Basic insect characteristics




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Invertebrate Zoology Lecture 19, June 2. 2008

Phylum Arthropoda, Part 3 (Hexapoda)

  1. Basic insect characteristics


    1. Three main body region (tagma) = head, thorax, abdomen

    2. Single pair of antennae

    3. Mouthparts variable

    4. Three pairs of thoracic legs; no abdominal appendages

      1. Uniramous

      2. Wings exist in addition to legs, not considered “appendages”
  2. How did insects evolve and further differentiate into current groups?

    1. Entognathous apterygotes were the earliest insects

      1. Entognathous: mouthparts pulled into head


      2. Apterygotes: “without wings”
    2. Development of ectognathous forms: mouthparts extended


      1. Derived condition. All common insects have this.
      2. Silverfish are common ectognathous apterygotes

    3. Development of winged forms, the Pterygota


      1. Developed without loss of legs (unlike birds and bats…)

      2. From what structure are they derived?

        1. Hyopoth. 1: Expansion of notum (dorsal exoskeleton)

          • Function?

        2. Hyopoth. 2: Gill origin

      3. First wing types were held laterally, unable to fold over abdomen at rest (=Paleopterous condition or “primitive winged”)

        1. Paleopterous ("primitive-winged" ) orders

          • Odonata (dragonflies and damselflies)

          • Ephemeroptera (mayflies)
    4. Evolution of wing flexion (neopterous insects)

    5. Development of complete metamorphosis (endopterygotes)


      1. Those with “simple” or incomplete metamorphosis are the exopterygotes, those with complete metamorphosis = endopterygotes.

  3. Paleopterous exopterygotes (primitive-winged, incomplete metamorphosis)

    1. Order Odonata: dragonflies and damselflies

      1. Large compound eyes; good vision

      2. Chewing mandibles

      3. Two pairs of membranous wings

        1. Each pair of wings used separately

        2. Some of the best fliers in the insect world

      4. Elongate abdomen: serves as rudder in flight

      5. Aquatic, predatory larvae; adults usually found near water

      6. Adults prey on flying insects, capture by extending forelegs

      7. Adults mate in flight

      8. Earliest dragonflies were gigantic, with wingspans up to 30 inches!

    2. Order Ephemeroptera (briefly…)

  4. Neopterous exopterygotes (have wing-flexion, incomplete metamorphosis)

    1. Order Plecoptera (briefly)

    2. Order Orthoptera (“ortho” = straight): grasshoppers, katydids, crickets, etc…

      1. Adult characteristics

        1. Chewing (mandibulate) mouthparts; primarily herbivorous

        2. Pronotum is enlarged, extends over thorax and (sometimes) abdomen

        3. Femurs and associated muscles of hindlegs are greatly enlarged and adapted for jumping. (Just how good are they?)

        4. Wings

          • Forewings are toughened

          • Hindwings are membranous, fan-shaped

          • More often gliders than fliers

        5. Ears: Difference in location, grasshoppers vs. crickets

        6. Sound production for mate attraction via stridulation

          • How does sound production differ between grasshoppers and crickets?

    3. Order Hemiptera: true bugs: Includes water striders, water boatmen and backswimmers, plant bugs, stink bugs, assassin bugs, box elders, etc…

      1. Adult characteristics

        1. Mouthparts are piercing-sucking. They extend from the anterior portion of the head to form a long, multisegmented rostrum (or “beak”). The beak is tucked beneath the thorax and abdomen, often within a groove when not in use, swings forward.

        2. Triangular-shaped scutellum on mesothorax between the forewing bases (distinctive)

        3. Forewings: basal portions are thickened and colored while the distal portions are membranous

        4. Hindwings are membranous

        5. Wings held flat against body.

      2. Habitat

        1. Water bugs: water scorpions, giant water bugs, water boatman, backswimmers

          • Adaptations to aquatic environment?

        2. Surface-dwelling bugs (i.e. water striders)

          • What enables them to walk on water?

        3. Terrestrial bugs (diverse terrestrial habitats)

      3. Food/Feeding behavior (some select examples)

  5. Neopterous endopterygotes (wing-flexion, complete metamorphosis = within pupae)

    1. Order Coleoptera (“coleo” = sheath): beetles Most diverse of the insect orders

      1. Chewing (mandibulate) mouthparts

      2. Hardened pronotum

      3. Forewings are called “elytra”—hardened (scleratized); usually meet in a central, vertical line.

        1. Adaptive value of hardening?

        2. Oher function(s) of space beneath elytra?.

      4. Hindwings membranous

      5. Robust abdomen

      6. Distinctive larval features (all are Neopterous)

        1. The larvae usually have three pairs of legs; some have no legs

      7. Groups (more common and/or unique groups)

        1. Curculionidae (weevils or “snout beetles”)

          • Most common family of insects

          • Females often use their long snout to bore holes in plant tissues, into which they deposit their eggs. Once hatched, the young feed on the plants.

          • Often infest important crops such as cotton (i.e. boll weevil)

          • Rapid reproduction: Up to 25 generations per year.

        2. Buprestidae (jewel beetles)

          • Metallic, wood-boring beetles

          • Eggs are laid in wood.

