internal_structure

University of Michigan Biological Station

Biology 442 - Biology of Insects

Lecture Notes - Internal Structure

Skeleton.
1. Tentorium. X or H-shaped. Serves as brace for head and for muscle attachment.
2. Phragmata. Thoracic, large in pterygotes. Attachment of dorsal longitudinal muscles.
3. Furca and Plueral apophysis. Muscle attachment.
4. Tonofibrillae. Small ridges for muscle attachment.
Digestive System.
1. Foregut.
  1. Ectodermal origin, lined with cuticle (intima), shed at each molt.
  2. X-sec. Intima, epidermis, longitudinal and circular muscles.
  3. Head glands.
    1. May be associated with mandibles, maxillae, labium, hypopharynx.
    2. Most often labial developed, usually as salivary glands. These are large and extend into thorax. Secretions lubricate mouthparts and start digestion. Amylase (starch to sugar) and invertase (sucrose to glucose plus fructose). Blood feeders may produce anticoagulant, produces silk in Lepidoptera and Trichoptera.
  4. Divisions.
    1. Mouth. Food ingestion.
    2. Pharynx. Muscular, sometimes modified into pump for sucking.
    3. Esophagous. Tube for passing food.
    4. Crop. Primarily for food storage, some digestion due to salivary enzymes and regurgitated midgut enzymes.
    5. Proventriculus. Often just a valve to prevent backflow of food, may be a grinding or rasping organ (teeth from intima).
2. Midgut.
  1. Not cuticle lined. Lined with peritrophic membrane, then columnar cells that secrete enzymes and absorb nutrients, then longitudinal and circular muscles.
  2. Peritrophic membrane protects gut cells against abrasion and infection. Permeable to enzymes and digestive products, sometimes adsorbs secondary chemicals.
  3. Divisions. Gastric caecae and ventriculus.
3. Hindgut.
  1. Ectodermal in origin, cuticle lined and shed at molt.
  2. Divsions.
    1. Pylorus. May be only a valve, Malphigian tubules arise here.
    2. Ileum. In most insects is just a tube, may contain symbionts.
    3. Rectum. Thin-walled, reabsorbs water, salts, amino acids. Sometimes contains gills in aquatic groups.
    4. Malphigian tubules. Lie in body cavity. 1 cell thick, may or may not have muscles. 2 to 250, absent only from Collembola, Aphids, small in Protura, Diplura, Strepsiptera. Collect nitrogenous waste from hemolymph, function in water and salt balance.
Circulatory System.
1. Open system with hemolymph as transport medium.
2. Divisions. Hemocoel, Posterior heart and anterior aorta (both dorsal).
3. Heart relaxes and blood allowed in thru ostia (valves). Contracts in wave to circulate blood, aided by diaphragm and pulsitile organs at base of appendages.
4. Dorsal vessel is closed posteriorly and open anteriorly.
5. Hemolymph. Consists of two fractions, plasma and cellular.
  1. Functions.
    1. Phagocytosis.
    2. Coagulation.
    3. Some metabolism.
  2. Plasma.
    1. Uses
      1. Transport of materials.
      2. Storage of sugars and proteins.
      3. Maintains hydrostatic pressure.
      4. Does not carry oxygen.
    2. Composition
      1. 90% water. Used to control osmotic pressure and pH.
      2. Inorganic ions. Most abundant are Cl-, Na+, K+ (may be responsible for quiescence), Mg++ (common in herbivores because of diet).
      3. Organic constituents. Amino acids (concentration varies with use), nitrogenous wastes, hormones, carbohydrates (trehalose for energy), glycerol (antifreeze).
      4. Pigments. Few have hemoglobin (Chironomidae, Notonectidae, Gasterophilidae), insectoverdin (gives green color - combination of carotenoid and bile).
  3. Cellular fraction.
    1. Uses
      1. Phagocytosis.
      2. Encapsulation.
      3. Coagulation.
      4. Some metabolism. Formation of connective tissue, basement membrane, may help form fat body.
    2. Fat body.
      1. Made of cells similar to blood cells called trophocytes. Floats in hemolymph.
      2. Major center for metabolic activity.
      3. Prominent in immatures.
      4. Stores glycogen, fat, protein for use during periods of non-feeding.
      5. Urate cells occur in insects without Malphigian tubules to store uric acid.
      6. May have mycetocytes with symbionts that sythesize nutritional compounds (e.g. cockroaches).
      7. Light organs are generally derived from fat body.
Respiratory System.
1. System of internal tubes which can usually be opened or closed (to prevent water loss) to the environment.
2. Diffusion is often good enough to circulate air but others use muscular movements.
3. Parts.
  1. Tracheae. Large tubes, down to about 2 microns diameter. Ectodermal and cuticle lined and must be shed. Taenidia are spiral thickenings. Air sacs spread throughout for storage.
  2. Tracheoles. Fine tubes, 1 to 0.1 microns diameter. Cuticle lined, but not shed. formed and enclosed by tracheoblasts of epidermal origin. Smallest are functionally intracellular.
Nervous System.
1. Central nervous system. Consists of brain, paired ventral nerve cords and ganglia, peripheral nerves.
  1. Brain.
    1. Protocerebrum. Bilobed, continuous with optic lobes. Receives ocellar nerves.
    2. Deutocerebrum. Contains antennal lobes.
    3. Tritocerebrum. Receives circumesophageal connective from subesophageal ganglion, frontal connective from frontal ganglion, labral nerve, tritocerebral commissure.
  2. Ventral nerve cord.
  3. Ganglia. Neuropile in center with axons and dendrites. Cell bodies in outer part. Giant fibers for quick long distance reaction.
    1. Major ganglion is subesophageal ganglion. Fusion of three, mandibular, maxillary, labial and receives these nerves.
    2. Paired along nerve cord or fused at each segment.
2. Stomogastric nervous system.
  1. Consists of occipital ganglion, frontal ganglion, corpora cardiaca, corpus allatum, stomodeal nerve, ventricular ganglion.
  2. Innervates gut muscles, also secretes hormones.

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