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Sense Organs
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Insects have sense organs for taste, touch, smell,
hearing, and sight--the same senses present in humans.
Some insects have sense organs for temperature and
humidity as well as stresses and movements of their
body parts.
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Most sense organs of insects are microscopic in size
and are found on their body wall. Many are small
hairs, and others are small domes or other shapes.
Regardless of their shape, all sense organs have one
or more nerves leading to them. These small sense
organs are called sensilla (a sensillum is one sense
organ). A single antenna of an insect can have more
than five thousand sensilla.
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Sense organs, or sensilla, that function for taste and
smell always have at least one small hole, or pore,
through which the chemical molecules enter the organ.
A single sense organ on the antenna of the
polyphemus moth can have 18,000 pores for the
chemicals to enter. There are always two or more
nerves inside these sensilla that respond to the
chemical.
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Sense organs that respond only to mechanical touch or
vibration do not have holes in them, and
they only have one nerve. Some of these sensilla
respond to changes in the body wall when the insect
moves.
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Taste
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The organs of taste may be found on all parts of the
insect's body, but they are located mainly on
mouthparts and their feet (or tarsi). Some insects,
including bees and wasps, have taste organs on their
antennae. Wasps and crickets know where to lay their
eggs because they have taste organs on their ovipositor.
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Most insects have the same four taste sensations as
humans--salty, bitter, sweet, and sour. Many insects
have taste organs for particular chemicals found in
only a few plants. The cabbage butterfly, for example,
has a taste organ for mustard.
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Smell
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Antennae sometimes are called "feelers."
However, antennae are primarily "smellers"--they
are the insect's "nose" because they are
covered with many organs of smell. These organs help
the insect to find food, a mate, and places to lay eggs.
Insects even can decide which direction to fly by
using their sense of smell.
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The organ of smell of an insect does not detect as
many different odors as a human's nose, but the
insect's organ is tuned more finely. It can detect
differences between very similar chemicals, and it can
smell much smaller amounts of a scent. In fact, the
male of the lesser emperor moth can smell the chemical
pheromone of the female at a distance of more than six
miles.
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Social insects, like ants and bees, know when an
unwanted visitor enters their nest because they
recognize the members of their own colony with their
sense of smell.
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Touch
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Most of the sense organs that respond to touch are
small hairs with a nerve at their base. The insect can
sense the movement of this hair if it touches another
object. These sensory hairs also help honey bees
orient to the earth's gravity when they are upside
down on their hive.
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These sense organs of touch can respond even to the
wind or a gentle breeze. This is one reason why it
is difficult to catch a fly. The fly can sense the
air being pushed towards it when your hand is moving.
One species of grasshopper can feel air that is moving
less than one-tenth mile per hour.
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Hearing
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Insects can hear sound passing as vibrations through
the air as well as through the ground, water, or the
leaf of a plant. Some insects can hear sounds that
people cannot. Insects have many different kinds of
"ears" or hearing organs. The most
simple hearing organs are the same hair-like sense
organs that respond to touch. Some insects, such as
cicadas and crickets, detect sound with a tympanum, a
large membrane like the ear drum in humans.
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Bats make sound that will echo from a flying insect,
and the bats use this echo-location to catch their
food. Many different moths have a tympanum on their
wings, thorax, or abdomen. These moths can
hear the clicking sounds of the bats and take evasive
action by dropping in the air or changing their flight
path to avoid being caught.
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Many insects have hearing organs inside their legs.
These ears in legs respond to vibrations passing
though the ground or a plant. This is why ants will
come out of their nest if you stomp the ground.
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Sight
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Adults and nymps of insects have two compound eyes and
up to three simple eyes on their head. Larvae
of insects with complete metamorphosis, such as
caterpillars and grubs, do not have compound eyes, but
they may have 1-6 simple eyes.
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A simple eye is a single lens that tells the
difference between light and dark. Larvae can also
see rough shapes with their simple eyes.
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A compound eye includes many lens that have six sides
and fit together like the cells of a honeycomb.
Compound eyes differ among insects in their ability to
see, but some can see sharp images and different
colors. All insects can see movement better than
shape.
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Insects with large compound eyes, like cockroaches and
dragonflies, have a wide field of view of 360
degrees. Color vision in insects differs from that
in humans. Many insects can see the ultraviolet color
not visible to humans, but most insects cannot see the
red color. If a red plastic film is placed over a
flashlight, insects can be observed at night without
their detecting the light.
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Other Senses
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Insects have special organs for sensing their
movements which cause internal
changes in pressure and stress inside their body.
These sense organs are similar to those for touch,
except they are dome-shaped and have no hair.
Insects have many of these pressure and stress organs
on their wings and legs, and they could not walk or
fly without them.
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Instinct and Learning
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Insect behavior is mostly instinctive. Instinct is
determined by genes before the insect hatches from the
egg. A caterpillar does not make a conscious choice
of which plant to eat. Rather, the caterpilar is
programmed to eat a certain kind of plant, even though
other nutritious plants might be available. Likewise,
a wasp does not choose to sting a person.
The wasp is reacting by instinct to a threat or
invasion of its territory.
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Insects also have the ability to learn. Some moths
first locate flowers instinctively by their scent.
These moths later learn to identify the flower
by their vision. Some kinds of wasps make
orientation flights to learn landmarks near their nest,
and these landmarks are remembered so they
can find their nest after a longer flight.
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honey bees can be trained, or conditioned, to associate
sugar water with a particular color or aroma.
honey bees also learn to come to food at certain
times during the day when there is nectar available.
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Insects can also learn by "trial and error."
When Colorado Potato Beetles first attempt to mate,
they are not very good at identifying their
own species or even distinguishing the head from the
tail ends of the body. With repeated attempts and
mistakes, they learn to recognize their
own species and to tell the head from the tail end.
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