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Biology 320
Invertebrate Zoology
Fall 2005
Chapter 21 – Phylum Arthropoda,
Superclass Hexapoda
Class Insecta
 Majority of all animals
 1,000,000 described species
 Estimated 10 to 30 million undescribed
 Do not live in oceans, with the
exception of a few intertidal species
 Well adapted for terrestrial existence
 2 – 250 Malpighian tubules
 Many other adaptations and behaviors
 Holometabolus (indirect) development
has led to success on land by
promoting resource partitioning
 Largest insect orders (Lepidoptera,
Hymenooptera, and Diptera) exhibit indirect
 Insect flight allows for:
 Dispersal
 Predator escape
 Access to food and optimum
 Many have interesting
coevolutionary relationships
with flowering plants
 Some are vectors of
 Herbivory by insects does
large amounts of damage to
Body Form
 Three tagmata
 Head
Two compound eyes
Three unpaired ocelli
One pair of antennae
Preoral cavity with complex
 Thorax
 Three pairs of legs
 Forelegs
 Middle legs
 Hind legs
 Thorax cont…
 Two pairs of wings in most
adults, while juveniles have
wing pads
 Some only have one pair
 Some lack altogether
 Some only possess wings
during reproductive periods
 Abdomen
 Typically 9 – 11 segments
 Female gonopore on
segment 8
 Male gonopore on segment 9
 Some have sensory
appendages called cerci on
last segment
Wings and Flight
 Wings composed of two sheets
of epidermis covered with cuticle
 Sclerotized areas called veins
offer support
 Hollow
 House tracheae, nerves, and
 Permanently outstretched wings
is a primitive trait
 The evolution of wing folding has
allowed insects to exploit
microhabitats where outstretched
wings would be a handicap
 Flight muscles fill most of the thorax
 Striated fibers with huge mitochondria, and profuse tracheal innervation
(to meet high oxygen demand)
 Wings move up, down, forwards, and backwards
 Wing angles are frequently changed
 Insects are probably the most skilled flyers
 Houseflies can outperform any bird
 Can turn in the distance of one body length
 Can fly upside down
 Hummingbird moths and botflies can fly 25 mph
 Display positive phototaxis, and often fly into lamps
 Diverse nutritional
strategies, and thus
diversity in mouthpart
structure and function
 Mandibulate mouthparts
 Grasshoppers and ants
 Large jaws for cutting and
 Found in many herbivores
and carnivores
 Sucking mouthparts
 Butterflies and moths
 Suck nectar and fruit juices with a
long proboscis formed from maxillae
 Rolled and coiled when not feeding
 Diet is N and Na deficient so they
drink other types of liquids (mud
puddles, carrion, animal wastes,
etc.) to supplement
 Piercing mouthparts
 Aphids drink plant juices
 Female mosquitoes require a blood
meal to produce eggs
 Stylet pierces prey
 Has a salivary channel and a food
 Proteins in saliva produce itching
associated with mosquito bites
 Surrounding structures support
 Sponging mouthparts
 Nonbiting flies
 Enlarged labium is used
to absorb liquids
 Liquefy food with
 Digestive system
 Salivary glands produce
many types of secretions
 Pectinase for breaking
down plant cell walls
 Typical secretions
 Some have silk glands in
place of salivary glands
 Used for making
 Digestive system cont…
 Peritrophic membrane
 Protects non-cuticularized midgut
 Partitions areas where digestive enzymes function
 Membrane is perforated, so some enzymes pass through
membrane and act on food bolus
 Hydrolyzed biomolecules pass from bolus side to lumen
 Additional enzymes facilitate further digestion
 Midgut has two to six gastric ceca
 Hemocoel contains a fat body that synthesizes
and stores biomolecules
Hemal System
 Not many vessels
 Accessory hearts pump blood to important
structures (antennae, mouthparts, legs,
wings, cerci, etc.)
