ALBERT

Notes: Summary 1) Comparable to the bee, but in contrast to the majority of ants, the desert antCataglyphis bicolor has been shown to exhibit a highly developed repertoire of visually guided behavioural responses. This paper deals with the anatomy and fine structure of the peripheral visual pathway of this ant. In the first visual neuropile, the lamina, first and second order neurons are classified by applying Golgi methods adapted for electron microscopy. Synaptic connections within the lamina are described and discussed. The results are summarised in developing a three-dimensional model of the ant's lamina (Fig. 17). 2) Eachretinula within the central eye region is composed of four large (nos. 2, 4, 6 and 8) and four small retinular cells (nos. 1, 3, 5 and 7) and a basal ninth cell. Visual cells nos. 2, 3, 4, 6, 7, and 8 form short unbranched axons (Rs), which terminate within the lamina. The visual cells nos. 1 and 5 (Rl), as well as the basal cell no. 9, show arborisations in the lamina, but terminate in the second visual neuropile, the medulla. Within the lamina all nine retinular cell axons, originating from one retinula, form a cartridge within which they interact with the second order neurons, the monopolar cells. Collaterals of second order neurons and side branches of retinular cell axons form local neuronal circuits. 3) Five types ofmonopolar cells have been classified by means of their dendritic fields within the lamina and the medulla (L1a, b, c, L2 and L4). They relay the retinular cells with higher order neurons within the medulla. In the distal layer of the lamina (stratum A) the spreads of the monopolar cells are restricted to a single cartridge, whereas in the proximal stratum C their collateral processes extend laterally through more than one cartridge. The collaterals of the L4-type of monopolar cells are exclusively confined to stratum C. There they are arranged bilaterally along the dorsoventral axis of the eye. Within stratum A, where all neurons are organised in well defined columns (cartridges), the axons of the short visual cells seem to be distributed over any cross section of a cartridge at random. In this layer, tangential fibres are the only candidates for inter-cartridge cross talk. In stratum C, the columnar organisation of the neuropile becomes less obvious because of the wide spread ramifications of the second order neurons. For instance, the collaterals of the L1a-type of monopolar cell extend over up to 18 neighbouring cartridges. 4) Three types ofcentrifugal fibres running from the medulla to the lamina are observed (T-fibres). Some of them form wide field arborisations either in stratum A (type T2) or in stratum C (type T3). In linear scale, their collaterals may extend over more than 40% of the large (dorsoventral) axis of the lamina. 5) Receptor terminals, especially Rs-fibres, are densely packed with elongated synaptic vesicles, whereas in second order neurons round vesicles are arranged around the presynaptic elements. Especially in Rs-fibres analyses of serial sections reveal T-shaped synaptic ribbons, which are the presynaptic sites as regards four postsynaptic elements. In case of rod-like presynaptic elements diadic and triadic arrangements of postsynaptic fibres can also be observed. Four main types of synaptic configurations are discriminated: (1) Receptor terminals synapse on second order neurons. (2) Second order neurons synapse on receptor cell axons as well as on other second order profiles. These synapses are sometimes observed in feedback configurations. (3) Synapses occuring between receptor axon terminals. (4) A small, probably efferent neurosecretory nerve fibre synapses on second order neurons. Neurosecretory fibres of larger diameters (to 1.5 Μm are frequently found in stratum C.

Notes: Summary Structurally specialized ommatidia at the dorsal rim of the compound eyes of honey bees have been shown to be indispensable for polarized skylight navigation. In this study numerous other hymenopteran genera belonging to various superfamilies are shown to exhibit similar specializations in this part of the eye: (1) The cornea is penetrated by pore canals, which affect the optics of the ommatidia by scattering the light falling into the eye. In Andrena and Ammophila the cornea contains extensive cavities. (2) Each retinula contains 9 long receptor cells as opposed to 8 long ones in the adjacent dorsal area, and the rhabdom area is increased by a factor of up to 2. In all ant species examined there are no corneal but only retinal specializations at the dorsal rim of the eye. They include a specially shaped rhabdom as in Cataglyphis, in which polarization vision has also been demonstrated.

