessed by a
given insect varies between species, ranging from only two tubules in some
insects to over 100 tubules in others.
Endocrine system
The salivary glands (element 30 in
numbered diagram) in an insect's mouth produce saliva. The salivary ducts lead
from the glands to the reservoirs and then forward through the head to an
opening called the salivarium, located behind the hypopharynx. By moving its
mouthparts (element 32 in numbered diagram) the insect can mix its food with
saliva. The mixture of saliva and food then travels through the salivary tubes
into the mouth, where it begins to break down. Some insects, like flies, have extra-oral digestion.
Insects using extra-oral digestion expel digestive enzymes onto their food to
break it down. This strategy allows insects to extract a significant proportion
of the available nutrients from the food source.
Reproductive system
The
reproductive system of female insects consist of a pair of ovaries, accessory glands, one or more spermathecae, and ducts connecting
these parts. The ovaries are made up of a number of egg tubes, called ovarioles, which vary in size and number by species.
The number of eggs that the insect is able to make vary by the number of
ovarioles with the rate that eggs can be develop being also influenced by
ovariole design. Female insects are able make eggs, receive and store sperm,
manipulate sperm from different males, and lay eggs. Accessory glands or
glandular parts of the oviducts produce a variety of substances for sperm
maintenance, transport and fertilization, as well as for protection of eggs.
They can produce glue and protective substances for coating eggs or tough
coverings for a batch of eggs called oothecae. Spermathecae are tubes or sacs in which
sperm can be stored between the time of mating and the time an egg is
fertilized.
For
males, the reproductive system is the testis, suspended in the body cavity by tracheae and
the fat body. Most male insects have a pair of testes, inside of which are
sperm tubes or follicles that are enclosed within a membranous sac. The
follicles connect to the vas deferens by the vas efferens, and the two tubular
vasa deferentia connect to a median ejaculatory duct that leads to the outside.
A portion of the vas deferens is often enlarged to form the seminal vesicle,
which stores the sperm before they are discharged into the female. The seminal
vesicles have glandular linings that secrete nutrients for nourishment and
maintenance of the sperm. The ejaculatory duct is derived from an invagination
of the epidermal cells during development and, as a result, has a cuticular
lining. The terminal portion of the ejaculatory duct may be sclerotized to form
the intromittent organ, the aedeagus. The remainder of the male reproductive
system is derived from embryonic mesoderm, except for the germ cells, or spermatogonia, which descend from the
primordial pole cells very early during embryogenesis.
Respiratory and circulatory systems
The
tube-like heart (green) of the mosquito Anopheles gambiae extends
horizontally across the body, interlinked with the diamond-shaped wing muscles (also green) and surrounded by pericardial cells (red). Blue depictscell nuclei.
Insect respiration is
accomplished without lungs. Instead, the insect
respiratory system uses a system of internal tubes and sacs through
which gases either diffuse or are actively pumped, delivering oxygen directly
to tissues that need it via their trachea (element
8 in numbered diagram). Since oxygen is delivered directly, the circulatory
system is not used to carry oxygen, and is therefore greatly reduced. The
insect circulatory system has no veins or arteries, and instead consists of little more than
a single, perforated dorsal tube which pulsesperistaltically. Toward the thorax, the
dorsal tube (element 14) divides into chambers and acts like the insect's
heart. The opposite end of the dorsal tube is like the aorta of the insect
circulating the hemolymph, arthropods' fluid
analog of blood, inside the body
cavity. Air is taken in through openings on the sides of the abdomen
called spiracles.
The
respiratory system is an important reason that limits the size of insects. As
insects get bigger, this type of oxygen transport gets less efficient and thus
the heaviest insect currently weighs less than 100 g. However, with
increased atmospheric oxygen levels, as happened in the late Paleozoic, larger
insects were possible, such as dragonflies with wingspans of more than two
feet.
There
are many different patterns of gas exchange demonstrated by
different groups of insects. Gas exchange patterns in insects can range from
continuous and diffusive ventilation,
to discontinuous
gas exchange During
continuous gas exchange, oxygen is taken in and carbon dioxide is released in a
continuous cycle. In discontinuous gas exchange, however, the insect takes in
oxygen while it is active and small amounts of carbon dioxide are released when
the insect is at rest. Diffusive ventilation is simply a form of
continuous gas exchange that occurs by diffusion rather than physically taking in the
oxygen. Some species of insect that are submerged also have adaptations to aid
in respiration. As larvae, many insects 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
.
