The morphology and physiology of a "mini-ommatidium" in the median optic nerve of Limulus polyphemus.

Examination of the Limulus median optic nerve with low-magnification light microscopy allows clear visualization of an ultraviolet-sensitive mini-ommatidium enshrouded by pigment cells, glial cells, and guanophores. Serial 1-micron sections of median optic nerves containing mini-ommatidia revealed the presence of a single, heavily pigmented photoreceptor (retinular) cell and a single, unpigmented arhabdomeric cell. Computer-assisted serial reconstructions from 1-micron sections confirmed the presence of two cells, each bearing a nucleus, and two axons leaving the mini-ommatidium. The retinular cell is morphologically similar to retinular cells from the median and lateral eyes. Its rhabdomere appears to be a continuous sheet of microvilli with much infolding. The structure of the arhabdomeric cell is nearly identical to those found in the median ocellus. As in other photoreceptors in Limulus, the retinular cell of the mini-ommatidium is innervated by efferent fibers from the brain. Each mini-ommatidium generates a single train of nerve impulses in response to light, presumably from the arhabdomeric cell. Measurement of the spectral sensitivity of the mini-ommatidium based upon a constant-response criterion indicated that the retinular cell is maximally sensitive to near ultraviolet light with lambda max = 380 nm. Comparison of intensity-response functions revealed that those of the mini-ommatidium are significantly steeper than those of the ocellus almost certainly as the result of neural processing in the ocellus which is absent in the mini-ommatidium.