 Located at bases of extremities
 Many insects can survive subfreezing
temperatures (-30°C)
 Compounds in blood, such as glycerol, function as
Respiratory System
 Elaborate tracheal system
 Branch and diverge into a
network of smaller and smaller
 Smallest tubes are called
tracheoles (make direct contact
with cells)
 10 pairs of spiracles
 Two pairs on thorax
 Spiracles can be closed, which
 Water loss
 Dust from entering
 Parasites from entering
 Cartilagenous support rings
Sensory Organs
 Most are derived from setae and
are associated with appendages
 Tarsi of many have gustatory (taste)
or tactile (touch) receptors
 Flies taste food by walking on it
 Eyes
 Form color images in some
 Some (especially pollinators) are
sensitive to UV light
Reproductive System
 Gonochoric with internal fertilization
 Female system
Two ovaries produce eggs with a waterproof capsule
Two oviducts converge and empty into a vagina
Oviduct opens on segment 8
Tubular ovipositor for depositing eggs in appropriate substratum (i.e.
soil, wood, plant, animal, etc.)
 Ovipositor is modified to form a sting with poison glands in some
Hymenopteran females
 Male system
 Two testes with sperm ducts
 Accessory glands
 Median ejaculatory duct
 Penis is extensible /
 Everts through gonopore on
segment 9
 Releases spermatophore or
 Sperm plug (mating plug)
 May have claspers for holding
female during copulation
 Sperm enter egg through a
micropyle in egg capsule
Insect Development
 Under hormonal control
 Three main stages:
 Egg
 Juvenile
 Many instars separated by
 Main role is to feed and grow
 Adult
 Technically termed an imago
 Sexually mature with functional
 May / may not continue to molt
 Main role is dispersal and
Direct Development
 Three main types
 Paurometabolous
development is the only
type we will discuss
 Grasshopper, cockroaches,
 Juveniles are morphologically
similar to adults
 Adults don’t molt
 Does not promote resource
 Adults and nymphs in
competition with one another
Indirect Development
 Holometabolus development
 Hymenopterans, flies, beetles, and Lepidopterans
 Juveniles (larvae) are wormlike, and called grubs, maggots, or
 Many larval instars
 Pupa phase between last instar and imago
 Undergoes metamorphosis
 Imago has wings and looks nothing like juvenile
 Different mouthparts, and therefore they feed on different types of
 Promotes resource partitioning
 Definition - evolution of plants and insects in response to
one another
 Plants and insects are vital to each other
 Plants provide food (nectar, pollen, and plant tissues) and shelter
to insects
 Insects pollinate plants
 Bees, wasps, butterflies and
moths are the primary insect
 Insects pollinate 70% of all
flowering plants
 Plants adapt to make
improvements that minimize
costs to both organisms, and
maximize efficiency of pollination
 Characteristic colors, odors, and
nectars to attract insects
 Guide marks and UV-sensitive
patterns on flowers
 Time that flowers bloom
coincides with periods of insect
 Coevolution in response to
 Plants have evolved a suite of
defenses against insect herbivory
 Mechanical – spines, bristles, and
 Chemical – nicotine, caffeine,
cyanide, and tannins
 In turn, insects develop counter
 Enzymes that detoxify protected
 These insects specialize on
plants they can detoxify, and
have little competition
 Defensive chemicals may
become attractants
 Arms race continues
 Another interesting aspect of insectplant relationships is gall formation
 Many herbivorous insects cause
plants to form galls
 Swollen portion of plant tissue
containing eggs or larvae
 Forms in response to compounds
secreted by ovipositing female and
 Gall shape is characteristic of the
insect that laid eggs
 Gall protects larvae, larvae feed on
gall tissue; eventually emerge
 Fleas, lice and biting flies are
 Usually only one life phase is
 Human botfly (Dermatobia)
 Captures a mosquito or biting fly
 Deposits eggs on the captured fly
and releases
 Recently captured insect bites a
human or other animal
 Egg falls into wound
 Grub develops in a cavity called a
 Molts several times
 Crawls to surface, falls to ground and
 Insects that parasitize larger
 Typically deposit eggs in or
on host using an ovipositor
 Eggs hatch and consume
 Very common
 It has been hypothesized that
each insect species has a
 Parasitoids are important for
controlling insect populations
 Example 1:
 Wasp secretes chemical that
causes worker ants to fight each
 While ants fight, wasp enters
nest and oviposits on larvae
 Example 2:
 Apanteles (wasp) oviposits in
Manduca (Sphinx moth) larvae
 Up to 500 larvae can grow inside
 Rupture body wall and form
cocoon on surface
 Pheromones
 Chemicals used for:
 Attracting potential mates
 Marking territories and trails
 Locating and removing dead
individuals from colony
 Defense
 Warning conspecifics of
 Locating food
 Usually dispersed by wind
 Effective in very small
 Flashing
 Fireflies
 Species-specific flashing pattern
 Some female fireflies mimic
flashing patterns of other species
 Attracts male of a different
 Female preys upon male
 Sound
 Stridulation is produced by wings
and leg files
 Other types of sound produced
by wing beating, forcing air from
tracheae, etc.