Reproduction and development
The
majority of insects hatch from eggs. The fertilization and
development takes place inside the egg, enclosed by a shell (chorion) that consists of maternal tissue. In
contrast to eggs of other arthropods, most insect eggs are drought resistant.
This is because inside the chorion two additional membranes develop from
embryonic tissue, the amnion and the serosa. This serosa secretes a cuticle rich in chitinthat protects the embryo against
desiccation. In Schizophora however the serosa
does not develop, but these flies lay their eggs in damp places, such as
rotting matter. Some species of insects, like the cockroach Blaptica dubia, as well as juvenile
aphids and tsetse flies, areovoviviparous. The eggs of
ovoviviparous animals develop entirely inside the female, and then hatch
immediately upon being laid. Some other species, such as those in the
genus of cockroaches known as Diploptera, are viviparous, and thus gestate inside the mother and areborn alive. Some insects, like parasitic wasps,
show polyembryony, where a single
fertilized egg divides into many and in some cases thousands of separate
embryos. Insects may be univoltine, bivoltine or multivoltine,
i.e. they may have one, two or many broods (generations) in a year
.The different forms of the male (top) and female (bottom) tussock mothOrgyia recens is an example of sexual dimorphism in insectsThe different forms of the male (top) and female (bottom) tussock moth Orgyia recens is an example of sexual dimorphism in insects.
.
.The different forms of the male (top) and female (bottom) tussock mothOrgyia recens is an example of sexual dimorphism in insectsThe different forms of the male (top) and female (bottom) tussock moth Orgyia recens is an example of sexual dimorphism in insects.
.
Other
developmental and reproductive variations include haplodiploidy, polymorphism,paedomorphosis or peramorphosis, sexual dimorphism, parthenogenesis and more
rarely hermaphroditism. In haplodiploidy,
which is a type of sex-determination
system,
the offspring's sex is determined by the number of sets of chromosomes an individual receives. This system
is typical in bees and wasps. Polymorphism is where a species may have
different morphs or forms, as in the oblong
winged katydid, which has four different varieties: green, pink and
yellow or tan. Some insects may retain phenotypes that are normally only seen in
juveniles; this is called paedomorphosis. In peramorphosis, an opposite sort of
phenomenon, insects take on previously unseen traits after they have matured
into adults. Many insects display sexual dimorphism, in which males and females
have notably different appearances, such as the moth Orgyia recens as an exemplar of
sexual dimorphism in insects.
Some
insects use parthenogenesis, a process in which the female
can reproduce and give birth without having the eggs fertilized by amale. Many aphids undergo a form of parthenogenesis,
called cyclical parthenogenesis, in which they alternate between one or many
generations of asexual and sexual reproduction. In summer, aphids are
generally female and parthenogenetic; in the autumn, males may be produced for
sexual reproduction. Other insects produced by parthenogenesis are bees, wasps
and ants, in which they spawn males. However, overall, most individuals are
female, which are produced by fertilization. The males are haploid and the females arediploid. More rarely, some insects
display hermaphroditism, in which a given individual
has both male and female reproductive organs.
Insect
life-histories show adaptations to withstand cold and dry conditions. Some
temperate region insects are capable of activity during winter, while some
others migrate to a warmer climate or go into a state of torpor. Still other insects have evolved
mechanisms ofdiapause that allow eggs or
pupae to survive these conditions.
Like other insects that develop through incomplete metamorphosis, this Southern Hawker dragonfly molts its exoskeleton (shown above) several times during its pre-adult life.
Metamorphosis in insects is the
biological process of development all insects must undergo. There are two forms
of metamorphosis: incomplete metamorphosis and complete metamorphosis.
