UNIV of CreteDept of BiologyLibrary English I English II English IIIcourse curriculumlist of course English IV DictionariesUseful LinksONEDIN

ENGlISH FOR BIOLOGY

THE EYESIGHT OF INSECTS

 

Nikolaidou - Neokosmidou Barbara

 

 

 

Someone might ask themselves: "How do insects see?" or "Do they sense their environment in the same way we do?" In order to answer these questions we should firstly examine what kind of photoreceptive organs they possess and which are their anatomy.

The photoreceptive structures that insects have, and which may be changed during their life cycle, are mainly of three types:
1. Compound eyes
2. Simple eyes (or dorsal occuli)
3. The stemmata (or lateral occuli)

1) Compound eyes are the major photoreceptive organs of adult insects with incomplete and complete metamorphosis and also the primary eyes of immature insects with incomplete metamorphosis. When present, there are two, one located on either side of the head. Each is composed of a number of individual sensory units, called ommaditia. Ommatidia, which are marked externally by hexagonal facets, vary in size and number among insects groups.

An individual ommatidioum is divisible in two parts:
i. The Dioptric Apparatus, which acts as the "lens".
ii. The Receptor Apparatus, which leads to the initiation of a nervous impulse.
 These two parts rest on the Basement Membrane.

    i. The Dioptric Apparatus, is composed of :
  • The corneal lens, which forms the outer surface.
  • The crystalline cone, which lies immediately beneath the corneal lens.
  • The corneal pigment cells, which surround the crystalline cone.
    ii. The Receptor apparatus is composed of:
  • 6-7 retinular (nerve) cells, that are usually surrounded by rather darkly pigmented cells. Each retinular cell gives rise to an axon, that passes through the Basement Membrane and enters the "brain". Each axon contributes to the formation of a centrally located retinal rod, the rhabdom. The rhabdom contains the light absorbing pigments, rhodopsin and metarodopsin.

We can distinguish two main types of compound eyes:
a) The apposition type: In this type the retinoular cells lie immediately beneath the crystalline cone. That is characteristic of daylight active insects.
b) The superposition type: In this type there is a space between the retinoular cells and the crystalline cone. This type of compound eyes is common between insects that are active during dark.

Research has shown that insects see a mosaic of "pictures". According to the "mosaic theory" (which was initially proposed by MŸller in 1826) each ommatidium "sees" only a small portion of the insectÕs surroundings. The combination of these portions forms a mosaic view of the external environment. There is also behavioral evidence that insectsÕ compound eyes can perceive not only form, but colour, movement, and distance as well. However insects understand these perceptions in a different way than that of humans. For example honeybees do not see red or orange, but they can see at the ultraviolet spectrum that man cannot.

 

2) Simple eyes (or Dorsal occuli) are found on nymphs and adults of insects with incomplete metamorphosis. There are 1 to 3 simple eyes located on the top and front of the insectÕs head, between the compound eyes.
They are composed of:

  • A corneal lens.
  • A layer of corneagen cells.
  • And the retina, which consists of up to 1000 photosensitive cells, that form the nerve.
    Simple eyes do not really form images. Their purpose is:
  • The regulation of the daily rhythms of the insect, and secondly
  • they seem to be responsible for insectÕs response to large or sudden changes of light intensity.

     

    3) Stemmata (or lateral occuli) are found on larvae of insects with complete metamorphosis and on insects without compound eyes. They are simple eyes on the sides of the insectÕs head. There are no more than 6 to each side and they donÕt form groups as the compound eyes do.
    Each eye consists of:
  • A corneal lens.
  • A crystalline body
  • And a number of retinoular cells forming a rhabdom, which concludes to the nerve. In various insects, stemmata seem to be involved with colour, form and distance perception.

    However the larvae of certain higher dipterans (true flies) possess specialized photoreceptive organs that differ from the ones we have already mentioned. They consist of photosensitive cells, which are located in small cavities in the anterior end of the larvae. Moreover many insects, in addition to discrete organs associated with light perception, possess a light sensitivity over the general body surface. To sum up, we can conclude that insects use many different ways in order to make their environment visible. This excellent understanding of their surroundings is one of the many reasons which lead them to be the most successful living organisms on earth.