Termination profiles of photoreceptor cells in the larval eye of the swallowtail butterfly.

Profiles of the axonal endings of seven retinular cells (R1-7) from each of six stemmata (I-VI) of the butterfly Papilio xuthus were examined. Their axons enter the brain anteroventrally and project dorsally along the brain's lateral surface before entering the optic neuropil. Larval optic neuropil has two distinct areas, a (distal) lamina and a (proximal) medulla, connected by a chiasma. Intracellular administration of cobalt shows that distal retinular cells (R1-3 of stemmata I-IV, R1-3 and R6 of stemmata V and VI) terminate as short axons with plug or toothbrush-like endings in the lamina. Proximal retinular cells (R4-7 of stemmata I-IV, R4, R5 and R7 of stemmata V and VI) terminate as long axons in the chiasma or in the medulla. The long axons have fine branches in the lamina, chiasma and medulla. The two proximal cells (R5 and R7) are distinguished by their deeper endings and longer branches. The terminals of photoreceptors correspond to their photoreceptive properties and provide further evidence for the functional specialization of the photoreceptors.