Incomplete metamorphosis
Insects
that show hemimetabolism, or incomplete
metamorphosis, change gradually by undergoing a series of molts. An insect molts when it outgrows its
exoskeleton, which does not stretch and would otherwise restrict the insect's
growth. The molting process begins as the insect's epidermis secretes a
new epicuticle. After this new
epicuticle is secreted, the epidermis releases a mixture of enzymes that
digests the endocuticle and thus detaches the old cuticle. When this stage is
complete, the insect makes its body swell by taking in a large quantity of water
or air, which makes the old cuticle split along predefined weaknesses where the
old exocuticle was thinnest Other arthropods have a much different process
and only molt; though must accommodate for the difference in exoskeleton
structure and make up with other enzymes.
Immature
insects that go through incomplete metamorphosis are called nymphs or in the case of
dragonflies and damselflies, naiads. Nymphs are similar in
form to the adult except for the presence of wings, which are not developed
until adulthood. With each molt, nymphs grow larger and become more similar in
appearance to adult insects.
Gulf Fritillary life cycle, an example ofholometabolism.
Gulf Fritillary life cycle, an example ofholometabolism.
Holometabolism, or complete
metamorphosis, is where the inseGulf Fritillary life cycle, an example ofholometabolism.ct changes in four stages, an egg or embryo, a larva, a pupa and theadult or imago. In these species, an egg hatches to
produce a larva, which is generally
worm-like in form. This worm-like form can be one of several varieties:
eruciform (caterpillar-like), scarabaeiform (grub-like), campodeiform
(elongated, flattened and active), elateriform (wireworm-like) or vermiform
(maggot-like). The larva grows and eventually becomes a pupa, a stage marked by reduced movement and
often sealed within a cocoon. There are three types
of pupae: obtect, exarate or coarctate. Obtect pupae are compact, with the legs
and other appendages enclosed. Exarate pupae have their legs and other
appendages free and extended. Coarctate pupae develop inside the larval
skin. Insects undergo considerable change in form during the pupal stage,
and emerge as adults. Butterflies are a well-known example of insects that
undergo complete metamorphosis, although most insects use this life cycle. Some
insects have evolved this system to hypermetamorphosis.
Some
of the oldest and most successful insect groups, such Endopterygota, use a system of
complete metamorphosisComplete metamorphosis is unique to a group of certain
insect orders including Diptera, Lepidoptera and Hymenoptera. This form of
development is exclusive and not seen in any other arthropods.
.jpg)
Insects have compound eyes and two antennae.
Senses and communication
Senses and communication
Many
insects possess very sensitive and, or specialized organs of perception. Some insects such as bees can perceive ultravioletwavelengths, or
detect polarized light, while the antennae of male moths can
detect the pheromones of female moths
over distances of many kilometers. There is a pronounced tendency for
there to be a trade-off between visual acuity and chemical or tactile acuity,
such that most insects with well-developed eyes have reduced or simple
antennae, and vice versa. There are a variety of different mechanisms by which
insects perceive sound, while the patterns are not universal, insects can
generally hear sound if they can produce it. Different insect species can have
varying hearing, though most insects can
hear only a narrow range of frequencies related to the frequency of the sounds
they can produce. Mosquitoes have been found to hear up to 2 kHz., and
some grasshoppers can hear up to 50 kHz Certain predatory and
parasitic insects can detect the characteristic sounds made by their prey or
hosts, respectively. For instance, some nocturnal moths can perceive the ultrasonic emissions of bats, which helps them avoid
predation. Insects that feed on blood have special sensory structures that
can detect infrared emissions, and use
them to home in on their hosts.
Some
insects display a rudimentary sense of numbers, such as the solitary
wasps that prey upon a single species. 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 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.
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Light production and vision
A
few insects, such as members of the families Poduridae and Onychiuridae
(Collembola), Mycetophilidae (Diptera) and the
beetle familiesLampyridae, Phengodidae, Elateridae and Staphylinidae are bioluminescent. The most familiar group
are the fireflies, beetles of the family
Lampyridae. Some species are able to control this light generation to produce
flashes. The function varies with some species using them to attract mates,
while others use them to lure prey. Cave dwelling larvae of Arachnocampa (Mycetophilidae,
Fungus gnats) glow to lure small flying insects into sticky strands of
silk. Some fireflies of the genus Photuris mimic the flashing of female Photinus species to attract
males of that species, which are then captured and devoured. The colors of
emitted light vary from dull blue (Orfelia fultoni, Mycetophilidae) to
the familiar greens and the rare reds (Phrixothrix tiemanni,
Phengodidae).
