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 l ight microscopy allows clear visualization of an ultraviolet-sensitive mini-ommatidium enshrouded by pigment cells, glial cells, and guanoph ores. Serial 1-mu m sections of median optic nerves containing mini-om matidia revealed the presence of a single, heavily pigmented photorece ptor (retinular) cell and a single, unpigmented arhabdomeric cell. Com puter-assisted serial reconstructions from 1-mu m sections confirmed t he presence of two cells, each bearing a nucleus, and two axons leavin g the mini-ommatidium. The retinular cell is morphologically similar t o retinular cells from the median and lateral eyes. Its rhabdomere app ears to be a continuous sheet of microvilli with much infolding. The s tructure of the arhabdomeric cell is nearly identical to those found i n the median ocellus. As in other photoreceptors in Limulus, the retin ular cell of the mini-ommatidium is innervated by efferent fibers from the brain. Each mini-ommatidium generates a single train of nerve imp ulses in response to light, presumably from the arhabdomeric cell. Mea surement of the spectral sensitivity of the mini-ommatidium based upon a constant-response criterion indicated that the retinular cell is ma ximally 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 a lmost certainly as the result of neural processing in the ocellus whic h is absent in the mini-ommatidium.