There are over 25,000 ommatidia in the eye of a hawk moth.
Watase has shown, in a very convincing way, how by deepening the pit-like set of cells beneath a simple lens the more complex ommatidia of the compound eyes of Crustacea and Hexapoda may be derived from such a condition as that presented in the lateral eyes of Limulus and Scorpio.
In the specialized ommatidia of the compound eyes of Crustacea and Hexapods the rhabdom is an important structure.'
- Section through a portion of the lateral eye of Limulus, showing three ommatidia - A, B and C. hyp, The epidermic cell-layer (so-called hypodermis), the cells of which increase in volume below each lens, 1, and become nerve-end cells or retinula-cells, rt; in A, the letters rh point to a rhabdomere secreted by the cell rt; c, the peculiar central spherical cell; n, nerve fibres; mes, mesoblastic skeletal tissue; ch, chitinous cuticle.
Like the single elements (ommatidia) of a compound insect eye, in others like simple ocelli.
They consist of a varying number of ommatidia or visual elements, covered by a transparent region of the external cuticle forming the cornea.