ENTM 340 Insect Pests of Trees Turf and Ornamentals�

�C. Sadof� Purdue University

Lecture 2� Moulting and Introduction to External Anatomy: The Insect Head

 

The exoskeleton of an insect provides the set of tools that insects use to survive in their habitat.� As such, the particular form of an insect should provide a clue to how a particular insect eats, its capacity for long or short distance movement, whether it lives above ground, in water, or in the ground.� As you study the insect external body plan in the next 2 lectures I expect you to:

�        Understand the structure of the insect shell

�        Learn how an insect moults

�        Become familiar with the general body plan, and function of the head, thorax and abdomen.

�        Be able to see the relationship between the structure and orientation of the insect mouth and the kinds of food it eats (predator, herbivore).

�        Learn adaptations of the mouths of insects who

o       �pierce and suck plants

o       feed on nectar

o       chew leaves

o       chew wood

�        Understand why the meso- and meta-thoracic segments are reinforced with a pleural suture.

�        The relation between leg structure and function (swim, dig, crawl etc)

�        How the number of articulation points at a joint affect its range of movement (eg single, vs double, mono or dichondyllic, shoulder vs knee).

�        The homology between an insect antenna and a leg, and a maxilla,

�        Adaptations of the abdomen for oviposition.

�        Know what a spiracle is and how it functions

�

 

 

A. Exoskeleton and molting

    1. Exoskeletal Structure

                                                               i.      Epidermis (basement membrane)

                                                             ii.      Cuticle components

1.      Endocuticle � (dissolved, resorbed)

2.      Epicuticle � shed

3.      Exocutile -shed

������������������������������������������ 2. Chemical composition of Endo, Epi and Exocuticle

                                                            iii.      Chitin- cellulose like with amino group

                                                           iv.      Proteins

1.      Sclerotin for hardening

2.      Resilin for resiliency

                                                             v.      Wax and cement cover on Exocuticle

 

����������������� 2. Exoskeletal processes

����������������������� �� i. Exocrine glands secrete waxes, pheromones and other compounds to cuticle surface

�ii.� Setae come up through the integument from the basement membrane.� They have 2 parts, a socket like structure called a tormogen cell, that holds the hair like structure called a trichogen cell.

 

3. Moulting Process �(AKA ecdysis) Under control of growth hormones commonly refered to as insect growth regulators ( juvenile hormone and ecdysone)� -� some insecticides work by disrupting this process.- safe for humans but not crustaceans.

                                                                                 i.            Insects blow air to separate cuticle from epidermis (= Apolysis)

                                                                               ii.            Enzymes (chitinase, proteinase) secreted from cuticle dissolves endocuticle

                                                                              iii.            Epidermal cells multiply and secrete new cuticle

                                                                             iv.            Waxy layer secreted

                                                                               v.            Old insects cuticle splits along specialized wrinkles called ecdysial lines, and insect crawls out

                                                                             vi.            Exposure to air and other chemicals (tyrosine) produced by insect causes sclerotization (hardening) and later melanization (browning)

���������������� ���

 

 

B.      The Insect Head �

1. Why study the Insect Head?

    1. Body region devoted to ingestion, and perception. Morphology determines the kinds of food it can eat.
    2. Modifications of mouthparts are typical of certain groups of insects.

                                                               i.      True bugs have piercing sucking mouth parts

                                                             ii.      Beetles have chewing mouth parts

c.� As with the rest of this course I will try to examples that are relevant to plant pests and the insects that feed on those pests.�

  1. Insect Head
    1. Areas of the head

                                                               i.      Vertex (top)

                                                             ii.      Frons� ( area between antennae and clypeus)

                                                            iii.      Clypeus (area below frons)

                                                           iv.      Gena (cheeks, below eyes)

    1. Sutures of the head

                                                               i.      Sutures-crease

                                                             ii.      Epicranial stem suture is the ecdysial cleavage line

    1. Appendages of the head

                                                               i.      Paired antennae

                                                             ii.      Paired compound eyes

                                                            iii.      Ocelli (simple eyes, sense light or dark)

                                                           iv.      Mouthparts�

    1. Orientation of the head determines how mouthparts are oriented to the source of food .

                                                               i.      Hypognathous� (mouthparts directed ventrally)- Great for herbivores feeding on leaves (think grazing cows)

1.      Grasshoppers, caterpillars

 

                                                             ii.      Prognathous� (mouthparts directed toward anterior)- Great for predators trying to eat an insect it catches.

