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Body Regions and Exoskeleton
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The insect's body is a series of segments that are grouped
into three sections, the head, thorax, and abdomen. The six
segments in the head cannot be seen because they are fused
together. The thorax has three segments, each bearing a pair
of legs. The abdomen of most insects has 10 segments,
although primitive insects may have as many as 12.
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The segments of the insect's body are hardened to form an
external skeleton, or exoskeleton. The insect's exoskeleton
provides the support for the body and connections for muscles,
similar to the internal skeleton of humans.
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The exoskeleton is divided into separate hardened plates on
some segments. A segment can have a plate on top, one on
each side, and one on the bottom. In the abdomen, only the
top and bottom plates of each segment are large and visible.
The plates of each segment are often connected by soft and
flexible membranes to allow movement of the body parts. These
membranes between the plates also can stretch to allow
expansion of the abdomen, as occurs when a mosquito feeds on
blood.
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Head
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The head bears the antennae, eyes, and mouthparts. The
antennae and eyes are the main sensory organs. The two
compound eyes each have many individual facets, or lens. In
addition, there may be as many as three simple eyes, or ocelli,
but these have only a single facet. Antennae are organs of
touch, smell, and sometimes hearing. Many different forms of
antennae are present in insects. Some are hair-like and
others are like a string of beads. Butterflies and many
beetles have antennae with enlarged ends that appear clubbed.
Moths have antennae that are either feathery or thread-like.
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The major mouthparts include the labrum, or upper lip,
mandibles, maxillae, and labium, or lower lip. The maxillae
and labium have small sensory appendages, termed palpi. These
mouthparts are modified in various insects to form two major
types: chewing and sucking. In chewing mouthparts, like
the grasshopper, the mandibles are used for cutting and
grinding food that the other mouthparts grab and bring towards
the mouth. Bees have a combination of chewing and
sucking mouthparts.
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Insects with sucking mouthparts include those with a beak or
proboscis. The beak in Hemiptera has four needle-like
mouthparts for piercing a plant or animal and sucking the
liquid food. The proboscis in mosquitoes and related biting
flies has six needle-like mouthparts for piercing and sucking.
In some flies, like the tse-tse fly, only one mouthpart is
used for piercing and the other mouthparts form a tube for
sucking. In many flies as well as moths and butterflies, the
mouthparts do not pierce the host, but are used only for
sucking or sponging the food.
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Thorax
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The thorax includes three segments: the first segment
(prothorax), middle segment (mesothorax), and last segment
(metathorax). All three segments have legs in adult insects,
but many larvae, such as fly maggots, do not have legs.
Wings may be present on the middle and last segment of the
thorax. Spiracles are openings in the body for air, and
these are often present on the middle and last segment of the
thorax.
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The human leg has a femur and tibia, and the foot has a tarsus
and toes. The insect leg also includes a femur and tibia, the
foot is the tarsus, and the claws are rather like the insect's
toes. The large femur of insects is connected to the body by
two other segments, the coxa and trochanter. The tarsus is
divided into as many as five smaller units, which gives it
more flexibility.
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Legs are used for many functions besides walking. The front
legs often are modified to catch prey, like a praying mantis,
or to burrow through soil, like a mole cricket. The hind legs
can be designed for jumping, like the flea, or for swimming,
like the water boatman. The hind legs of some insects, such
as the grasshopper, also are used for making sound.
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Most adult insects have two pairs of wings, but some have only
one pair. Some insects, like lice, fleas, and worker ants, do
not have wings. The inside of each wing has many veins that
give it support. In beetles, the front wings form protective
elytra, and the hind wings are used for flying. Flies, such
as the mosquito, use only the front wings for flying. The
hind wings are modified to form very small halteres that help
the fly to keep balance in the air. Most insects have
membranous wings, but some are leathery or hard. Some wings
are covered with hairs or scales.
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Abdomen
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Adult insects do not have walking legs on the abdomen. Some
larvae, such as caterpillars, have fleshy appendages, called
prolegs, that are used for walking. The abdomen usually has a
spiracle for air on each segment. The end of the abdomen has
appendages for mating and laying eggs. The part of the
female abdomen that deposits eggs is called an ovipositor.
Some insects, such as cockroaches and grasshoppers, also have
horn-like sense organs at the end of their abdomens.
A membranous tympanum, or hearing organ, is on the abdomen in
some insects.
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Internal Organs and Body Parts
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The food canal of insects includes a foregut, midgut, and
hindgut. Some insects shred their food in the foregut. Most
digestion takes place in the midgut. The hindgut absorbs any
remaining water before the food remains are excreted.
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In people, blood is pumped by the heart through arteries and
veins. Insects have a long, tube-like heart for pumping blood,
but do not have arteries or veins. The insect heart is on the
top side of the insect. The blood is pumped through the heart
from the tail end to the front of the body, where it empties
into the head. The blood then filters through open areas of
the insect's body back to the tail, where it once again is
pumped forward. It is possible to see the heart pumping in
insects with a transparent skin, such as many caterpillars and
other larvae.
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Insects are like people and other animals in needing oxygen to
live. Most insects get their oxygen from the air that enters
their spiracles, tiny openings on the side of their bodies.
The air passes through air tubes, called trachea, that divide
into smaller and smaller tubes that finally reach each cell in
the body. Many insects have areas in the air tubes that are
large and can help pump the air when the insect moves.
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Aquatic insects have special ways of breathing. Some aquatic
insects have gills that are filled with air tubes, and they
get their oxygen directly from the water like a fish. Others
come to the water surface and obtain a bubble of air that they
carry under their abdomen or under their wings.
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Humans have about 800 muscles. An adult grasshopper has about
900 muscles, and a butterfly caterpillar has about 4,000 muscles.
Humans get tired after running because of a substance, lactic
acid, that builds up in the muscles. Insects can fly for a
long time without getting tired because their muscles do not
build up lactic acid.
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The nervous system in insects includes a brain in the head and
a bundle of nerves in each segment of the body. The nerve
bundles, or ganglia, are connected by a nerve cord on the
bottom part of the body.
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Diagrams of the internal body parts and organ systems are
available at The University of Nebraska Department of Entomology.
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