Ommatidium

Rhabdomeres

In true flies, the rhabdom has separated into seven independent rhabdomeres (there are actually eight, but the two central rhabdomeres responsible for color vision sit one atop the other), such that a small inverted seven-pixel image is formed in each ommatidium. The angle between adjacent rhabdomeres within a single ommatidium (the acceptance angle) is similar to the angle between adjacent ommatidia (the inter-ommatidial angle), giving the eye a continuous field of view with areas of overlap between neighboring ommatidia; the advantage of this arrangement is that the same visual axis is sampled from a larger area of the eye, increasing overall sensitivity by a factor of seven, without increasing the size of the eye or reducing its acuity. Achieving this has also required the rewiring of the eye such that the axon bundles are twisted through 180 degrees (re-inverted), and each rhabdomere is united with those from the six adjacent ommatidia that share the same visual axis. Thus, at the level of the lamina – the first optical processing center of the insect brain – the signals are input in exactly the same manner as in the case of a normal apposition compound eye, but the image is enhanced. This visual arrangement is known as neural superposition.

Since an image from the compound eye is created from the independent picture elements produced by ommatidia, it is important for the ommatidia to react only to that part of the scene directly in front of them. To prevent light entering at an angle from being detected by the ommatidium it entered, or by any of the neighboring ommatidia, six pigment cells are present. The pigment cells line the outside of each ommatidium. Pigment cells are located at an ommatidium's vertices, thus each pigment cell lines the outside of three mutually-adjacent ommatidia. Light entering at an angle passes through the thin cross-section of the photoreceptor cell, with only a tiny chance of exciting it, and is absorbed by the pigment cell, before it can enter a neighboring ommatidium. In many species, in low-light situations, the pigment is withdrawn, so that light entering the eye might be detected by any of several ommatidia. This enhances light detection but lowers resolution.

The size of the ommatidia varies according to species, but ranges from 5 to 50 micrometres. The rhabdoms within them may cross-section at least as small as 1.x micrometres, the category of "small" being assigned in some cross-species studies to those under 2 micrometers. A microlens array can be seen as an elementary, biomimetic analogy of ommatidia.

Mechanism of eye determination

Retinal cell fate determination relies on positional cell–cell signaling that activates signal transduction pathways, rather than cell lineage. Cell–cell signals released from R8 photoreceptors (already differentiated retinal cells) of each ommatidium are received by neighboring progenitor retinal cells, stimulating their incorporation into developing ommatidia. The undifferentiated retinal cells select their appropriate cell fates based on their position with their differentiated neighbors. The local signal, Growth Factor Spitz, activates the epidermal growth factor receptor (EGFR) signal transduction pathway, and initiates a cascade of events that will result in transcription of genes involved in cell fate determination. This process leads to the induction of cell fates, starting from the R8 photoreceptor neurons and progresses to the sequential recruitment of neighboring undifferentiated cells. The first seven neighboring cells receive R8 signaling to differentiate as photoreceptor neurons, followed by the recruitment of the four non-neuronal cone cells.