Hexapods

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Hexapods

• Chapter 21

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Subphylum Hexapoda

• Members of the subphylum Hexapoda are named for
  the presence of six legs.
• All legs are uniramous.

• Hexapods have 3 tagmata
• Head
• Thorax
• Abdomen
• Appendages attach to head and thorax.

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Subphylum Hexapoda

• Two classes within Hexapoda
• Entognatha
• Insecta

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Subphylum Hexapoda

• Entognathans are a small group characterized by
  having the bases of mouthparts enclosed within
  the head capsule.
• 3 orders Protura, Diplura, and Collembola.
• Protura and Diplura - tiny, eyeless, and inhabit
  soils or dark, damp places.
• Collembola are commonly called springtails
  because of ability to leap.
• Animal 4 mm long may leap 20 times its body
  length.
• Members live in soil, decaying plant matter, on
  freshwater pond surfaces, and along seashore.
• Can be very abundant, reaching millions per
  hectare.

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Class Insecta

• Insecta is an enormous class whose members have
  ectognathous mouthparts bases of mouthparts lie
  outside the head capsule.

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Class Insecta

• Insects are the most diverse and abundant of all
  arthropods.
• 26 orders
• Most are terrestrial or inhabit freshwater.
• Few are marine.

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Class Insecta - Distribution

• Found in nearly all habitats except the sea.
• Common in freshwater, brackish water, and salt
  marshes.
• Abundant in soils, forest canopies, and can be
  found in deserts and wastelands.
• Most animals and plants have insects as parasites
  externally and internally.

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Class Insecta - Adaptive Traits

• Flight and small size makes insects widely
  distributed.
• Well-protected eggs withstand rigorous conditions
  and are readily dispersed.
• Wide variety of structural and behavioral
  adaptations gains them access to every possible
  niche.

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Class Insecta

• Insects have
• 3 Tagmata head, thorax, abdomen.
• 3 pairs of legs and usually 2 pairs of wings on
  their thorax.

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Class Insecta

• Insects show a diverse array of morphological
  variation.
• They consistently have 3 tagmata.
• Head compound eyes, one pair antennae, 3
  ocelli, mouthparts (including mandibles
  maxillae)

• Thorax 3 segments each with a pair of legs, the
  last 2 segments usually have wings as well.
• Abdomen 9-11 segments

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Class Insecta

• Antennae can act as tactile organs, olfactory
  organs, and sometimes auditory organs.

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Class Insecta

• Legs have also become highly specialized for
  walking, grasping, skating over water, and
  specialized jobs like gathering pollen.

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Class Insecta

• Insects also have highly variable body forms.
• Land beetles are thick and shielded.
• Aquatic beetles are streamlined.
• Cockroaches are flat and live in crevices.

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Class Insecta

• Flight is one key to the great success of
  insects.
• An animal that can fly can escape predators, find
  food, and disperse to new habitats much faster
  than organisms that can only crawl.

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Class Insecta

• Insects are the only invertebrates that can fly.
• Insect wings not homologous with bird and flying
  mammal wings.

• Insect wings are outgrowths of cuticle from the
  mesothoracic and metathoracic segments.

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Class Insecta - Power of Flight

• Most have two pairs of wings.
• Some are ancestrally wingless silverfish.
• Some are secondarily wingless fleas.
• Recent fossil evidence suggests insects may have
  evolved fully functional wings over 400 million
  years ago.

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Class Insecta - Modifications of Wings

• Wings for flight are thin and membranous.
• The thick and horny front wings of beetles are
  protective.
• Butterflies have wings covered with scales.
• Caddisflies have wings covered with hairs.

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Class Insecta - Wing Thrust

• Direct flight muscles alter the angle of wings to
  twist leading edge to provide thrust.
• Figure-8 movement moves insect forward.
• Fast flight requires long, narrow wings and a
  strong tilt, as in dragonflies and horse flies.

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Class Insecta

• The internal anatomy of an insect includes
  several complex organ systems.

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Insects - Nutrition

• Most insects are herbivorous, feeding on plant
  juices and/or tissues.
• Some are specialized, others will eat almost any
  plant.

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Insects - Nutrition

• Some insects are predaceous, catching eating
  other animals.

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Insects - Nutrition

• Other insects are scavengers or parasites.