1.      (Tiger beetles, lacewing larvae)

 

                                                            iii.      Opistognathous ( mouthparts directed toward posterior)Some Sucking insects, great for drilling through wood

1.      ����������� (Cicada)

 

    1. Types of mouthparts

                                                               i.      Chewing (grasshopper, caterpillars, beetles, sawflies)

1.      Labrum- upper lip

2.      Mandible, pliers like jaw

3.      Maxilla� below mandible with appendages

4.      Labium lower lip.

5.      Hypopharynx forms the opening for the oral cavity.

                                                             ii.      Piercing Sucking of plant feeders (True bugs, lacebugs, aphids, scale insects, mealybugs)

Mouthparts are shaped to brace the proboscis (beak) as the needle like mouthparts (stylets) penetrate the leaf tissue.� The head is musculated with a cybarial pump that helps insect suck plant fluid

1.      Labrum of cicada reinforces proboscis which is a tube made of the rolled up labrum(lower lip)

2.      Mandibular stylets lay curved in the tube and are used to pierce the plant tissue

3.      Maxillary stylets are contained within the mandibular tube and form 2 tube.� The salivary tube is used to secrete enzymes that prevent the plant phloem sieve tubes from sealing or plant tissue from suberizing.� The food channel is used to suck up the plant fluid.

4.      Note that some predaceous bugs have a similar morphology that allows them to skewer insects and suck out their body fluids.

                                                            iii.      Rasping sucking mouthparts (Thrips)

1.      These insects have an unusual adaptation in that the mouth has 3 stylets and lacks bilateral symmetry.� The left mandible is modified into a stylet as well as the part of each� of the maxillae (lacinea). In the western flower thrips the right mandible functions as a rasp..�

                                                           iv.      Piercing sucking blood feeder (female mosquito)

1.      Note that the same mouthparts are used to produce a penetrating proboscis.�

a.       Adaptations are to allow continual tissue cutting by maxillary and mandibular stylets, while a much larger food canal is used to suck up the blood.

b.      Salivary ducts are key to secreting anticoagulant enzymes.

                                                             v.      Siphoning (moths and butterflies)

1.      Note this modification is just for sucking nectar out of plants, and water out of puddles.��� There is no need to pierce anything.

a.       How would this compare with the mouth of a caterpillar?� Would this have some implications for developing a control program?

2.      The curled tube of the mouth is formed by a specialized, musculated,� part of the maxilla called the galea.

                                                           vi.      Sponging (house flies)

1.      Adult house flies are modified to form a sponge at the bottom with the hypopharynx that extracts fluids from the feeding surface.�

                                                          vii.      Chewing lapping (honey bee)

1.      These mouthparts are modified to extract nectar and to chew pollen and shape wax to construct the hive.

    1. Types of eyes

                                                               i.      Compound

                                                             ii.      Simple (ocelli)

                                                            iii.      Stematta (primitive on caterpillars)

    1. Internal skeleton of head (Tentorium) used as points of articulation for muscles associated with mouthparts.
    2. Insect Antennae

                                                               i.      Parts. (Note that layout is similar to legs)

1.      Scape (basal)

2.      Pedicel (second segment)

3.      Flagellum ( third segment, but may be secondarily segmented).

                                                             ii.      Modifications of the antennae, useful for identification

1.      Filiform,- like a thread

2.      Pectinate- like half a feather

3.      Serrate � sawlike

4.      Plumose- like a feather

5.      Aristate, with an arista, a setaceous appendage

6.      Geniculate- bent

7.      Lamellate- like a waving hand

8.      Setacious-� like a seta or spine

9.      Capitate � with ball or head at the end

10.  Clavate- with an expanding end, not shaped like a ball.

                                                            iii.      Functions

1.      Smelling

2.      Touching

3.      Hearing

    1. Insect neck

                                                               i.      Needs a cervical sclerite or heads would bob like a bobble head doll

                                                             ii.      Connects to the thorax- the body region responsible for locomotion.

 

Review Questions:����

  • What is the morphological structure of the insect cuticle.
  • What is the chemical composition of the endo, epi and exo-cuticle?
  • Describe how insects molt.��
  • If insects lack endoskeletons, where do muscles attach?
  • Where do insect spines come from?
  • How do insect mouthparts influence feeding?
  • Give an example of a piercing sucking plant feeder, and how it penetrates plant tissue.
  • How does the tubular shape of butterfly mouths influence the depth of the flowers from where they can get nectar?
  • Explain how mouth structure forces butterflies and their caterpillars to consume different food.� What is the significance to the insect stage that is targeted in an insecticide control program?
  • What kind of adaptations do sucking and chewing insects have?
  • What do insects use antennae for?
  • What is the difference between a compound eye and a simple eye?