Insects   All Materials © Cmassengale  

Phylum Arthropoda        Subphylum Uniramia          Class Insecta

Characteristics

• Largest arthropod group

• Found in freshwater & terrestrial habitats, especially tropical areas

• Legs, mouthparts, & antenna jointed

• Body segmented into three sections --- head, thorax, & abdomen

• Six legs & up to two pairs of wings located on thorax

• Have compound & simple eyes

• One pair of antennae on head

• Abdomen has 11 segments

• Exoskeleton, covering & protecting body, is made of chitin & must be molted to grow

• Elaborate mouthparts include:
       *  Mandibles - jaws
       *   Maxillae - paired sensory structures that move food to mouth
       *   Labium - lower lip
       *   Labrum - upper lip
       *   Palpi - used for tasting

• Known as mandibulates

• Spiracles on abdomen open into tracheal tubes for oxygen & carbon dioxide exchange

• Tympanic membranes on 1st abdominal segment aid in hearing

• Thorax divided into 3 sections --- prothorax, mesothorax, & metathorax

• One pair of legs on each thoracic segment

• Wings located on mesothorax & metathorax

• Ovipositor located on the end of the abdomen in female insects & used to dig hole & lay eggs

Common Insect Orders

• Orthoptera - grasshoppers, crickets, & cockroaches 2 pairs of straight wings & chewing mouthparts)

• Isoptera - termites (feed on wood)

• Dermaptera - earwigs (pincers on end of abdomen)

• Anoplura - sucking lice (wingless parasites)

• Hemiptera - true bugs (have triangular-shaped scutellum & last 1/3 of wings membranous)

• Homoptera - aphids & cicadas (membranous wings held roof-like over body

• Ephemeroptera - mayflies (have 2 cerci on tail, membranous wings, & nonfunctional mouthparts in adults)

• Odonata - dragonflies & damselflies (2 pairs of equal size, membranous wings, strong fliers, feed on other insects)

• Neuroptera - Dobson flies &  lacewings (2 pairs of membranous wings)

• Coleoptera - beetles (hard forewings or elytra, membranous hindwings)

• Lepidoptera - butterflies & moths (powdery scales covered wings

• Diptera - flies & mosquitoes (one pair of wings, 2nd pair modified into balancing structure called halteres)

• Siphonaptera - fleas (parasites on birds & mammals, wingless as adults)

• Hymenoptera - bees, ants, & wasps (stinger on abdomen for protection, may live together in groups, pollinators)

        Click Here for Pictures of Insect Orders

Success of Insects

• Found everywhere except in deep part of ocean

• Very short life span & rapidly adapt to new environments

• Small size helps minimize competition in habitats

• Flight helps escape predators & move into other environments

Environmental Impact

• Pollinate almost 2/3's of all plants

• Serve as food for fish, birds, & mammals

• Help recycle materials (termites recycle wood)

• Make useful byproducts such as silk & honey

• Some spread disease

• Agricultural pests

Grasshoppers

External Structure

• Head with antenna, compound eyes, & chewing mouthparts

• Walking legs on prothorax & mesothorax; jumping legs on metathorax

• Tarsus are lower leg segments with spines, hooks, & pads

• Leathery, protective forewings on mesothorax & membranous hindwings for flight on metathorax

• Covering over thorax called pronotum

Internal Structure
Digestive & Excretory Systems

• Cutting & chewing mouthparts (labium, labrum, mandibles, & maxillae)

• Saliva added to food in mouth

• Esophagus carries food to crop for temporary storage

• Gizzard has chitinous plates to grind food

• Midgut (insect's stomach) has gastric caeca (pouches) to secrete digestive enzymes to break down food

• Food is absorbed into the body cavity or coelom in the hindgut (composed of the colon & rectum)

• Malpighian tubules filter chemical wastes from the blood & deposit them in the rectum where they leave through the anus

Circulatory System

• Open circulation of blood

• Aorta is the largest blood vessel carrying blood to the body cells

• Hearts are muscular regions of the aorta in the posterior end of the abdomen that pump blood toward head