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Insects - Nutrition

• Some insect parasites are parasitized by other
  insects hyperparasitism.
• Parasitoids are a lethal type of parasite.
• A tiny wasp lays eggs on the tomato hornworm.
  The wasp larvae will consume the hornworm.

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Insects - Nutrition

• Insects have mouthparts specialized for the many
  different foods they eat.
• Sucking mouthparts (mosquitoes) form a tube,
  can pierce animal or plant tissues.

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Insects - Nutrition

• Sponging mouthparts (house flies) liquid food
  is lapped up, food may be liquefied first.

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Insects - Nutrition

• Chewing mouthparts (grasshoppers) strong plates
  can tear food.

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Insects Circulation Gas Exchange

• Insects have an open circulatory system.
• Gas exchange is accomplished with a tracheal
  system an extensive network of thin-walled
  tubes that branch into every part of the body.
• Tracheal trunks open to the outside by spiracles.

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Insects Circulation Gas Exchange

• Insects spiders have independently evolved an
  excretory system of Malpighian tubules blind
  tubules opening into the hindgut.
• Potassium is secreted into the tubules and water
  diffuses in after it. Other solutes and wastes
  are secreted or diffuse into the tubules as well.

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Insects Circulation Gas Exchange

• Insects that feed on dry grains must conserve
  water and excrete salts.
• Leaf-feeders ingest excrete lots of fluid.
• Aphids pass fluid as honeydew that is consumed by
  other insects.

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Insects Nervous System

• The nervous system resembles that of larger
  crustaceans, with fusion of ganglia.
• A stomadeal system corresponds to the autonomic
  system of vertebrates.
• Neurosecretory cells in brain function to control
  molting and metamorphosis.

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Insects Sensory Organs

• Mechanoreception Mechanical stimuli are
  received by sensilla (simple or complex)
  distributed over the antennae, legs, and body.

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Insects Sensory Organs

• Auditory Reception Very sensitive setae or
  tympanal organs detect vibrations that come
  through the substrate or the air.
• Some moths detect ultrasonic pulses emitted by
  bats. They drop toward the ground in response to
  avoid the bats.

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Insects Sensory Organs

• Chemoreception Chemoreceptors for taste and
  smell are located in sensory pits on the
  mouthparts, antennae or sometimes the legs.

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Insects Sensory Organs

• Visual Reception Simple eyes (ocelli) are used
  to monitor light intensity, they do not form
  images.
• Compound eyes in insects, similar to those of
  crustaceans, consist of thousands of ommatidia,
  each having its own pigment cells and lens.

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Insects Sensory Organs

• Different insects have different capability to
  see color.
• Bees can distinguish most colors (they dont see
  red) beginning in the ultraviolet range.
• To us a flower may look uniformly colored, but to
  the bee there are lines that appear in the UV
  range that act as nectar guides.
• Other insects, like butterflies, can see red.

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Insects - Reproduction

• Sexes are separate, some are parthenogenetic.
• Fertilization is internal.
• In some, like butterflies, nutrients are passed
  to the female as well as sperm.

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Insects - Reproduction

• Insects have a variety of methods for attracting
  mates.
• Pheremones play an important role in many
  species.
• Fireflies communicate using light flashes.
• Crickets communicate using sound.

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Insects - Reproduction

• Female insects deposit eggs on a specific habitat
  that will provide food for larvae.
• Monarch butterflies lay eggs on milkweed plants.
• Parasitoid wasp species lay eggs on tomato
  hornworms.
• Mosquitoes lay eggs in standing water where the
  larvae will live as filter feeders.

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Insects - Metamorphosis

• Many insects undergo metamorphosis during their
  development.
• Each stage between molts is called an instar.

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Insects - Metamorphosis

• Insects with complete metamorphosis have larval
  stages specialized for eating and growing that
  are known by such names as maggot, grub, or
  caterpillar.
• The larval stage looks entirely different from
  the adult stage.

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Insects - Metamorphosis

• Female butterflies lay eggs on the plant that the
  caterpillars will feed on.
• After the eggs hatch, the larvae (caterpillars)
  eat and grow, molting many times.
• When it reaches a certain size, the larva will
  molt one more time, becoming a pupa (chrysalis
  in butterflies).

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Insects - Metamorphosis

• Metamorphosis from the larval stage to the adult
  stage occurs during a pupal stage.

