The unit structure of the locust compound eye.

1. The ommatidium or functional unit of the locust compound eye comprises a compound corneal lens, 4 cone cells, 2 primary pigment cells, 16 secondary pigment cells and 8 retinula cells. 2. All retinula cells run the entire length from the cone to the basal lamina, although two, called the proximal cells, only contribute to the lowest third of the rhabdom, and one of either cell 6 or cell 7 on our arbitrary numbering system forms its axon one third the way up the ommatidium. 3. 84% of the 417 ommatidia examined had five cone cell processes. The position of three cone cell processes (cone threads) is almost invariable with respect to numbered retinula cells but the remaining threads can take any of three intercellular locations. 4. The position of these threads correlates with the number of the cell distally displaced from the rhabdom. We suggest that cone thread position in the developing ommatidium determines some features of retinula cells and we propose a simple model to account for this.