Chapter 20
Terrestrial
Mandibulates
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Diversity of Uniramia
 The subphylum Uniramia is a controversial
taxon including the millipedes, centipedes and
insects.
Uniramians are primarily terrestrial with a few
found in freshwater habitats.
Characteristics
 The myriapods include the centipedes, millipedes, pauropods
and symphylans.
 The insects have reduced their body tagmata to head, thorax
and abdomen.
– Abdominal appendages are greatly reduced or absent.
– The common ancestor of insects probably resembled the myriapod
body form.
 Uniramia only have one pair of antennae and appendages are
always uniramous.
 Uniramian gills in aquatic larvae are not homologous with
crustacean gills.
 Uniramians use tracheae to distribute respiratory gases, similar
to onychophorans and annelids.
 Excretion usually involves Malpighian tubules.
Class Chilopoda
Characteristics
 Centipedes are terrestrial and have flattened bodies with up to
177 somites.
 Each somite, except the one behind the head and the last two,
bears a pair of jointed legs, the last pair of which serves a
sensory function.
 Appendages of the first body segment form poison claws.
 The head has one pair of antennae, a pair of mandibles and one
or two pairs of maxillae.
 Eyes on either side of the head consist of groups of ocelli.
 Salivary glands empty into the anterior end of the straight
digestive tract.
Chiloped Characteristics
 Two pairs of Malpighian tubules
empty into the hind intestine.
 The elongated heart has a pair of
arteries in each somite; ostia
provide return flow of
hemolymph.
 A pair of spiracles in each somite
allows air to diffuse through
branched air tubes of the tracheae.
 The arthropod nervous system
includes a portion that serves as a
visceral nervous system.
http://www.goldenphoenixexotica.com/cent.html
Reproduction
 Sexes are separate with unpaired gonads and
paired ducts.
Some lay eggs and others are viviparous.
Young resemble adults.
Natural History
 Centipedes are found under logs, bark and
stones.
They are carnivorous, eating earthworms,
cockroaches and other insects.
The house centipede has 15 pairs of long legs
and is common in bathrooms and damp cellars.
Most are harmless to humans but a few large,
tropical centipedes are dangerous.
Class Diplopoda
 Millipedes have 2 pairs of legs
per somite, probably from fusion
of 2 segments.
 Their cylindrical bodies have
from 25 to 100 somites.
 The head has two clusters of
simple eyes and a pair each of
antennae, mandibles and
maxillae.
 Each abdominal somite has two
pairs of spiracles opening into air
chambers and tracheal air tubes.
 The two genital apertures are
toward the anterior end.
Reproduction
 The appendages of the seventh somite are
specialized for copulatory organs.
After copulation, the female lays eggs in a nest
and guards them.
Larvae have only one pair of legs to each
somite.
Natural History
 Millipedes are less active
than centipedes; they walk
with a graceful rather than
wriggling motion.
 Most eat decayed plants but
a few eat living plant tissue.
 Most are slow moving and
roll into a coil for defense.
 Some secrete toxic or
repellant fluids from special
repugnatorial glands on
the side of the body.
http://abc.net.au/cgi-bin/common/printfriendly.pl?/science/news/stories/s1031780.htm
Class Insecta
Diversity
 Insecta are the most diverse and abundant of all
arthropods.
The number of known species is estimated at one
million.
There is continued evolution among modern insects;
the fossil record indicates they are a stable group.
Insects play major medical and economic roles with
humans, and are critical to animal ecology.
Characteristics
 Insects have three pair of legs and often two
pair of wings on the thoracic region of the
body.
Insects range from less than 1 mm to 20 cm in
length; the larger insects are tropical.
Distribution
 Insects are found in nearly all habitats except
the sea.
Insects are common in freshwater, brackish
water and salt marshes.
Insects are abundant in soils, forest canopies,
and can be found in deserts and wastelands.
Most animals and plants have insects as
parasites externally and internally.
Adaptive Traits that
Promote Wide Distribution
 Flight and small size.
Their well-protected eggs withstand rigorous
conditions and are readily dispersed.
A wide variety of structural and behavioral
adaptations gains them access to every
possible niche.
Adaptability
 Most structural modifications are in wings, legs, antennae,
mouthparts and alimentary canal.
 Specialization for eating only one part of a host plant allows
many insect species to coexist on a plant.
 The hard and protective exoskeleton is well-adapted to life in
desert regions.
 The exoskeleton holds in water which, along with metabolism
that saves water, allows for desert survival.
