University of Michigan Biological Station

Biology 442 - Biology of Insects

Lecture Notes - Integument and Molting

1. Parts of the Integument.
   1. Basement membrane - continuous sheet of connective tissue.
   2. Epidermis - single cell layer that secretes constituents of cuticle.
   3. Cuticle.
      1. Procuticle - main structural element.
         1. Endocuticle - flexible, non-sclerotized inner layer.
         2. Exocuticle - hard, sclerotized outer layer.
      2. Epicuticle - thin outer layer.
         1. Cuticulin - hard layer of lipoprotein.
         2. Wax - provides watertight surface, made of long chain hydrocarbons and the esters of fatty acids and alcohols.
         3. Cement - May protect wax layer, acts as shellac.
2. Chemistry of the Cuticle.
   1. Chitin - polysaccharide made up of primarily N-acetylglucosamine with some glucosamine.
      1. Linked by 1-4 beta linkages - all residues in same direction.
      2. Held together by hydrogen bonds between chains, run in opposite directions.
      3. Chitin is only 25-50% of cuticle by weight, rest is protein linked to chitin covalently. Together called arthopodin.
      4. Protein may be very flexible then structure is called resilin.
   2. Sclerotization (Tanning). See handout.
      1. Protein molecules in arthropodin are cross-linked using quinones (derived from tyrosine).
      2. Links using Quinone sclerotization at terminal amino ends of proteins and then using Beta sclerotization at Lysine sites using 2nd amino group on side chains.
   3. Melanization. Provides dark color. See handout.
3. Process of molting.
   1. Epidermal cell proliferation.
   2. Apolysis - separation of old endocuticle from epidermis.
   3. Secretion of inactive molting fluid.
   4. Secrection of cuticulin and tanning.
   5. Activation of molting fluid. Digests endocuticle for resorption.
   6. Formation of new procuticle starts.
   7. Digestion and resorption of old endocuticle continues.
   8. Wax secrection.
   9. Ecdysis - Swallow air or water to increase hemolymph pressure. Old exoskeleton splits and is shed.
   10. Bloating continues to stretch new cuticle and expand wings.
   11. Wax secretion contines, procuticle secretion continues, cement deposited, exocuticle tanned, melanization occurs.
4. Hormonal control of molting.
   1. Neurosecretory cells in protocerebrum produce "brain hormone" (prothoracatropic hormone). Usually in response to stretch receptors.
   2. Travels via axons to corpora cardiaca. Stored there.
   3. Threshold reached or stimulus causes release into hemolymph. Travels to target organ - prothoracic gland.
   4. Prothoracic gland produces molting hormone (ecdysone). Target organ is epidermis. Molting sequence is started.
   5. Ecdysone must be synthesized using sterol components that are obtained in the diet. Insects cannot make own steroids.

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