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Insect Development

Insect Development. Lecture 16. Basic terms to know…. Molting – periodic formation of new cuticle of greater surface area Ecdysis – shedding of the old cuticle Instar – growth state between two successive molts Stadium – time between two successive molts

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Insect Development

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  1. Insect Development Lecture 16

  2. Basic terms to know… • Molting – periodic formation of new cuticle of greater surface area • Ecdysis– shedding of the old cuticle • Instar– growth state between two successive molts • Stadium – time between two successive molts • Imago – reproductively mature individual • Metamorphosis – change in body form between immature and adult instar

  3. Two types of growth • Indeterminate – continued molting until death (no terminal molt), found in collembolans, diplurans, and apterygote insects • Determinate – distinctive instar (final instar) that marks the cessation of growth and molting, found in pterygote insects

  4. Three types of developmental pattern • Ametaboly– hatchling to adult very similar, except genitalia, found in Archaeognatha and Zygentoma • Hemimetaboly– egg  nymph  winged adult; exopterygote (externally visible wing bud, wing pad in nymphal stage) • Holometaboly– egg  larva  pupa  winged adult; endopterygote (internal imaginal discs in larval stage)

  5. Exopterygote Endopterygote

  6. Embryonic Development (From egg to hatchling) • Superficial cleavage – zygote nucleus subdivides by mitotic division to produce many daughter cells • Blastoderm formation – migration of cleavage nuclei (energids) to egg periphery to form one-cell thick layer • Germ cell formation – later forms spermatocytes and oocytes • Gastrulation– infolding of blastoderm • Germ band formation – a group of blastoderm cells make germ band from which embryo’s body forms and differentiates; the rest of the cells form membrane that makes up the yolk sac (serosa)

  7. Embryonic Development (From egg to hatchling) Developmental genetics: http://bcs.whfreeman.com/thelifewire/content/chp19/1902003.html

  8. Larval or nymphal phase • Larvae – all immature holometabolous insects • Nymphs – Immature terrestrial hemimetabolous insects • Naiads – Immature aquatic hemimetabolous insects

  9. Larval types Short thoracic legs + abdominal prolegs Functional thoracic legs + no abdominal prolegs + prognathous mouthparts No legs, worm-like

  10. Pupal types Non-articulated mandibles Articulated mandibles Appendages cemented to body, cuticles heavily sclerotized Appendages not closely appressed to body

  11. Special adult type • Ephemeroptera (mayflies): • emerges as a subimago • shorter legs and caudal filaments, wings translucent • files off and molts again to the imago • only known example of fully winged insects undergoing a molt

  12. Molting

  13. Molting • Complex process involving hormonal, behavioral, epidermal, and cuticular changes leading to the shedding of the old cuticle • Apolysis– separation of the old from the new cuticle during molting • Mitotic division of epidermal cells leading to increases in size and volume for the subsequent instar

  14. Hormonal control of molting Prothoracicotropic hormone (PTTH) CC stimulate release release Juvenile Hormone (JH) Ecdysteroids Initiate changes in epidermis high titre: larval-larval Lower titre: larval-pupal No titre: pupal-adult

  15. Hormonal control of molting Eclosionhormone:help escape from old cuticle Bursicon: control sclerotization of exocuticle and deposition of endocuticle Commitment peak: larval cuticular protein no longer produced, insect committed to metamorphosis JH level at critical periods determines the type of cuticle produced

  16. Cuticular control of molting Tension generated at epidermal cell surface resulting in its separation from cuticle Enzymes digest all unsclerotized cuticles; digested cuticles absorbed Shedding due to increased hemolymph pressure

  17. Hormonal and cuticular control of molting

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