Most
insects, except some species of cave crickets, are able to perceive
light and dark. Many species have acute vision capable of detecting minute
movements. The eyes may include simple eyes or ocelli as well as compound eyes of varying sizes.
Many species are able to detect light in the infrared, ultraviolet and
the visible light wavelengths. Color
vision has been demonstrated in many species and phylogenetic analysis suggests
that UV-green-blue trichromacy existed from at
least the Devonian period between 416
and 359 million years ago.
Sound production and hearing
Insects
were the earliest organisms to produce and sense sounds. Insects make sounds
mostly by mechanical action of appendages. Ingrasshoppers and crickets, this
is achieved by stridulation. Cicadas make the loudest sounds among the
insects by producing and amplifying sounds with special modifications to their
body and musculature. The African cicada Brevisana brevis has
been measured at 106.7 decibels at a distance of 50 cm
(20 in) Some insects, such as the Helicoverpa zeamoths, hawk moths and Hedylid butterflies, can hear ultrasound and take evasive action when they
sense that they have been detected by bats. Some moths produce ultrasonic
clicks that were once thought to have a role in jamming bat echolocation.
The ultrasonic clicks were subsequently found to be produced mostly by
unpalatable moths to warn bats, just as warning colorations are
used against predators that hunt by sight Some otherwise palatable moths
have evolved to mimic these calls
Very
low sounds are also produced in various species of Coleoptera, Hymenoptera, Lepidoptera, Mantodea and Neuroptera. These low sounds are simply the sounds
made by the insect's movement. Through microscopic stridulatory structures
located on the insect's muscles and joints, the normal sounds of the insect
moving are amplified and can be used to warn or communicate with other insects.
Most sound-making insects also have tympanal organs that can perceive
airborne sounds. Some species in Hemiptera, such as the corixids (water boatmen), are known to
communicate via underwater sounds. Most insects are also able to
sense vibrations transmitted through
surfaces.
Communication
using surface-borne vibrational signals is more widespread among insects
because of 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. Vibrations
can also be used to communicate between entirely different species; lycaenid (gossamer-winged butterfly)
caterpillars which are myrmecophilous (living in a
mutualistic association with ants) communicate with ants in this
way The Madagascar
hissing cockroach has the ability to press air through its spiracles
to make a hissing noise as a sign of aggression; the Death's-head
Hawkmoth makes
a squeaking noise by forcing air out of their pharynx when agitated, which may
also reduce aggressive worker honey bee behavior when the two are in close
proximity
Chemical communication
Chemical
communications in animals rely on a variety of aspects including taste and
smell. Chemoreception is the physiological response of a sense organ (i.e.
taste or smell) to a chemical stimulus where the chemicals act as signals to
regulate the state or activity of a cell. A semiochemical is a message-carrying
chemical that is meant to attract, repel, and convey information. Types of
semiochemicals include pheromones and kairomones. One example is the
butterfly Phengaris arion which
uses chemical signals as a form of mimicry to aid in predation.
In
addition to the use of sound for communication, a wide range of insects have
evolved chemical means for communication. These chemicals,
termed semiochemicals, are often derived from
plant metabolites include those meant to attract, repel and provide other kinds
of information. Pheromones, a type of
semiochemical, are used for attracting mates of the opposite sex, for
aggregating conspecific individuals of both
sexes, for deterring other individuals from approaching, to mark a trail, and
to trigger aggression in nearby individuals. Allomonea benefit their producer by the effect
they have upon the receiver. Kairomones benefit their receiver instead of
their producer. Synomones benefit the producer and the receiver. While some
chemicals are targeted at individuals of the same species, others are used for
communication across species. The use of scents is especially well known to
have developed in social insects.