          • The larvae tunnel through the wood

          • Some species have heat sensors that allow them to detect freshly burned forests that make ideal egg-laying locations

          • Adults feed on flowers, nectar and pollen

        3. Carabidae (ground beetles)

          • Ground dwellers

          • Primarily hunters with well-developed legs & large mandibles

          • The bombardier beetles, expel hot chemicals from their abdomen as a defense

        4. Many more Coleoptera families!

    2. Order Neuroptera (briefly)

      1. Snakefly: Chewing mandibles; predatory!

    3. Order Diptera: flies

      1. Compound eyes have interommatidial hairs

        1. Most have 3 ocelli in addition to compound eyes

      2. Variable antennae: some are quite bizarre with fleshy lobes

      3. Mouthparts (adult): considered “sucking”, highly modified & quite variable.

        1. Single pair of membranous wings, the forewings

          • Thorax often robust  well-developed flight muscles.

        2. Hindwings highly reduced and modified as halteres, which

          • Sensory: detect minute air currents, giving flies greater flight control and maneuverability than other insects

          • Serve as stabilizers for flight

          • Why are halteres not considered “vestigial” since they are so reduced in size compared to wings?

      4. Distinctive larval features

        1. No legs; called “maggots”

        2. Chewing mouthparts (mandibulate)

      5. Habitats

        1. Wide variety of ecological niches

        2. Some with aquatic larvae

      6. Food/feeding behavior

        1. “A survey of the feeding habits of the order could well apply to the entire Class Insecta.”

          • Pollinators, predators. blood suckers, etc…

          • Housefly (Family Muscidae) feeding: Feed on solid organic matter by regurgitating saliva with proteolytic enzymes and then sponging it back up with their mouthparts.

      7. Several groups transmit disease-causing pathogens, most of which are prevalent in the tropics

        1. Mosquitoes transmit more different diseases than any other insect group (such as…?)

        2. Black flies transmit the nematode that causes river blindness

        3. House flies: More than 100 pathogens associated with the house fly may cause disease in humans and animals, including typhoid, cholera, dysentery, tuberculosis, infantile diarrhea, and parasitic worms.

          • What features of houseflies cause them to spread so much disease?

      8. Abundant black flies and midges  poor water quality.

    4. Order Trichoptera (briefly)

    5. Order Lepidoptera (“lepido”= scale): butterflies, moths, 2nd largest order

      1. large compound eyes; may have 2 ocelli

      2. mouthparts: long, coiled proboscis for sucking liquids (not piercing); larvae have chewing mandibles

      3. Two pairs of membranous wings covered with small scales.

        1. large wings in proportion to body

        2. forewings usually larger than hindwings

        3. often colorful (aposematic for many)

        4. wings are coupled to each other

      4. Larvae: Generally three pairs of true legs on the thorax, plus 5 pairs of prolegs on the abdomen

      5. Food/feeding behavior

        1. Most adults feed on nectar while larvae feed on the stems, leaves and buds.

        2. Most Lepidoptera are quite specific to their host plant, at least to the particular plant family, but often to species.

          • Example: Monarchs and milkweeds. Why is this relationship critical for monarch survival?

      6. Flight: Glide on thermals to travel long distances

      7. Males and females are often sexually dimorphic.

        1. Males of many species have specialized glands associated with some of their wing scales (androconia) and legs or abdomens (eversible hair-pencils) that produce and emit pheromones to attract females.

      8. Often migratory, with different generations in different locations based on temperature and plant blooming cycles (i.e. monarch migratory circuits)

      9. Endangered Lepidoptera in Oregon

        1. Fender’s blue: its host plant, the Kincaid’s lupine, is also endangered

          • Current planting efforts at Baskett Slough and elsewhere.

        2. Oregon silverspot: threatened

          • Found in coastal grassland and dunes (i.e. Cascade head); feeds on early blue violets

    6. Order Hymenoptera (“hymen”=membrane): ants, bees, wasps, sawflies, horntails, etc…

      1. Well-developed compound eyes with interommatidial hairs; usually have 3 dorsal ocelli

      2. Mouthparts: some with chewing mouthparts (ants and some wasps) others with sucking mouthparts (i.e. honeybees)

      3. Wings

        1. two pairs, membranous, forewings larger than hindwings

        2. wings linked together

      4. Abdomen:

        1. Some with “petiole” first abdominal segment fused to thorax and constricted. Allows for greater maneuverability

        2. Some with stingers at distal end of abdomen, homologous with ovipositor

      5. Distinctive larval features

        1. Usually legless

        2. Chewing (mandibulate) mouthparts

        3. Pupae usually within a cocoon

      6. Food/feeding behavior

        1. Difficult to generalize even within a family

          • For example, ants show all modes of feeding

            • Predators: i.e. army ants

            • Fungus gardeners: i.e. leafcutter ants

            • Other species are herbivores or omnivores

      7. Selected aspects of reproduction

        1. Many species build nests either above or underground

        2. Some species exhibit sociality or even eusociality

          • In eusociality, individuals give up their own reproduction to help raise relatives (usually sisters raising their siblings)

      8. Other

        1. Many wasp species are parasitoids on other insects as larvae (primary insect group with this behavior)

          • Have been used as biological control agents for destructive insect species. Usually the parasitoids are species-specific.

        2. Many are important pollinators

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