Hexapods
Chapter 21
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.
Subphylum Hexapoda
 Two classes
within
Hexapoda:
 Entognatha
 Insecta
Subphylum Hexapoda
 Entognathans are a small group characterized
by having the bases of mouthparts enclosed
within the head capsule.
 Includes the order Collembola, commonly called
springtails because of ability to leap.
 Can be very abundant, reaching millions per
hectare.
Class Insecta
 Insecta is an enormous class whose members
have ectognathous mouthparts – bases of
mouthparts lie outside the head capsule.
Class Insecta
 Insects are the most
diverse and
abundant of all
arthropods.
 26 orders
 Most are terrestrial
or inhabit
freshwater.
 Few are marine.
Class Insecta - Distribution
 Found in nearly all habitats except the sea.
 Most animals and plants have insects as
parasites externally and internally.
Class Insecta - Adaptive
Traits
 Flight and small size are traits that lead to wide
distribution.
 Well-protected eggs withstand rigorous
conditions and are readily dispersed.
 Wide variety of structural and behavioral
adaptations gains them access to every
possible niche.
Class Insecta
 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
Class Insecta
 Antennae can act as
tactile organs,
olfactory organs,
and sometimes
auditory organs.
Class Insecta
 Legs have also
become highly
specialized for
walking, grasping,
skating over water,
and specialized jobs
like gathering pollen.
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.
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.
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.
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.
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.
Class Insecta - Wing Thrust
 Figure-8 movement
moves insect forward.
 Fast flight requires long,
narrow wings and a
strong tilt, as in
dragonflies and horse
flies.
Class Insecta
 The internal anatomy of an insect includes
several complex organ systems.
Insects - Nutrition
 Most insects are
herbivorous, feeding
on plant juices
and/or tissues.
 Some are
specialized, others
will eat almost any
plant.
Insects - Nutrition
 Some insects are
predaceous,
catching & eating
other animals.
Insects - Nutrition
 Other insects are
scavengers or
parasites.
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.
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.
Insects - Nutrition
 Sponging
mouthparts (house
flies) – liquid food is
lapped up, food may
be liquefied first.
Insects - Nutrition
 Chewing
mouthparts
(grasshoppers) –
strong plates can
tear food.
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.
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.
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.
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.
Insects – Sensory Organs
 Mechanoreception – Mechanical stimuli are
received by sensilla (simple or complex)
distributed over the antennae, legs, and body.
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.
Insects – Sensory Organs
 Chemoreception – Chemoreceptors for taste
and smell are located in sensory pits on the
mouthparts, antennae or sometimes the legs.
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.
Insects – Sensory Organs
 Different insects have different capability to see
color.
 Bees can distinguish most colors (they don’t 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.
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.
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.
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.
Insects - Metamorphosis
 Many insects undergo metamorphosis during
their development.
 Each stage between molts is called an
instar.
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.
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).
Insects - Metamorphosis
 Metamorphosis from the larval stage to the
adult stage occurs during a pupal stage.
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.
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.
Diapause
 Insects are able to undergo dormancy during
harsh conditions.
 Hibernation – winter dormancy
 Estivation – summer dormancy
 Diapause – arrested growth that occurs regardless
of conditions.
Defense
 Aposematic
coloration –
many insects
utilize bright colors
as a warning that
they are toxic.
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.
Defense
 Cryptic coloration – often insects are colored
and patterned very much like the plants they
are found on, making them very difficult to see.
Defense
 Other defensive features include the
exoskeleton, offensive odor (as with the stink
bug), bites and stings.
Insect Behavior
 Insects exhibit a wide range of behaviors
involving innate behaviors, pheromones, and
learning.
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.
Social Insects
 Honey bees, ants and
termites have complex
social groups.
 In honeybees:
 The queen is the
reproductive female.
 Workers are nonreproductive females.
 Drones are haploid
males.
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.
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
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.
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.
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.
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.
 Some modern apterygote
orders have abdominal styli that
are considered vestigial legs.
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.