 The exoskeleton is made of complex plates, or sclerites,
connected by hinge joints.
– Muscles attaching sclerites allow precise movement.
– The rigidity is due to scleroproteins and not mineral matter; this lightness
allows flight.
External Form and Function
 The insect is more homogenous in
tagmatization than the variable crustaceans.
The cuticle of a somite is composed of a dorsal
notum, a ventral sternum and a pair of lateral
pleura.
Fig. 20.4a
Head
 Usually there is a pair
of large compound eyes.
One pair of antennae
varies greatly in form;
they can feel, taste and
hear.
Mouthparts consist of a
labrum, a pair of
mandibles and maxillae,
a labium and a
hypopharynx
Thorax
 The thorax consists of the prothorax,
mesothorax and metathorax; each has a pair of
legs.
Wings
If two pairs of wings are present, they are on
the mesothorax and metathorax.
Wings consist of a double membrane.
Veins serve to strengthen the wing; the vein
pattern is used to identify insect taxa.
Legs
 Walking legs end in terminal pads and claws.
 Hindlegs of grasshoppers and crickets are enlarged for
jumping.
 Mole crickets have front legs adapted for burrowing in the
ground.
 Forelegs of the praying mantis allow it to grasp prey.
 Honeybees have leg adaptations for collecting pollen.
Abdomen
 The insect abdomen has from nine to 11
segments; the last is reduced to a pair of cerci.
Larval and nymphal forms may have
abdominal appendages lacking in adults.
The external genitalia are usually at the end of
the abdomen.
Locomotion: Walking
Insects walk using the first
and last leg on one side and
the middle leg on the opposite
side in alteration with the
reverse; this provides stability.
A water strider has nonwetting footpads that do not
break the surface water
tension.
http://www.mtbaker.wednet.edu/harmony/
ditch/water_strider.htm
Power of Flight
 Insect wings are not homologous with bird and
flying mammal wings.
Insect wings are outgrowths of cuticle from the
mesothoracic and metathoracic segments.
Most flying insects have two pairs of wings; the
Diptera (true flies) have one pair.
Halteres are reduced wings that provide the fly with
balance during flight.
Non-reproductive ants and termites are wingless; lice
and fleas have also lost their wings.
Modifications of Wings
http://www.fotosearch.com/BDX161/bxp31931/
 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.
http://www.internet-atwork.com/hos_mcgrane/butterflies/hannah_melissa2.html
Flight Muscles of Insects
 Direct flight muscles attach to a wing
directly.
 Indirect flight muscles alter the shape
of the thorax to cause wing
movement.
 The wing is hinged on a pleural
process that forms a fulcrum; all
insects cause the upstroke with
indirect muscles that pull the tergum
downward.
 Dragonflies and cockroaches contract
direct muscles to pull the wing
downward.
 Bees, wasps and flies arch the tergum
to cause the downstroke indirectly.
 Beetles and grasshoppers use a
combination of direct and indirect
muscles to move wings.
Flight Muscle Contraction
 Synchronous muscle control uses a single volley of
nerve impulses to stimulate a wing stroke.
Asynchronous muscles stretch the antagonistic
muscle and cause it to contract in response.
Asynchronous muscles only need occasional nervous
stimulation.
Potential energy can be stored in resilient tissues.
Wing beats may vary from a slow 4/second in
butterflies to over 1000/second in midges.
Wing Thrust
 Direct flight muscles also
alter the angle of wings to
twist the leading edge to
provide thrust.
 This figure-8 movement
moves the insect forward.
 Fast flight requires long,
narrow wings and a strong
tilt, as in dragonflies and
horse flies.
Nutrition-Digestive System
 The foregut consists of the mouth with salivary
glands, esophagus, crop and gizzard.
Some digestion, but no absorption, occurs in the crop
as salivary enzymes mix with food.
The gizzard grinds food before it enters the midgut,
the main site of digestion and absorption.
The ceca may increase the digestive and absorptive
area.
The hindgut is primarily a site for water absorption.
Overview of Digestive Processes
 Ingestion of foods and their reduction by digestion only
begins the steps in nutrition.
 Foods reduced by digestion are absorbed into the circulatory
system.
 Foods are transported to the tissues of the body.
 They are assimilated into the structure of cells.
 Oxygen is also transported to tissues where food products are
oxidized to yield energy and heat.
 Food not immediately used is stored for future use.
 Wastes produced by oxidation must be excreted.
 Food products unsuitable for digestion are egested in the form
of feces.
Overview of Digestion
 Digestion mechanically and chemically
breaks food into small units for
absorption.
