Ommatidium

The compound eyes of arthropods like insects, crustaceans and millipedes[1] are composed of units called ommatidia (sg.

The number of ommatidia in the eye depends upon the type of arthropod and range from as low as 5 as in the Antarctic isopod Glyptonotus antarcticus,[3] or a handful in the primitive Zygentoma, to around 30,000 in larger Anisoptera dragonflies and some Sphingidae moths.

The portion of the R cells at the central axis of the ommatidium collectively form a light guide, a transparent tube, called the rhabdom.

[6] A hexagonal lattice of pigment cells insulates the ommatidial core from neighboring ommatidia to optimize coverage of the visual field, which therefore affects the acuity of Drosophila vision.

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.

In many species, in low-light situations, the pigment is withdrawn, so that light entering the eye might be detected by any of several ommatidia.

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.