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Insect eyes are made of thousands of hexagonal columns called ommatidia, each of which focuses light through a lens down a transparent tube called a rhabdom to a set of photosensitive cells at the bottom.

B, Rhabdom of the same, consisting of five confluent rhabdomeres.

C, Transverse section of the rhabdom of a retinula of the scorpion’s central eye, showing its five constituent rhabdomeres as rays of a star.

Whilst each unit of the lateral eye of Limulus has a rhabdom of ten3 pieces 292 forming a star-like chitinous centre in section, each lateral eye of Scorpio has several rhabdoms of five or less rhabdomeres, indicating that the Limulus lateral eye-unit is more specialized than the detached lateral eyelet of Scorpio, so as to present a coincidence of one lens with one rhabdom.

From the distal ends of the cells are secreted chitinous rhabdomeres, forming a rhabdom which occupies and fills up the central portion of the cup beneath the middle of the corneal lens.