ALBERT

Description / Table of Contents: Summary 1. The brain of the ant Formica rufa has been studied (mushroom bodies; unstructured protocerebral neuropil, central body and deutocerebrum) with normal stains, silver impregnations, electron microscopy and lesions with subsequent degeneration of fibers. 2. The mushroom bodies mainly consist of four groups of nerve cells, differing in the location of the cell bodies and the structure of the dendritic arborizations. The axons of all these cells pass down the pedunculus; bifurcate and send branches to the \ga- and \gb-lobe. Little endings all over the axons are claimed to be synaptic regions. Lesions with following degeneration of the fibers show that axons of cell groups I and IV pass down to the \ga- and \gb-lobe in the middle of the neuropil. Groups II and III follow each other in the outer parts. \ldCentral endings\rd described by Trujillo-Cenoz and Melamed (1962) seem to belong to neurons from other lobes of the brain to the calyces, while the \ldthin fibers\rd are parts of mushroom body neurons. Terminations of neurons arising from other cells, connecting the mushroom bodies with sensory and motor lobes of the brain, are present all over the mushroom body neuropil. Endings in the calyx all show same structure. It could be pointed out that neurons of tracts originating from the antennal glomeruli and the lateral neuropil send arborizations in both calyces. Terminations of tracts ending in pedunculus, α- and β-lobe all are developped in the same manner, but differ from those in the calyx. Each ending is connected with many mushroom body neurons, covering a great part of the area of those lobes in cross sections. 3. Mushroom bodies, protocerebral bridge and central body are surrounded by a large mass of unstructured neuropil of the protocerebral lobes. In the anterior part of the protocerebrum the unstructured neuropil of both hemispheres is closely connected, but in the other parts a clear boundary between the two hemispheres exists and no connections \3- except few commissures \3- can be seen. Several different neuron systems have been found in the lateral lobes One system connecting the lobes with the calyces of the mushroom bodies and other tracts with arborizations in the pedunculus, one part entering the commissure dorsal of the central body, the other part terminating in the β-lobe of the other hemisphere. Further tracts are passing between ventral parts of the lateral neuropil and the central body. 4. The central body consists of three parts of neuropil, separated by cell-bodies. \ldHorizontal-systems\rd, fibers from the antennal glomeruli, lateral neuropil and \gb-lobes enter the sides of the central body neuropil, while \ldvertical systems\rd enter from ventral between the two \gb-lobes and dorsal, leaving the commissure between the two hemispheres. Two systems have been found connecting central body and mushroom bodies. Impregnations only show parts, so it is assumed that arborizations of these fibers also penetrate the unstructured protocerebral neuropil. 5. The antenno-motor center of the deutocerebrumis closely connected with protocerebral and subesophageal structures and shows equal distribution of neuropil unlike the sensory center, where synaptic glomerulus regions differ from the other parts of neuropil. The antennal nerve consists of about 60000 fibers, the major part entering the sensory center. Some fibers pass by and terminate in the antenno-motor neuropil. Three groups of fibers could be detected in the antennal nerve, but it only seems to be shure for one group (40000 fibers with 0,05-0,2 μ diameter per fiber) to function as sensory fibers. Each fiber only enters one glomerulus, so that each glomerulus is claimed to receive about 200 incoming antennal fibers. Outgoing tracts to other lobes of the brain are in connection with all parts of the sensory center and each fiber has its terminations in only one deutocerebral glomerulus. Segmental internuncials with cell-bodies lateral and medial of the neuropil unite several deutocerebral glomeruli. 6. Structure and function. It is tried to give some histological explanations for physiological results obtained by stimulation and lesions. Possible tracts for inhibition between the mushroom bodies and for connections between the mushroom bodies and the central body are enumerated. Structures and physiological results in the neuropil of the lateral protocerebral neuropil and their relations to the central body, pedunculi and calyces of the mushroom bodies are in accordance. The unstructured parts of the protocerebral neuropil seem to play an important coordinating role concerning integrative processes in the brain of insects.