Social Insects
 Isopterans (termites) and
Hymenopterans (bees, wasps, and ants)
 All groups share several characteristics
 One queen that mates with other fertile
individuals (drones)
 Physical labor is carried out by sterile
 Caring for juveniles
 Foraging
 Soldiers
 No individual can exist independent of
 No individual can be accepted into a
colony other than its own
 However, some ants raid nests of other
species, kidnap pupae, raise juveniles,
and force them to work as slaves
 Characteristics…
 Caste system
 Individuals with different morphologies
 Have different duties
 Examples: soldiers, workers, and drones
 Ants and termites usually inhabit
elaborate colonies with many
tunnels, chambers, entrances and
 Army ants are an exception
 Leaf cutter ants have chambers for
growing fungus
 Colony formation
 In bees, new queens are produced
when larvae eat a substance known as
“royal jelly” that is produced by the
hypopharyngeal glands of nursery
 Queen usually takes part in nuptial flight
in order to start new colony
 In bees 20,000 to 60,000 individuals will
swarm and search for a new colony
 There are around 30 insect
 We will briefly cover 14
 Order Odonata
 Dragonflies and damselflies
 Voracious predators found near
 Nymphs are aquatic
 Dragonflies are heavy bodied with
outstretched wings
 Damselflies are more slender and
delicate, with wings held over
abdomen while at rest
 Order Orthoptera
 Grasshoppers, crickets, and
 Large insects with large
compound eyes
 Large hind legs for jumping
 Stridulating and auditory
 Herbivores
 Order Phasmida
 Walking sticks
 Mimic sticks or vegetation
 Elongate and cylindrical
 Usually lack wings
 Some up to 30cm long
 Order Dermaptera
Nocturnal and omnivorous
Elongate body
Large pincer-like cerci
 Order Isoptera
 Termites
 Soft bodied and pale in color
 Build large mounds or nests in
 Can infest buildings
 Have endosymbiotic microbes
that produce cellulase
 Order Mantodea
Large and slow moving
Cryptic coloration
Ambush predators with
raptorial forelegs
 Large eyes on a freely
movable head
 Order Blattaria
Fast runners
Endosymbionts similar to
those of termites
 Order Phthiraptera
 Lice
 Ectoparasites of mammals and birds
 Prehensile legs for clinging to hair or
 Wingless
 All life stages are spent on host
 Host to host physical contact is
required for transmission
 Order Hemiptera
 True bugs
 Stink bugs, water striders, aphids, and
 Sucking mouthparts for ingestion of
 Herbivory is of economic significance
 Some are vectors of plant or animal
 Order Coleoptera
 Beetles
 Largest order at
approximately 600,000
 Extremely sclerotized
 Forewings are called elytra
 Most are herbivores, but
ladybird beetles are
 Order Hymenoptera
 Ants, wasps and bees
 Chewing mouthparts
 Most have wings, but worker
ants lack
 In females, ovipositor may be
a sting
 Maggot-like larvae
 Order Lepidoptera
Butterflies and moths
Adults have a coiled proboscis
Drink nectar and fruit juices
Larvae are herbivorous caterpillars
Cocoon or chrysalis is constructed
for pupa phase
 Order Siphonaptera
Small and wingless
Laterally compressed
Ectoparasites of mammals and
 Excellent jumpers
 Larvae aren’t parasitic
 Some are vectors of disease like
bubonic plague
 Order Diptera
 True flies
 Mosquitoes, houseflies,
gnats, and craneflies
 Large compound eyes
 Feed on animal or plant
 Some are vectors of
 Eggs deposited in wet
substrata such as flesh,
fruit, or water
 Larvae called maggots