1. Ommatidium structure and function

1. Ommatidium structure and function


What? Do insects really see — since the compound eyes of insects are comprised of a mosaic of small facets? The popular media likes to represent insect mosaic vision like this. However, insects actually see this. Let us examine the structure of the insect compound eye and learn how the compound eyes visualize the landscape for insects. The main visual structure of insects is the compound eye. The compound eye appears to be comprised of many hundreds or thousands of tiny facets. Each facet is actually the cornea lens for an individual photoreceptor unit called an
ommatidium. Let us look at the detailed structure of
an ommatidium. an ommatidium is divided into light gathering and light detecting
components. The corneal lens is transparent cuticle
that is secreted by two modified epidermal cells called corneagen cells. Corneagen cells secrete the corneal lens when new cuticle is formed at the
time of the molt. The corneagen cells secrete the corneal lens, then later differentiate into the primary pigment cells. Below the corneal lens is the crystalline cone. This structure is either secreted or formed by four cells called Semper’s cells. Light enters the corneal lens and is focused on the light detecting apparatus by the crystalline cone. The light detecting apparatus of each ommatidium contains seven to eight light-sensitive photoreceptor cells
called retinula cells Retinula cells are actually photoreceptor neurons that detect wavelengths of light. The light-sensitive dendritic region of the retinula cell is the rhabdomere. Together all of the individual rhabdomeres are referred to as the rhabdom. Rhabdomeres consist of parallel microvilli containing light detecting visual pigment molecules embedded into
their plasma membrane. Light entering the cornea and cone of an
ommatidium is focused on and detected by visual pigments embedded in the rhabdomere membranes of the retinula cells. Retinula cells are surrounded by 12 to 18 secondary pigment cells so that each ommatidium can be functionally isolated
from its neighbors. Ommatidia sit on a basement membrane in the retina and axons from the retinula cells combine below the membrane to form the optic nerve that leads to the protocerebrum of the insect brain. Let us see how insect compound eyes function to detect light and perceive visual information

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