The structure of the cabbage looper ocellus was examined by light and electron microscopy. Rhabdoms were found in the distal region of the retinula cells. These were formed by the apposition of microvillar borders of adjacent cells, thus, an interrupted network of rhabdoms was found to exist in cross section. No convergence of first order neurons to second order neurons was found anywhere above the protocerebrum as is typical of insect photoreceptors in general. No synapses were found to exist anywhere above the brain. It is hypothesized that the cabbage looper ocellus plays a major role in determining the light intensity at which crepuscular flight is initiated. This hypothesis was tested in the following manner. Laboratory reared moths were divided into five groups each with a different or no treatment. These were: 1. Compound eyes and ocelli occluded. 2. Compound eyes occluded. 3. Ocelli occluded. 4. Controls. 5. Controls sham treated. All groups were placed in transparent flight chambers and submitted to a simulated sunset. Test results were compared using the Duncan Multi-Range Test and Standard errors. The results indicate that the cabbage looper ocellus plays a role in determining the light intensity at which crepuscular flight is initiated. / Master of Science