Social insects, such as termites, ants and many bees and wasps, are the most familiar species of eusocial animal They live together in large well-organized colonies that may be so tightly integrated and genetically similar that the colonies of some species are sometimes considered superorganisms. It is sometimes argued that the various species of honey bee are the only invertebrates (and indeed one of the few non-human groups) to have evolved a system of abstract symbolic communication where a behavior is used to represent and convey specific information about something in the environment. In this communication system, called dance language, the angle at which a bee dances represents a direction relative to the sun, and the length of the dance represents the distance to be flown
Only
insects which live in nests or colonies demonstrate any true capacity for
fine-scale spatial orientation or homing. This can allow an insect to return
unerringly to a single hole a few millimeters in diameter among thousands of
apparently identical holes clustered together, after a trip of up to several
kilometers' distance. In a phenomenon known as philopatry, insects that hibernate have shown the
ability to recall a specific location up to a year after last viewing the area
of interest. A few insects seasonally migrate large distances
between different geographic regions (e.g., the overwintering areas of
the Monarch butterfly).
Care of young
The eusocial insects build nest, guard eggs, and
provide food for offspring full-time (see Eusociality). Most insects, however,
lead short lives as adults, and rarely interact with one another except to mate
or compete for mates. A small number exhibit some form of parental care, where they will at
least guard their eggs, and sometimes continue guarding their offspring until
adulthood, and possibly even feeding them. Another simple form of parental care
is to construct a nest (a burrow or an actual construction, either of which may
be simple or complex), store provisions in it, and lay an egg upon those
provisions. The adult does not contact the growing offspring, but it
nonetheless does provide food. This sort of care is typical for most species of
bees and various types of wasps.
Locomotion
Flight
Insects
are the only group of invertebrates to have developed flight.[Note 1] The evolution of insect wings has
been a subject of debate. Some entomologists suggest that the wings are from
paranotal lobes, or extensions from the insect's exoskeleton called the nota, called theparanotal theory. Other
theories are based on a pleural origin.
These theories include suggestions that wings originated from modified gills,
spiracular flaps or as from an appendage of the epicoxa. The epicoxal
theory suggests the insect wings are modified epicoxal exites, a
modified appendage at the base of the legs or coxa. 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
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, many small insects are swept along by
the prevailing winds although many of
the larger insects are known to makemigrations. Aphids are known to be 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 radar network,
often represent large groups of insects
.White-lined sphinx moth feeding in flight
.White-lined sphinx moth feeding in flight
a wings
b joints
c dorsoventral muscles
d longitudinal muscles.
Walking
Many
adult insects use six legs for walking and have adopted a tripedal gait. The tripedal gait allows for rapid
walking while 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, while 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 high speeds. 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 among the fastest insect runners and, at full speed, adopt a bipedal run to
reach a high velocity in proportion to their body size. As cockroaches move
very quickly, they need to be video recorded at several hundred frames per
second to reveal their gait. More sedate locomotion is seen in the stick
insects or walking sticks (Phasmatodea). A few insects have
evolved to walk on the surface of the water, especially members of the Gerridae family, commonly
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.
Use in robotics
Insect
walking is of particular interest as an alternative form of locomotion in robots. The study of insects and bipeds has a significant impact on possible
robotic methods of transport. This may allow new robots to be designed that can
traverse terrain that robots
with wheels may be unable to
handle
A large number of insects live either part or the whole of their lives underwater. In many of the more primitive orders of insect, the immature stages are spent in an aquatic environment. Some groups of insects, like certain water beetles, have aquatic adults as well.
Many
of these species have adaptations to help in under-water locomotion. Water beetles
and water bugs have legs adapted into paddle-like structures. Dragonfly naiads use jet propulsion,
forcibly expelling water out of their 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 pygidial gland
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)
Ecology
Insect
ecology is
the scientific study of how insects, individually or as a
community, interact with the surrounding environment or ecosystem. Insects play one of the most
important roles in their ecosystems, which includes many roles, such as soil
turning and aeration, dung burial, pest control, pollination and wildlife
nutrition. An example is thebeetles, which are scavengers that feed on dead animals and fallen
trees and thereby recycle biological
materials into forms found useful by other organisms. These insects, and others, are
responsible for much of the process by which topsoil is created.
Defense and predation
Insects
are mostly soft bodied, fragile and almost defenseless compared to other,
larger lifeforms. The immature stages are small, move slowly or are immobile,
and so all stages are exposed to predation and parasitism. Insects then have a variety of defense
strategies to avoid being attacked by predators or parasitoids. These include camouflage, mimicry, toxicity and active defense.
