University of Michigan Biological Station

Biology 442 - Biology of Insects

Lecture Notes - Vision

1. Types of visual sensors include compound eye (in adults and hemimetabolous nymphs), ocelli (in adults and hemimetabolous nymphs), stemmata (in larvae of holometabolous insects), general body light sensitivity.
2. Compound eye.
   1. Basic insect eye - apposition eye.
      1. Made up of ommatidia - 1 (worker army ants) to 10,000 (dragonflies). Can see up to 360 degrees (holoptic).
      2. L. s. of ommatidium. Corneal lens (clear cuticle), corneagen cells (2, form lens, become primary pigment cells), semper cells (4, form crystalline cone), retinula cells (6-8, actually modified neurons, axon extends to optic lobe), rhabdom (central core of retinula cells, contains visual pigments on microvilli), secondary pigment cells (12-18, surround retinula cells), basement membrane.
      3. X. s. of ommatidium. Through retinula cells, microvilli in any one cell are roughly parallel and so are those in opposite cell.
      4. Tapetum may be present, most often in night flying insects. Made up of tracheae around each ommatidium. Acts to reflect light back across rhabdom and increase sensitivity. Why moth eyes shine.
   2. Image formation.
      1. Mosaic theory. Images are made up of small parts which are integrated in the CNS. Each ommatidium detects light intensity over small area. Ommatidial angle is usually about 1-2 degrees, but other light can affect.
      2. Bees and butterflies and definitely distinguish shapes. Hostplant shapes including leaves.
      3. Movement acuity is much greater - up to 0.1 degree arc.
      4. Function of rhabdom.
         1. Visual pigment primarily rhodopsin - made up of retinene, aldehyde of vitamin A, conjugated protein. It is produced continuously. Broken down into free retinene when struck by light causing action potential.
         2. Speed of firing of neurons varies with group (Flicker fusion freqency). Slow = up to 40 - 50 per second (Orthoptera, Lepidoptera; low light insects). Fast - up to 300 per second (Hymenoptera, Odonata, Diptera; high light/speed insects).
   3. Adaptation to light and dark. Many can handle a change up to 1000 times difference in intensity.
      1. Changes in visual pigment availability. How much around.
      2. Movement of other pigment in secondary pigment cells - superposition eyes.
      3. Adaptation of nervous impulses.
      4. Cytological changes in retinula cells (mitochondria and ER changes positions changing refractive index).
      5. Changes in cell shape.
   4. Color vision.
      1. Generally more responsive to blue through ultraviolet. Some butterflies see red but because most insects do not nocturnal behavior can be studied using red light.
      2. Blowflies have 3 pigments, 1 per retinula cell. Of 6 cells, 1 has A, 5 have B, 1 has C. combinations of 3 pigments in bees allow them to see 6 basic colors (yellow, blue-green, blue, violet, ultraviolet, bee purple [mixture of yellow and ultraviolet]).
      3. Effect of diffraction. Small ommatidia get greater resolution within a wavelength, but diffraction also increases with decreasing size (especially among longer wavelengths). More light also reduces resolution.
   5. Distinguishing polarized light.
      1. Occurs in some insects notably bees and ants.
      2. Only some ommatidia can do it. Usually requires several, each with microvilli directed in a particular direction. These are activated only when light is in same plane and so can determine position of sun. Need blue sky because it is polarized. Clouds scatter light.
3. Dorsal ocelli.
   1. Structure.
      1. Cuticular lens made from corneagen cells.
      2. Sense cells with rhabdoms as in compoud eyes. Axons form ocellar nerve.
      3. Pigment may be present between sense cells.
   2. Might be able to form image but probably only for light detection.
   3. May be useful in controlling physiological activity including circadian rhythyms.
4. Stemmata. Occur laterally on head.
   1. Structure.
      1. Lens of endocuticle.
      2. Some with, some without crystalline lens.
      3. Sensory cells may be like ocelli or be fewer in number and form rhabdom like compound eye.
   2. May be able to form images but probably used for detecting objects by using back and forth movements of the head. May be used for locating host plants in larvae.

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