Ocellar system of the swallowtail butterfly larva: II. Projection of retinular axons in the brain

Abstract

The retinular axons from the lateral ocelli have been examined in the brain of the larva of the swallowtail butterfly, Papilio xuthus . The ocellar nerve, which joins the anteroventral part of the brain, is comprised of 42 axons originating from the six ocelli. In the brain these axons are divided into six bundles, each corresponding to one ocellus. The six bundles travel dorsally for 150–200 μ m along the surface of the brain, and then turn inward to enter their respective neuropils. Each neuropil is divided into a distal neuropil and a proximal neuropil, connected by a narrow intermediate region. Of seven retinular axons three occur centrally in the distal neuropil and extend to the proximal neuropil. The central retinular axons synapse with fine processes in both distal and proximal neuropils. Reciprocal synapses between axons are also observed. The remaining four axons occur peripherally in the distal neuropil, where they appear swollen and project many branches centrally. The branches synapse extensively with surrounding processes. These peripheral retinular axons could not be followed into the proximal neuropil. Several second-order processes (greater than 1 μ m in thickness) are observed together with three retinular axons in the intermediate region. Only the thickest process can be followed in the distal neuropil, where it branches extensively. These results are discussed in relation to the regular retinular pattern of the ocellus and in comparison with the optic neuropils of the insect compound eye.