Perhaps one of the most well-known examples of mimicry, the viceroy butterfly(top) appears very similar to the noxious-tasting monarch butterfly (bottom).
Camouflage is an important defense strategy, which involves the use of coloration or shape to blend into the surrounding environment. This sort of protective coloration is common and widespread among beetle families, especially those that feed on wood or vegetation, such as many of the leaf beetles (family Chrysomelidae) or weevils. In some of these species, sculpturing or various colored scales or hairs cause the beetle to resemble bird dung or other inedible objects. Many of those that live in sandy environments blend in with the coloration of the substrate. Most phasmids are known for effectively replicating the forms of sticks and leaves, and the bodies of some species (such as O. macklotti and Palophus centaurus) are covered in mossy or lichenous outgrowths that supplement their disguise. Some species have the ability to change color as their surroundings shift (B. scabrinota, T. californica). In a further behavioral adaptation to supplement crypsis, a number of species have been noted to perform a rocking motion where the body is swayed from side to side that is thought to reflect the movement of leaves or twigs swaying in the breeze. Another method by which stick insects avoid predation and resemble twigs is by feigning death (catalepsy), where the insect enters a motionless state that can be maintained for a long period. The nocturnal feeding habits of adults also aids Phasmatodea in remaining concealed from predators.
Camouflage is an important defense strategy, which involves the use of coloration or shape to blend into the surrounding environment. This sort of protective coloration is common and widespread among beetle families, especially those that feed on wood or vegetation, such as many of the leaf beetles (family Chrysomelidae) or weevils. In some of these species, sculpturing or various colored scales or hairs cause the beetle to resemble bird dung or other inedible objects. Many of those that live in sandy environments blend in with the coloration of the substrate. Most phasmids are known for effectively replicating the forms of sticks and leaves, and the bodies of some species (such as O. macklotti and Palophus centaurus) are covered in mossy or lichenous outgrowths that supplement their disguise. Some species have the ability to change color as their surroundings shift (B. scabrinota, T. californica). In a further behavioral adaptation to supplement crypsis, a number of species have been noted to perform a rocking motion where the body is swayed from side to side that is thought to reflect the movement of leaves or twigs swaying in the breeze. Another method by which stick insects avoid predation and resemble twigs is by feigning death (catalepsy), where the insect enters a motionless state that can be maintained for a long period. The nocturnal feeding habits of adults also aids Phasmatodea in remaining concealed from predators.
Another
defense that often uses color or shape to deceive potential enemies is mimicry.
A number of longhorn beetles (family
Cerambycidae) bear a striking resemblance to wasps, which helps them avoid predation even
though the beetles are in fact harmless. Batesian and Müllerian mimicry complexes are commonly found in
Lepidoptera. Genetic polymorphism and natural selection give rise to otherwise
edible species (the mimic) gaining a survival advantage by resembling inedible
species (the model). Such a mimicry complex is referred to as Batesian and
is most commonly known by the mimicry by the limenitidine Viceroy butterfly of the
inedible danaine Monarch.
Later research has discovered that the Viceroy is, in fact more toxic than the
Monarch and this resemblance should be considered as a case of Müllerian
mimicry. In Müllerian mimicry, inedible species, usually within a
taxonomic order, find it advantageous to resemble each other so as to reduce
the sampling rate by predators who need to learn about the insects'
inedibility. Taxa from the toxic genus Heliconius form one of the
most well known Müllerian complexes.
Chemical
defense is another important defense found amongst species of Coleoptera and
Lepidoptera, usually being advertised by bright colors, such as the Monarch butterfly.
They obtain their toxicity by sequestering the chemicals from the plants they
eat into their own tissues. Some Lepidoptera manufacture their own toxins.
Predators that eat poisonous butterflies and moths may become sick and vomit
violently, learning not to eat those types of species; this is actually the
basis of Müllerian mimicry. A predator who has previously eaten a poisonous
lepidopteran may avoid other species with similar markings in the future, thus
saving many other species as well. Some ground beetles of the Carabidae
family can spray chemicals from their abdomen with great accuracy, to repel
predators.