• Blood flows back toward abdomen carrying digested food & re-enters the aorta through openings called ostia

Respiratory System

• Air enters through openings called spiracles along the sides of the abdomen & enters into tracheal tubes that branch into smaller tracheoles where gas exchange with body cells occurs 

• Tracheal tubes carry oxygen to body cells & return carbon dioxide to leave the body though spiracles

Nervous System

• Simple brain, nerve cords, & ganglia 

• Three simple eyes or ocelli (detect light) & a pair of compound eyes (can detect movement but not images)

• Tympanic membrane on 1st abdominal segment

• Pair of antenna contains sense organs for touch, taste, & smell detects sound

• Sensory hairs found on parts of the body

• Palpi for taste

Reproductive System

• Reproductive organs (ovaries & testes) located  in abdomen

• Male deposits sperm into female's seminal receptacle

• Stored sperm fertilizes eggs as they  are released by female

• Ovipositor on tip of female's abdomen is used to lay eggs

• Separate sexes

• Lay large number of eggs to ensure survival

Development

• Most insects go through changes in form & size called metamorphosis

• Some insects such as silverfish don't go through metamorphosis

• Incomplete metamorphosis goes from egg to nymph (immature form that looks like adult but without fully developed wings) to adult (3 stages)

• Instars are growth periods between molts of nymphs & larva

• Grasshoppers, termites, & true bugs go through incomplete metamorphosis

HEMIPTERAN (TRUE BUG) NYMPH

• Complete metamorphosis goes from egg to larva (segmented & wormlike) to pupa  to adult (4 stages)

BUTTERFLY LARVA (CATERPILLAR)

• Butterflies, beetles, & flies go through complete metamorphosis

• In pupal stage, larval tissues break down & cells called imaginal disk develops into tissues of the adult

• Cocoon or chrysalis is a protective case formed around the pupa

BUTTERFLY COCOON

• Metamorphosis controlled by hormones
       * Brain hormone stimulates the release of molting hormone (ecdysone)
       * When juvenile hormone level high, larva molts
       * When juvenile hormone level low, larva pupates
       * When juvenile hormone absent, adult emerges from pupal case

• Different stages of metamorphosis eliminates competition between larva & adults for food & space

• Multi-stage life cycle helps insects withstand harsh weather

• Different stages have different functions (caterpillar/growth & adult/reproduction)

Defense Mechanisms

• Bombardier beetle sprays noxious chemical

BOMBARDIER BEETLE

• Wasps & bees can sting

• Some insects use camouflage to blend into their environments

• Some insects taste bad & have warning colorations 

PAPER WASP

• Mullerian mimicry - poisonous or dangerous species have similar patterns of warning coloration so predators avoid all the species (black & yellow stripes on bees & wasps)

• Batesian mimicry - species that are nonpoisonous or not bad tasting have colorations that mimic other poisonous or bad tasting species (Viceroy butterfly mimics bad tasting Monarch)

Insect Communication

• Insects may communicate with each other using sound (cricket chirps), light (firefly), or "dances" (honeybee)

• Pheromones are chemicals released by some insects to attract mates or mark trails

Insect Behavior

• Insects may be solitary or social

• Social insects (bees, ants, & some wasps) live together in groups & share work (division of labor)

• Social insects have a caste system with different individuals doing different jobs

• Honeybee caste system:
       * Workers
                  - sterile females
                  - care for queen & feed her honey and pollen
                  - make beeswax for hive
                  - fan wings to cool hive
                  - eat honey
                  - collect nectar, pollen, & royal jelly
                  - live about 6 weeks
                  - nurse bees care for larva
                  - secrete royal jelly to feed new queen
       * Drones
                  - males
                  - mate with queen
                  - feed by workers
                  - driven out of hive to conserve food during winter
      * Queen
                  - reproductive female
                  - mate only once but store sperm for up to 5 years in seminal receptacles
                  - feed by workers
                  - secretes chemical called queen factor that prevents other females from sexually maturing
                  - leaves hive with 1/2 the workers if there is overcrowding

HONEYBEE HIVE