 Food solids contain carbohydrates,
proteins and fats that must be reduced
to simpler molecules.
 An animal must then reassemble the
digested and absorbed units into the
animal’s own compounds.
 Intracellular digestion is limited in the
size of food particle that can be utilized.
– Digestion in sponges and protozoa is
entirely intracellular
Extracellular Digestion
 The invention of the alimentary
system allowed extracellular
digestion to take place.
 This allowed cells lining the
lumen of the alimentary canal to
specialize for digestion or
absorption.
 Development of mouth-to-anus
flow-through systems allowed
regional specialization of
digestion.
Action of Digestive Enzymes
 Digestive enzymes are hydrolytic enzymes or
hydrolases; molecules are split by adding water.
Proteins must be split into hundreds or thousands of
small amino acid molecules.
Carbohydrates must be reduced to simple sugars.
Fats are reduced to glycerol and fatty acids although
some are absorbed without being hydrolyzed.
Specific enzymes form an “enzyme chain” so one
may complete what another has started.
Motility in the Alimentary Canal
 Food moves through the digestive tract by cilia,
specialized musculature or both.
Acoelomate and pseudocoelomate animals lack
mesodermally derived gut musculature and use cilia.
Most molluscs also use cilia; the coelom is weakly
developed.
In coelomic animals, the gut is lined with circular and
longitudinal layers of smooth muscle.
Gut movements cause segmentation; this mixes food
but does not move it through the gut (See Fig 32-8).
Peristalsis moves food down the gut.
Regional Function of the
Alimentary Canal
Receiving Region
 Mouthparts may include mandibles, jaws,
teeth, radula or bills.
The buccal cavity and pharynx are inner
chambers.
Most metazoans, other than suspension
feeders, have salivary glands to produce
lubricating secretions.
Receiving Region continued
Salivary Glands
Specialized saliva may contain toxins to quiet
struggling prey.
Leech saliva contains an anaesthetic and enzymes to
prevent blood coagulation and increase flow.
Salivary amylase is found in herbivorous molluscs,
insects and primate mammals.
Salivary amylase breaks starch into two-glucose
fragments of maltose.
Conduction and Storage
 The esophagus of vertebrates and many
invertebrates moves food to the digestive
system.
In annelids, insects and octopods, the
esophagus is expanded into a crop, a food
storage area.
Among vertebrates, only birds have a crop; it
softens grain and allows mild fermentation.
Grinding and Early Digestion
 The stomach is a region for initial digestion and
storage of food in vertebrates and some invertebrates.
Herbivorous animals often continue the grinding and
crushing of plants in the stomach.
Swallowed stones and grit assist the muscular gizzard
of oligochaete worms and birds.
The insect proventriculus has chitinous teeth, and
crustaceans have a gastric mill.
Digestive diverticula are blind tubules or pouches that
supplement the stomach and secrete enzymes and/or
absorb nutrients.
The Problem with Cellulose
 The woody cellulose that encloses plant cells is a
very abundant molecule.
Only the enzyme cellulase can break down the
cellulose molecule.
No metazoan animal can produce cellulase for
direct digestion of cellulose.
Many herbivorous animals harbor bacteria and
protozoa in their gut that do produce cellulase.
These microorganisms ferment cellulose under
anaerobic conditions of the gut, producing fatty acids
and sugars.
Terminal Digestion and
Absorption: The Intestine
 In invertebrates with digestive diverticula, the
intestine may serve only to carry wastes away.
In invertebrates with simple stomachs and in
vertebrates, intestines digest and absorb nutrients.
One method to increase digestive surface is to
increase the length of the intestine.
A coiled intestine is rare in invertebrates but may be
eight times body length in some mammals.
Invertebrates may use infolding to increase surface
area as in the typhlosole in oligochaetes.
Absorption
 Little food is absorbed in the stomach;
digestion is not complete and absorptive
surface is limited.
Most digested food is absorbed by the villi of
the small intestine.
Region of Water Absorption and
Concentration of Solids
 The large intestine consolidates the
undigested material as semisolid feces.
Reabsorption of water is the main function and
is critical in desert species.
Some animals have specialized rectal glands to
absorb water and ions, leaving nearly dry fecal
pellets.