Notes: Summary The superposition eye of the cockchafer, Melolontha melolontha, exhibits the typical features of many nocturnal and crepuscular scarabaeid beetles: the dioptric apparatus of each ommatidium consists of a thick corneal lens with a strong inner convexity attached to a crystalline cone, that is surrounded by two primary and 9–11 secondary pigment cells. The clear zone contains the unpigmented extensions of the secondary pigment cells, which surround the cell bodies of seven retinula (receptor) cells per ommatidium and a retinular tract formed by them. The seven-lobed fused rhabdoms are composed by the rhabdomeres of the receptor cells 1–7. The rhabdoms are optically separated from each other by a tracheal sheath around the retinulae. The orientation of the microvilli diverges in a fan-like fashion within each rhabdomere. The proximally situated retinula cell 8 does not form a rhabdomere. This standard form of ommatidium stands in contrast to another type of ommatidium found in the dorsal rim area of the eye. The dorsal rim ommatidia are characterized by the following anatomical specializations: (1) The corneal lenses are not clear but contain light-scattering, bubble-like inclusions. (2) The rhabdom length is increased approximately by a factor of two. (3) The rhabdoms have unlobed shapes. (4) Within each rhabdomere the microvilli are parallel to each other. The microvilli of receptor 1 are oriented 90° to those of receptors 2–7. (5) The tracheal sheaths around the retinulae are missing. These findings indicate that the photoreceptors of the dorsal rim area are strongly polarization sensitive and have large visual fields. In the dorsal rim ommatidia of other insects, functionally similar anatomical specializations have been found. In these species, the dorsal rim area of the eye was demonstrated to be the eye region that is responsible for the detection of polarized light. We suggest that the dorsal rim area of the cockchafer eye subserves the same function and that the beetles use the polarization pattern of the sky for orientation during their migrations.

Notes: Abstract. The distribution of histamine-like immunoreactivity has been analyzed in the visual system and brain of the cricket Gryllus campestris and of the bee Apis mellifera by using an antiserum against histamine. Specific immunolabeling of the photoreceptors has been found in the compound eyes and ocelli of both examined species. Intense immunostaining can be also detected in the midbrain of these species. The axons of immunoreactive cells innervate almost every area in the protocerebrum. Most of the reactive neurons are typically wide-field neurons with bilateral ramifications that form dense arborizations. Numerous small buttons on the arborizations probably represent pre- and postsynaptic sites. The histamine-like immunoreactive neurons are apparently connected to many postsynaptic neurons. In both bees and crickets, some regions of the nervous system such as the first two optic neuropils and the central body show the same labeling pattern, whereas the mushroom bodies exhibit no immunoreactivity. Nevertheless, several differences in the staining pattern can be seen: the glomeruli of the antennal lobe are invaded by histamine-like immunoreactive fibers in the bee but not in the cricket. Furthermore, an interneuron connects the second and third optic neuropil in the cricket, whereas no histamine-like immunoreactive interneuron is found in the second optic neuropil in the bee. In accord with the work of other authors on the distribution histamine in the insect nervous system, we suggest that histamine is not only a transmitter within the visual system, but also a transmitter or co-transmitter in the insect midbrain.

Notes: Abstract We have examined the fine structure of dorsal rim ommatidia in the compound eye of the three odonate species Sympetrum striolatum, Aeshna cyanea and Ischnura elegans. These ommatidia exhibit several specializations: (1) the rhabdoms are very short, (2) there is no rhabdomeric twist, and (3) the rhabdoms contain only two, orthogonally-arranged microvillar orientations. The dorsal rim ommatidia of several other insect species are known to be anatomically specialized in a similar way and to be responsible for polarization vision. We suggest that the dorsal rim area of the odonate compound eye plays a similar role in polarization vision. Since the Odonata are a primitive group of insects, the use of polarized skylight for navigation may have developed early in insect phylogeny.

Notes: Summary The fine structure of the cornea in an anatomically and functionally specialized part of the honey bee's compound eye (dorsal rim area) was examined by light microscopy, transmission electron and scanning electron microscopy. Under incident illumination the cornea appears grey and cloudy, leaving only the centers of the corneal lenses clear. This is due to numerous pore canals that penetrate the cornea from the inside, ending a few μm below the outer surface. They consist of (1) a small cylindrical cellular evagination of a pigment cell (proximal), and (2) a rugged-walled, pinetree-shaped extracellular part (distal). The functional significance of these pore canals is discussed. It is concluded that their light scattering properties cause the wide visual fields of the photoreceptor cells measured electrophysiologically in the dorsal rim area, and that this is related to the way this eye region detects polarization in skylight.

Notes: Summary The tracheal systems of five insect species (two species of ants, worker bee, housefly and the cabbage butterfly) have been studied by scanning electron microscopy of corrosion casts. This technique, which is commonly used for the investigation of vertebrate vasculature, is adapted to demonstrate the ultrastructure of the insect respiratory organ. The problem of filling a “blind ending system” was solved by injecting the resin Mercox into the evacuated tracheae through a thoracal spiracle. After polymerization of the resin, the tissue was digested enzymatically and chemically. The three-dimensional structure of the tracheal system was investigated by scanning electron microscopy. The technique used here displays for the first time the complex morphology of the entire tracheal system in fine detail, especially the structure of spiracles, airsacs, tracheae and tracheoles. Smooth-walled terminal tracheoles show up in flight muscles. The finest tracheoles that could be identified have diameters of approximately 70 nm. This approaches the finest tracheoles portrayed by transmission electron micrographs.