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.
There are over 25,000 ommatidia in the eye of a hawk moth.
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.
The compound eye of the king-crab (Limulus) is the only recognized instance of ommatidia in their simplest state.
In Crustacea and Hexapoda of all grades we find compound eyes with the more complicated ommatidia described above.
But they seem to point to a community of origin of Hexapods and Crustacea in regard to the complicated ommatidia of the compound eye, and to a certain isolation of the Arachnida, which are, however, traceable, so far as the eyes are concerned, to a distant common origin with Crustacea and Hexapoda through the very simple compound eyes (monostichous, polymeniscous) of Limulus.