16'3 Insects

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16.3 Insects
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Objectives

• Describe the characteristics of insects.
• Distinguish between incomplete and complete
  metamorphosis.
• Explain how social insects differ from other
  insects.

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• These six-legged invertebrates are arthropods.
  They belong to a single class, but there are more
  species in this class than in all the other
  arthropod classes put together.
• In fact, in their number of species, insects
  outnumber all other forms of life combined!

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The Success of Insects

• Insects appeared about 400 million years ago.
• By the beginning of the Cenozoic era, they had
  evolved an amazing variety of shapes, sizes, and
  behaviors.
• The great number of insect species is a sign that
  insects have been very successful as a life form.
  

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• They have been able to adapt to many different
  habitats, from high mountains to dry deserts to
  rivers and wetlands to your backyard.
• They have been able to survive when other
  organisms
• could not.
• Insects owe their success to a body plan that has
  many advantages over others.

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• One important characteristic of the insect body
  is its flexibility.
• In the process of evolution, insect body parts
  have been modified for different ways of living.
• Changes in mouthparts, for example, have resulted
  in the adaptation of different species for eating
  different food.

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• Another major insect advantage is the ability to
  fly.
• Flight helps insects find food, escape predators,
  locate mates, and reach new places to live.

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Body Structure of Insects

• Insects, like all arthropods, have exoskeletons,
  segmented bodies, and jointed appendages.
• They differ from other arthropods in having three
  pairs of legs.

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• In addition, they are the only arthropods with
  wings.
• All insect bodies are divided into the three
  basic arthropod body regions head, thorax, and
  abdomen.

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• The grasshopper is a good example of an insect.
• Its body is divided into the three regions.

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• The grasshopper has a pair of compound eyes and
  three simple eyes.
• Its antennas are for touch and smell.
• The grasshopper's mouthparts are adapted for
  chewing.

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• The grasshopper's thorax is divided into three
  segments.
• It has a pair of legs attached to each segment.
• The first two pairs are for walking.

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• The hind pair are powerful legs used for jumping.
  
• The grasshopper has two pairs of wings, attached
  to the second and third segments of the thorax.
• Many other insects have two pairs of wings, but
  some have only one pair, and others are wingless.

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• A grasshopper's abdomen has ten segments.
• Notice the tympanum (TIHM peh nuhm), which is
  like an eardrum it senses sound.
• Other insects have tympanums on their thorax or
  legs.

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• Like other insects, a grasshopper has tiny
  openings in its thorax and abdomen.
• Air enters these openings and flows through tubes
  called tracheae (TRAYkee EE).
• Oxygen diffuses from tiny tubes into the body
  cells.

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Insect Development

• All insects reproduce sexually.
• In most species the eggs are fertilized inside
  the female's body by a male's sperm.
• Many insects lay eggs on a food source so the
  young can begin to eat right after they hatch .

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• Most insects don't look exactly like their
  parents when they hatch from their eggs.
• As they grow, they undergo a process called
  metamorphosis (MEHT uh MOR fuh sihs).
• During metamorphosis, a young organism changes
  its appearance to become an adult.

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Incomplete Metamorphosis

• Some insects, such as grasshoppers and lice, go
  through an incomplete metamorphosis.
• This is a series of molts in which the insect
  changes from an egg to a nymph (NIHMF) to an
  adult.
• A nymph looks like a little adult, but it has no
  wings and is not yet able to reproduce.

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Complete Metamorphosis

• Most insects go through complete metamorphosis,
  which involves a more complete change of
  appearance.
• Bees, wasps, flies, beetles, butterflies, and
  moths are among the insects that go through
  complete metamorphosis.
• They change from eggs to larvae to pupae to
  adults.

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• In the larva stage, the insect usually looks
  something like a worm.
• You have probably seen caterpillars, which are
  the larvae of butterflies or moths.
• During the pupa (PYOO puh) stage, the insect does
  not eat or move around.

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• In many species the pupa is surrounded by a
  cocoon.
• During this stage, amazing changes are taking
  place.
• Most of the larva's cells are absorbed.
• Others divide rapidly, forming adult structures.
• When metamorphosis is complete, an adult insect
  emerges.

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Social Insects

• Most insects live on their own, but some species
  live together in colonies.
• They are called social insects.
• Social insects include termites, most ants, and
  some wasps and bees.

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• A colony of social insects is in many ways like a
  single organism.
• The members of the colony have specialized
  functions, much like different cells or tissues.
• They also have systems of communication that
  allow them to work together smoothly.

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• In an ant colony, or nest, most of the ants are
  wingless female workers.
• They build the nest, gather food from outside,
  care for the young, and defend the nest against
  invaders.
• They communicate with each other through touch
  and chemicals.

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• The worker ants do not reproduce.
• That job is reserved for the queen ant.
• She lays eggs all during her long life.
• The eggs are fertilized by stored sperm the queen
  receives after mating with one or more winged
  male ants.

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End