Pollination
Pollination is the process by which pollen is transferred in the reproduction of plants, thereby enabling fertilisation and sexual reproduction. Most flowering plants require an animal to do the transportation. While other animals are included as pollinators, the majority of pollination is done by insects.[98] Because insects usually receive benefit for the pollination in the form of energy rich nectar it is a grand example ofmutualism. The various flower traits (and combinations thereof) that differentially attract one type of pollinator or another are known aspollination syndromes. These arose through complex plant-animal adaptations. Pollinators find flowers through bright colorations, including ultraviolet, and attractant pheromones. The study of pollination by insects is known as anthecology.
Parasitism
Many
insect are parasites of other insects such as the parasitoid wasps. These insects are known
as entomophagous
parasites.
They can be beneficial due to their devastation of pests that can destroy crops
and other resources. Many insects have a parasitic relationship with humans
such as the mosquito. These insects are known to spread diseases such as malaria and yellow fever and because of
such, mosquitoes indirectly cause more deaths of humans than any other animal.
Other biological interactions
Relationship
to humans
As pests
Many insects are considered pests by humans. Insects commonly regarded as pests include those that are parasitic (e.g. lice, bed bugs),transmit diseases (mosquitoes, flies), damage structures (termites), or destroy agricultural goods (locusts, weevils). Many entomologistsare involved in various forms of pest control, as in research for companies to produce insecticides, but increasingly rely on methods ofbiological pest control, or biocontrol. Biocontrol uses one organism to reduce the population density of another organism — the pest — and is considered a key element of integrated pest management.
Despite
the large amount of effort focused at controlling insects, human attempts to
kill pests with insecticides can backfire. If used carelessly, the poison can
kill all kinds of organisms in the area, including insects' natural predators,
such as birds, mice and other insectivores. The effects of DDT's use exemplifies how some insecticides
can threaten wildlife beyond intended populations of pest insects.
In beneficial roles
Because
they help flowering plants to cross-pollinate, some insects are critical to agriculture
.
This European honey bA robberfly with its prey, a hoverfly.Insectivorous relationships such as these
help control insect populations.
A robberfly with its prey, a hoverfly.Insectivorous relationships such as these help control insect populations
.
.
.jpg)
The
common fruitfly Drosophila
melanogaster is one of the most widely used organisms in
biological research.
Insects play important roles in biological research. For example, because of its small size, short generation time and high fecundity, the common fruit fly Drosophila melanogaster is a model organism for studies in the genetics of higher eukaryotes. D. melanogaster has been an essential part of studies into principles like genetic linkage, interactions between genes, chromosomal genetics, development, behavior and evolution. Because genetic systems are well conserved among eukaryotes, understanding basic cellular processes like DNA replication or transcription in fruit flies can help to understand those processes in other eukaryotes, including humans The genome of D. melanogaster was sequenced in 2000, reflecting the organism's important role in biological research.
As food
In
some cultures, insects, especially deep-fried cicadas, are considered to be delicacies, while in other places
they form part of the normal diet. Insects have a high protein content for
their mass, and some authors suggest their potential as a major source of protein in humannutrition. In most first-world countries,
however, entomophagy (the eating of
insects), is taboo. Since it is
impossible to entirely eliminate pest insects from the human food chain,
insects are inadvertently present in many foods, especially grains. Food safety laws in many
countries do not prohibit insect parts in food, but rather limit their quantity.
According to cultural materialist anthropologist Marvin Harris, the eating of insects
is taboo in cultures that have other protein sources such as fish or livestock.
Due
to the abundance of insects and a worldwide concern of food shortages,
the Food
and Agriculture Organisation of the United Nations considers that the
world may have to, in the future, regard the prospects of eating insects as a
food stable. Insects are noted for their nutrients, having a high content of
protein, minerals and fats and are eaten by one-third of the global population.
In culture
Scarab beetles held religious and
cultural symbolism in Old Egypt, Greece and some shamanistic Old World
cultures. The ancient Chinese regarded cicadas as symbols of rebirth or immortality.
In Mesopotamian literature, the
epic poem of Gilgamesh has allusions
to Odonata which signify the
impossibility of immortality. Amongst the Aborigines of Australia of the Arrernte language groups,
honey ants and witchety grubs served as personal clan totems. In the case of
the 'San' bush-men of the Kalahari, it is the praying mantis which holds much
cultural significance including creation and zen-like patience in waiting.
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