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Insects - Metamorphosis

• In incomplete metamorphosis, the young, called
  nymphs resemble adults but are smaller and go
  through a series of molts until they reach full
  size.

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Insects - Metamorphosis

• Direct Development
• Silverfish and springtails have young similar to
  adults except in size and sexual maturation.
• Stages are egg-juveniles-adult.
• Wingless insects.

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Diapause

• Insects are able to undergo dormancy during harsh
  conditions.
• Hibernation winter dormancy
• Estivation summer dormancy
• Diapause arrested growth that occurs regardless
  of conditions.

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Defense

• Aposematic coloration many insects utilize
  bright colors as a warning that they are toxic.

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Defense

• Batesian mimicry when a palatable species
  mimics the bright colors of an unpalatable
  species.
• Müllerian mimicry when two unpalatable species
  have come to resemble each other.

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Defense

• Cryptic coloration often insects are colored
  and patterned very much like the plants they are
  found on, making them very difficult to see.

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Defense

• Other defensive features include the exoskeleton,
  offensive odor (as with the stink bug), bites and
  stings.

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Insect Behavior

• Insects exhibit a wide range of behaviors
  involving innate behaviors, pheromones, and
  learning.

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Insect Behavior

• Fireflies use bioluminescence to signal each
  other. The female firefly attracts males by
  using a particular flash pattern.
• Another firefly species mimics the call of the
  female and then eats the males that arrive.

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Social Insects

• Honey bees, ants and termites have complex social
  groups.
• In honeybees
• The queen is the reproductive female.
• Workers are non-reproductive females.
• Drones are haploid males.

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Social Insects

• Ants have fascinating societies where they farm
  fungi, herd ant cows (aphids which they keep
  for the honeydew that they secrete), sew their
  nests with silk, and even use tools.

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Insects and Humans

• Insects can be beneficial, preying on harmful
  insects, fertilizing crops etc.
• Insects are critical components of most food
  chains and an important food source for many fish
  and birds

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Insects and Humans

• Or, they can be harmful, spreading disease,
  eating crops etc.
• Care must be taken when controlling pests that
  beneficial insects are not harmed.

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Insects and Humans

• Control of Insects
• Broad-spectrum insecticides damage beneficial
  insect populations along with targeted pest.
• Some chemical pesticides persist in the
  environment and accumulate as they move up the
  food chain.
• Some strains of insects have evolved a resistance
  to common insecticides.

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Insects and Humans

• Biological control use of natural agents,
  including diseases, to suppress an insect
  population.
• Bacillus thuringiensis - bacterium that controls
  lepidopteran pests.
• Gene coding for the B.t. toxin has been
  introduced to other bacteria and transferred to
  crop plants.
• Some viruses and fungi may be economical
  pesticides.
• Natural predators or parasites of insect pests
  can be raised and released to control pest.
• Release of sterile males can eradicate the few
  insect species that only mate once.

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Insects and Humans

• Integrated pest management - combined use of all
  possible, practical techniques listed above, to
  reduce reliance on chemical insecticides.

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Phylogeny

• Our understanding of the relationships among
  arthropods has changed over the past decade.
• Using molecular data, members of former subphylum
  Uniramia are now divided between subphylum
  Myriapoda and Hexapoda.
• The nature of the relationship, however, between
  hexapods and crustaceans is not well understood.

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Phylogeny

• Some phylogenies support a sister-taxon
  relationship between them, but others indicate
  that hexapods arose within Crustacea.
• Future studies may show that subphylum Crustacea
  is paraphyletic.
• Phylogenies that support hexapods arising from
  within Crustacea, find that hexapods are most
  similar to branchiopod, cephaplocarid, and
  remipedian crustaceans.

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Adaptive Diversification

• The first terrestrial arthropods were scorpions
  and millipedes that appeared in the Silurian
  period.
• The ancestral insect had a head and trunk of
  similar somites.
• Insects have specialized the first three
  post-cephalic somites as thorax and lost the
  remaining appendages.
• Some modern apterygote orders have abdominal
  styli that are considered vestigial legs.

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Adaptive Diversification

• Recent fossil evidence suggests winged insects
  were in existence about 400 million years ago.
• Ancestral flying insects may have derived from
  aquatic insects or insects with aquatic
  juveniles wings may be derived from external
  gills on the thorax.