Structure and Function of the Peripheral Visual Pathway in Hymenopterans

Mapping and ultrastructure of serotonin-immunoreactive neurons in the optic lobes of three insect species

With antibodies to serotonin (5-HT) we have mapped immunoreactive neurons in the optic lobes of three species, the blowfly Calliphora, the desert ant Cataglyphis, and the worker bee Apis. The main emphasis in this investigation is on a system of 5-HT-positive neurons connecting the most peripheral neuropil of the optic lobes, the lamina, to more central neuropil regions. To aid in electron microscopical identification of these neurons we used immunocytochemistry at the EM-level and Golgi-EM for Calliphora and horseradish peroxidase (HRP) labelling for the other two insects. The immunoreactive terminals in Calliphora and the HRP-labelled ones in the other insects contain large (c. 100 nm) granular vesicles and smaller (c.60 nm) clear vesicles. In Cataglyphis and Apis the profiles with granular vesicles are presynaptic to second order neurons of the lamina, whereas in Calliphora no synaptic contacts were found. In this animal the 5-HT-positive terminals are situated distal to the synaptic layer of the lamina, in a region of retinal photoreceptor axons and perikarya of the lamina monopolar neurons. In Catagylphis and Apis the interactions of the 5-HT-neurons with the laminar neurons might occur through chemical synapses, whereas in Calliphora neuroactive substance could be released non-synaptically from varicosities distal to the synaptic layer. The possible involvement of 5-HT in control of neuronal activity in the optic lobes is discussed.

Retrograde labelling of photoreceptors in different regions of the compound eyes of bees and ants

The geometry of retinal receptor arrays and the projection patterns of photoreceptor axons are unravelled in the compound eyes of bees and ants by backfilling large populations of photoreceptors with horseradish peroxidase or Lucifer yellow. Such retrograde labelling techniques are applied in the insect retina for the first time. They brilliantly label populations of specific receptor types within more than 100 ommatidia. Such preparations are obtained in three distinctly different parts of the eye: (1) the part of the retina that is positioned at the uppermost dorsal margin of the eye and specialized for the detection of polarized skylight; (2) the remainder of the dorsal retina; and (3) the ventral retina. As shown by the large populations of labelled photoreceptors, the retinal receptor arrays differ strikingly between different parts of the eye. Furthermore, there exist similarities as well as marked differences between bees and ants. Former hypotheses concerning the location of photoreceptor terminals in the first and second visual neuropil have all been based on a few individual Golgi-labelled photoreceptors. The results presented in this paper, based on retrograde mass impregnations of selected types of photoreceptors, confirm some of the former hypotheses and reject others. The new findings are important for understanding how insects analyse polarized skylight.

Neuroanatomy of the visual afferents in the horseshoe crab (Limulus polyphemus)

The central nervous system of Limulus consists of a circumesophageal ring of fused ganglia and a paired ventral nerve cord. The anterior portion, the protocerebrum, receives sensory inputs including visual information. Three optic nerves, one each from the lateral eye, median ocellus, and ventral eye enter each side of the protocerebrum. The central connections of each optic nerve were determined by staining cut nerve trunks with cobalt chloride. The lateral optic nerve innervates the lamina, medulla, optic tract, ventral central body, and ocellar ganglion. The branching patterns of single axons, probably those of eccentric cells in the lateral eye retina, were observed. Single, large-diameter axons in the lateral optic nerve ramify at seven loci including sites in each of the structures innervated by the lateral optic nerve as a whole. The median optic nerve innervates the ocellar ganglion, central body, optic tract, and medulla. Three types of branching patterns were observed for single, large-diameter fibers in the median optic nerve. One type bypasses the ocellar ganglion and innervates the central body. A second type passes through the ocellar ganglion and optic tract without branching and innervates the posterior medulla. A third type innervates the ocellar ganglion, ventral central body, optic tract, and medulla. The ventral optic nerve is composed of large-diameter axons of ventral photoreceptors. Each axon enters the ganglion cell layer of the medulla and branches over a planar area less than 150 micrograms in diameter. We also observed that axons from mechanoreceptors on the anterior carapace innervate the posterior neuropil of the medulla, and that about 5% of the neurons in the medullar ganglion cell layer send axons to the ipsilateral circumesophageal connective.