Various Feeding Strategies
Feeding strategy
Definition
Carnivore
Feeding on animals
Folivore
Feeding on leaves
Frugivore
Feeding on fruits
Graminivore
Feeding on grasses
Granivore
Feeding on seeds
Insectivore
Feeding on insects
Nectarivore
Feeding on nectar
Omnivore
Feeding on both animals and vegetation
Osteovore
Feeding on bones and marrow
Piscivore
Feeding on fish
Sanguinivore
Feeding on blood
Insect Nutrition
 Most insects feed on plant tissues or juices and are
herbivorous or phytophagous.
Many caterpillars are specialized to eat only certain
species of plants (ex. Monarchs).
Certain ants and termites cultivate fungus gardens for
food.
Many beetles and other insect larvae are saprophagous.
Some insects are predaceous on other insects or other
animals.
Fig. 20.14
Parasitism in Insects
 Many species are parasitic as adults and/or
larvae.
 Many parasitic insects, in turn, have parasites,
which is a condition called hyperparasitism.
Parasitoids live inside a host until they
eventually kill the host; they are important in
pest control.
Mouthparts
 Sucking mouthparts
form a tube to pierce
tissues of animals or
plants.
Houseflies and
blowflies have sponging
mouthparts; the soft
lobes at the tip absorb
food.
Biting mouthparts can
seize and crush food.
Circulation
 A tubular heart in the pericardial cavity moves hemolymph
forward through the dorsal aorta.
 The heartbeat is a peristaltic wave.
 Accessory pulsatile organs help move the hemolymph into
wings and legs.
 Hemolymph has plasma and amebocytes but does not function
with oxygen transport.
Gas Exchange
 Terrestrial animals are faced with the dilemma of
exchanging gases but preventing water loss.
The tracheal system is a network of thin-walled tubes
that branch throughout the insect body.
Spiracles open to the tracheal trunks; there are two
on the thorax and 7-8 on the abdomen.
A valve on the spiracle often cuts down on water loss;
the spiracle may also serve as a dust filter.
Tracheae
Composed of a single layer of
cells lined with cuticle that is
shed at each molt.
 The tracheae branch out into
fluid-filled tubules called
tracheoles that reach
individual body cells.
This system provides gas
transport without use of
oxygen-carrying pigments.
Excretion and Water Balance
 Both insects and spiders utilize Malpighian tubules
in conjunction with rectal glands.
Malpighian tubules vary in number but join between
the midgut and hindgut.
The blind ends of the tubules float freely in the
hemocoel bathed in hemolymph.
Potassium is actively secreted into the tubules; other
solutes follow the gradient.
The main waste product is uric acid; it flows across at
the upper end that is mildly alkaline.
In the lower end of the tubule, potassium combines
with carbon dioxide and is reabsorbed.
Rectal glands then reabsorb chloride, sodium and
water; the wastes pass on out.
Nervous System
 Insect nervous systems resemble that of larger
crustaceans, with fusion of ganglia.
Some have a giant fiber system.
A stomadeal system corresponds to the
autonomic system of vertebrates.
Neurosecretory cells in the brain function to
control molting and metamorphosis.
Sense Organs
 Many insects have keen sensory perception.
Most sense organs are microscopic and located
in the body wall.
Different organs respond to mechanical,
auditory, chemical, visual and other stimuli.
Visual Reception
 Insects have two types of eyes: simple and compound.
 Ommatidia structure is similar to that of crustaceans.
 Insects can see simultaneously in almost all directions; the
image is myopic and fuzzy.
 Flying insects have a higher flicker-fusion rate; they distinguish
200-300 flashes per second.
Reproduction
 Sexes are separate in insects and fertilization is
usually internal.
Parthenogenesis is common in Homoptera and
Hymenoptera.
Sexual Attraction
– Female moths secrete a powerful pheromone to attract
males from a great distance.
– Fireflies use flashes of light to detect mates.
– Some insects use sounds, color signals and other courtship
behaviors.
Reproduction continued
 Many insects deposit sperm in the vagina during
copulation.
In some orders, spermatophores are transferred or
deposited on substrate.
During evolution from aquatic to terrestrial life,
spermatophores were used first.
The female may only mate once and store the sperm
to fertilize eggs throughout her life.
Females may lay a few eggs and provide care of
young, or lay huge numbers.
Metamorphosis and Growth
 Various forms of metamorphosis produce
degrees of change among different insect
groups.
– Most insects change form after hatching from an
egg.
– Each stage between molts is called an instar.
– Insects develop wings during the last stage where
they are useful in reproduction.
Holometabolous Metamorphosis
 About 88% of insects undergo this complete
metamorphosis.
This separates the physiology of larval growth, pupal
differentiation and adult reproduction.
Larvae and adults often live in completely different
environments and therefore do not compete.
