University of Michigan Biological Station

Biology 442 - Biology of Insects

Lecture Notes - Development

1. Growth and Metamorphosis.
   1. General.
      1. Egg - Larva with several molts - adult.
      2. Other terms for larva (usually used for holometabolous insects) - immature (general term), nymph (hemimetabolous insects), naiad (aquatic hemimetabolous insects).
      3. Instar - what insect is called between molts. Stadium - time period between molts.
   2. Ametabolous. No obvious metamorphosis, only become sexually mature by obtaining functional genitalia. See simple increase in size. Occurs in apterygotes - Thysanura, Microcoryphia, Diplura, Protura, Collembola.
   3. Hemimetabolous. Incomplete metamorphosis.
      1. Nymphs resemble adults and usually have similar habits. Wings develop externally along with genitalia.
      2. Includes Hemiptera, Homoptera, Orthoptera, Mantodea, Grylloblattaria, Phasmida, Blattaria, Isoptera, Dermaptera, Embiidina, Psocoptera, Zoraptera, Phthiraptera, Thysanoptera.
      3. Plecoptera, Odonata and Ephemeroptera also here but nymphs (naiads) are more different, especially in habitats.
   4. Holometabolous. Complete metamorphosis.
      1. Larvae and adult usually do not look at all alike. Usually with different foods and habits. This is definite advantage.
      2. To do this the last immature stage becomes pupa.
         1. Physically inactive but metabolically very active, most cells are lysed and contents used to create new tissues.
         2. Imaginal discs present in larva. These are determined early in development (will always become the same tissue).
         3. Sometimes last part of last larval instar is prepupa, where insect rests. Occassionally a distinct stage with own molt.
      3. Includes Endopterygotes - Neuroptera, Coleoptera, Diptera, Hymenoptera, Lepidoptera, Siphonaptera, Mecoptera, Trichoptera, Strepsiptera.
      4. Pupal types.
         1. Exarate - free appendages. If active with moveable mandibles then called decticous (pupa leaves pupal chamber in Megaloptera), if inactive then adecticous (adult leaves pupal chamber).
         2. Obtect - appendages glued to body, not moveable. Most Lepidoptera, Neuroptera, primitive Diptera, some Coleoptera and Hymenoptera. All are adecticous.
         3. Coarctate - present in higher Diptera. Larva forms puparium from last larval skin which is hardened and not shed. Pupates inside.
   5. Hormonal control.
      1. Insect remains juvenile as long as level of juvenile hormone (terpene) is high enough during particular stage of last instar. Only sensitive for part of instar. Levels don't necessarily drop steadily throughout larval growth, but have sudden dip at that stage.
      2. Produced by corpora allata.
      3. As production drops below critical level, when ecdysone is released then adult emerges. Neat experiments can be done by adding juvenile hormone or removing corpora allata.
      4. Juvenile hornome is important for other functions as an adult, such as maturation of sexual structures - so name is actually misleading.
2. Dormancy.
   1. Quiescence. Temporary inactivity due to environmental factor. E.g. light or dark may cause inactivity as at night. Not prolonged or obligate.
   2. Diapause. Genetically determined state of depressed activity due to environmental cue other than that which is being avoided. E.g. overwintering or estivation during dry season.
      1. Cue is usually on earlier stage, commonly earlier instar, but may be transgenerational as in Bombyx mori - photoperiod which egg of mother is exposed to determines whether her eggs will diapause.
      2. Stages of diapause.
         1. Preparatory phase - normal activity but environmental cue is detected - often photoperiod because reliable, may be modified by temperature.
         2. Induction - build up of cryoprotectants (glycerol). May become up to 25% of body weight. Metabolic rate declines to about 2% of normal.
         3. Refractory period - period during which insect must be exposed to adverse condition before development can proceed.
         4. Activated phase - development will resume if conditions are appropriate in environment.
         5. Termination - Normal activity resumes.
      3. Obligate versus facultative diapause. Obligate occurs in univoltine (one generational) species, always must diapause. Facultative occurs in multivoltine (multigenerational) species, depends on environmental cues.

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