Ommatidia in the compound eyes of nocturnal insects
are referred to as scotopic ommatidia.
The structure of scotopic ommatidia is similar to that of photopic ommatidia
except that scotopic ommatidia appear to have an open space, referred to as the
"clear zone", between the crystalline cone and the retinula cells.
The clear zone actually contains transparent crystalline tracts that arise
from the retinula cells and extend to the crystalline cone.
The crystalllines tracts act as "light guides".
In scotopic ommatidia, incoming light is focused on
the base of the crystalline cone and conducted to the rhabdome by the crystalline tracts.
Unlike photopic ommatidia, scotopic ommatidia are sensitive to the changes
in light intensity that occur during the day and night cycles,
and they detect light differently during the light and dark phases of the daily circadian cycle.
During the daytime, when light is abundant,
scotopic ommatidia detect light similar to photopic ommatidia.
The shielding pigments in the secondary pigment cells of scotopic ommatidia
absorb light rays that enter the corneal lens at an angle,
and each ommatidium detects light independently of neighboring ommatidia,
just as in photopic ommatidia.
However, as night approaches and the intensity of light decreases,
the shielding pigments of scotopic ommatidia contract upward
toward the distal end of the secondary pigment cells.
Distal contraction of the shielding pigments in scotopic ommatidia opens
the basal portion of the pigment cells so that light entering the dioptric apparatus
of one ommatidium can pass through that ommatidium and strike
the rhabdomes in adjacent ommatidia.
In this manner, scotopic ommatidia interact cooperatively
to form multiple superimposed images on the rhabdomes of neighboring ommatidia.
This interaction compounds the sensitivity of the scotopic ommatidia to light.
Because scotopic ommatidia form multiple overlaid images on adjacent
ommatidia, the scotopic ommatidium is
sometimes referred to as a superposition ommatidium.
The superimposed images
are probably poorly defined, but detailed vision is less critical to nocturnal
insects than is the enhanced ability to detect dim light.
Hence, the night-phase superposition compound eye has low visual acuity but
high light sensitivity.
Let us observe how the shielding pigments react in this
scotopic ommatidia of the retina during several light and dark cycles to
illustrate the interactions between adjacent ommatidia.
Next we will see the mosaic eye perceives the visual scene.