Insect eyes are compound eyes: each eye is made of many small unit eyes (ommatidia), as opposed to a single large-unit eye as in vertebrates. 

Cajal’s drawing above details the many ommatidia present in a single fly eye, as well as the neural circuitry relaying visual signals in toward the brain. In the lamina complex, labeled C, we can see the axons of the photoreceptor cells crossing over each other, suggesting neural superposition.

Neural superposition, discovered by Valentino Braitenberg and by Kuno Kirschfeld in 1967, is a phenomenon where photoreceptors from different ommatidia that receive the same signals converge upon the same synapses in the brain to achieve high sensitivity and spatial resolution. Cajal was unaware of the phenomenon, but his drawings suggest that he recognized the organization of structures that make it possible. 

While visibly different from the vertebrate visual system, the insect visual system shares certain similarities in neural circuits: both systems have intermediary neurons that receive information from photoreceptor cells and direct it onwards towards the neuropil or “neural switchboard.”