After several larval instars, a larval moth or butterfly
becomes a pupa inside a cocoon or chrysalis.
Pupae often pass the winter in this stage; the final
molt occurs and the adult emerges in spring.
Stages are egg-larva-pupa-adult.
Fig. 20.24
Hemimetabolous Metamorphosis
 Some insects undergo a gradual metamorphosis.
Grasshoppers, cicadas, mantids, true bugs, mayflies
and dragonflies exhibit this metamorphosis.
Young are called nymphs.
Bud-like growths in early instars show where the
adult wings will eventually develop.
Stages are egg-nymph-adult.
Fig. 20.27
Direct Development
 Silverfish and springtails have young similar
to adults except in size and sexual maturation.
Stages are egg-juveniles-adult.
These are primitively wingless insects.
Physiology of Metamorphosis
Hormones regulate insect metamorphosis.
Regulation is via negative feedback control of
release of various hormones
– Ecdysone starts the molting process.
– Molting continues as long as juvenile hormone
(neotenine) is sufficiently present.
– Cessation of juvenile hormone production in the
pupa leads to an adult at the last molt.
Diapause
 Diapause is a period of dormancy in the annual life cycle
that is independent of conditions.
 Winter dormancy is called hibernation; summer dormancy is
called estivation.
 Any stage (eggs, larvae, pupae or adults) may remain
dormant to survive adverse conditions.
 This allows them to synchronize with the environment.
 Diapause is genetically determined but it may be triggered by
environmental cues such as day length.
 Diapause always occurs at the end of an active growth stage;
the insect is then ready for another molt.
 Therefore, many larvae do not develop beyond this point
until spring in spite of mild temperatures.
Defense
 Protective coloration, warning coloration and
mimicry are protective adaptations.
Stink bugs and others have repulsive odors and tastes.
Some insects are aggressive (e.g. bees and ants).
The monarch caterpillar incorporates a poisonous
substance from its food plant, milkweed.
The bombardier beetle can spray an attacking enemy
with irritating chemicals.
Behavior and Communication
 Due to very sensitive perception, many insects
respond to many environmental stimuli.
Responses are governed by both the physiological
state of the animal and its nerve pathways.
Many insect behaviors are complex sequences of
responses.
Most insect behavior is innate but some involve
simple learning.
Pheromones
These chemicals are secreted by one individual to
affect the behavior of another individual.
Pheromones attract the opposite sex, trigger
aggregation, fend off aggression and mark trails.
Bees, wasps and ants can recognize nestmates and
signal an alarm if strangers enter the nest.
Pheromones can be used to trap insects to monitor
populations.
Sound Production and Reception
 Sounds are used as warning devices,
advertisement of territory, and courtship songs.
Crickets chirp for courtship and aggression.
The male cicada vibrates paired membranes on
it abdomen to attract females.
Tactile Communication
 Tactile communication involves tapping,
stroking, grasping and antennae touching.
Some beetles, flies and springtails use
bioluminescence.
Some female fireflies mimic another species’
flash pattern and attract males and then eat
them.
Social Behavior
 Some social communities are temporary and
uncoordinated.
Other social groups are highly organized and depend
on chemical and tactile communication.
Caste differentiation is common in the most
organized social groups.
Groups exhibiting social behavior include bees,
wasps, termites, and ants. See your textbook for
detailed descriptions.
Insects and Human Welfare
Beneficial Insects
Insects produce honey, beeswax, silk and shellac.
Of more economic importance, bees pollinate $10
billion worth of food crops in the U.S. annually.
Pollinating insects and flowering plants are tightly
co-evolved.
Predaceous and parasitoid insects are vital in
controlling many pest insect populations.
Dead animals are rapidly consumed by fly maggots.
Insects are critical components of most food chains
and a central food for many fish and birds.
Insects and Human Welfare
Harmful Insects
Harmful insects eat and destroy our plants and
fruits.
Nearly every cultivated crop has several insect
pests; this requires substantial $ for insect control.
Bark beetles, spruce budworms, the gypsy moth
and others are serious forest pests.
Insects also destroy food, clothing and property.
Medically important insects include vectors for
disease agents.
Control of Insects
Broad-spectrum insecticides
– damage beneficial insect populations along with the
targeted pest
– persist in the environment and accumulate
– strains of insects have evolved a resistance
Biological control
– “B.t.” toxin
– Some viruses and fungi may be economical pesticides.
– Some natural predators or parasites of insect pests
Integrated pest management
– combined use of all possible, practical techniques