Insects (from Latin insectum, a calque of Greek ἔντομον [éntomon], "cut into sections")
are a class of living
creatures within the arthropodsthat have a chitinous exoskeleton, a three-part body (head, thorax, and abdomen), three
pairs of jointed legs, compound eyes, and twoantennae. They are among the most diverse
groups of animals on the
planet, including more than a million described species and 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 metazoan life forms on Earth .Insects may be found in nearly
all environments, although
only a small number of species occur in the oceans, a habitat dominated by another
arthropod group, the crustaceans .
The life
cycles of insects vary but most 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 ( Non Verbal
Sentence), 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 (verbal
sentence) .
Insects
typically move about by walking, flying or occasionally 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 (Verbal
Sentence). Many
insects spend at least part of their life underwater, with larval adaptations that include gills and some adult insects are
aquatic and have adaptations for swimming .Some species, like water
striders, are
capable of walking on the surface of water. Insects are mostly solitary, but
some insects, such as certain bees, ants, and termites are social and live in large,
well-organized colonies. Some insects, like 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 distances of many kilometers .
Many other
insects are considered ecologically beneficial as predators and a few provide
direct economic benefit .Silkworms and
bees have
been used extensively (Non Verbal Sentence ) by humans for the production of silk and honey,
respectively .
Insects have segmented bodies (Verbal
Sentence)supported by
an exoskeleton, a 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, if present in the species, 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 . There is considerable variation
and many adaptations in the body parts of insects especially wings, legs,
antenna, mouth-parts etc .
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 others 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
(Verbal Sentence), and this has played an important role in their success (Verbal Sentence) . 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 .
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 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 (Verbal Sentence)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 (Non Verbal Sentence), there is not a 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 (Verbal Sentence)and salivary reservoirs. These
structures usually reside in the thorax, adjacent to the foregut .
Other
developmental and reproductive variations include haplodiploidy, polymorphism, paedomorphosis orperamorphosis, 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 the 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 (Verbal Sentence). 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 a male. 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 are diploid . 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 of diapause (Verbal
Sentence) that allow
eggs or pupae to survive these conditions .
Senses and Communication
Many insects
possess very sensitive and or
specialized organs of perception. Some
insects such as bees can perceive ultraviolet wavelengths, 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 ( Non Verbal Sentence) to hear up to 2 MHz., and some grasshoppers can hear up to
50 MHz .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.
Light production and vision
Insects have
compound eyes and two antennae.
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 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 (Non Verbal Sentence) and phylogenetic analysis suggests that UV-green-bluetrichromacy 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. In grasshoppersand 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 (Non Verbal Sentence) decibels at a distance of 50 cm (20 in). Some insects, such as
the hawk
moths and Hedylid butterflies, can hear ultrasound and
take evasive action when they sense that they have been detected by bats (Non Verbal Sentence). 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 (Verbal Sentence) .
Insects are
the only group of invertebrates to have developed flight . The evolution of
insect wings has been a subject of debate (Non Verbal Sentence). Someentomologists suggest that the wings are from paranotal lobes, or extensions
from the insect's exoskeleton called the nota, called the paranotal 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 (Non Verbal Sentence). 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 (Non Verbal Sentence) 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 make migrations .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 .
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