The central complex is a group of modular neuropils across the midline of the insect brain. Hallmarks of its anatomical organization are discrete layers, an organization into arrays of 16 slices along the right-left axis, and precise inter-hemispheric connections via chiasmata. The central complex is connected most prominently with the adjacent lateral complex and the superior protocerebrum. Its developmental appearance corresponds with the appearance of compound eyes and walking legs. Distinct dopaminergic neurons control various forms of arousal. Electrophysiological studies provide evidence for roles in polarized light vision, sky compass orientation, and integration of spatial information for locomotor control. Behavioral studies on mutant and transgenic flies indicate roles in spatial representation of visual cues, spatial visual memory, directional control of walking and flight, and place learning. The data suggest that spatial azimuthal directions (i.e., where) are represented in the slices, and cue information (i.e., what) are represented in different layers of the central complex.

Many animals rely on a sun compass for spatial orientation and long-range navigation. In addition to the Sun, insects also exploit the polarization pattern and chromatic gradient of the sky for estimating navigational directions. Analysis of polarization-vision pathways in locusts and crickets has shed first light on brain areas involved in sky compass orientation. Detection of sky polarization relies on specialized photoreceptor cells in a small dorsal rim area of the compound eye. Brain areas involved in polarization processing include parts of the lamina, medulla and lobula of the optic lobe and, in the central brain, the anterior optic tubercle, the lateral accessory lobe and the central complex. In the optic lobe, polarization sensitivity and contrast are enhanced through convergence and opponency. In the anterior optic tubercle, polarized-light signals are integrated with information on the chromatic contrast of the sky. Tubercle neurons combine responses to the UV/green contrast and e-vector orientation of the sky and compensate for diurnal changes of the celestial polarization pattern associated with changes in solar elevation. In the central complex, a topographic representation of e-vector tunings underlies the columnar organization and suggests that this brain area serves as an internal compass coding for spatial directions.

Many animals use the sun as a reference for spatial orientation [1-3]. In addition to sun position, the sky provides two other sources of directional information, a color gradient [4] and a polarization pattern [5]. Work on insects has predominantly focused on celestial polarization as an orientation cue [6, 7]. Relying on sky polarization alone, however, poses the following two problems: E vector orientations in the sky are not suited to distinguish between the solar and antisolar hemisphere of the sky, and the polarization pattern changes with changing solar elevation during the day [8, 9]. Here, we present neurons that overcome both problems in a locust's brain. The spiking activity of these neurons depends (1) on the E vector orientation of dorsally presented polarized light, (2) on the azimuthal, i.e., horizontal, direction, and (3) on the wavelength of an unpolarized light source. Their tuning to these stimuli matches the distribution of a UV/green chromatic contrast as well as the polarization of natural skylight and compensates for changes in solar elevation during the day. The neurons are, therefore, suited to code for solar azimuth by concurrent combination of signals from the spectral gradient, intensity gradient, and polarization pattern of the sky.

Pfeiffer, Keram, Michiyo Kinoshita, and Uwe Homberg. Polarization-sensitive and light-sensitive neurons in two parallel pathways passing through the anterior optic tubercle in the locust brain.

The anterior optic tubercle is a small neuropil in the insect brain and a major target of visual interneurons from the optic lobe. The functional role of the tubercle is poorly understood, but recent evidence from locusts points to a possible involvement in polarization vision. The present study examines the organization of the anterior optic tubercle in the locust Schistocerca gregaria and its connections with other brain areas. The tubercle of the locust consists of an upper and a lower subunit. Both units are connected in parallel with the medulla and lobula of the optic lobe, with the contralateral tubercle, and with the lateral accessory lobe in the median protocerebrum. Wide-field transmedullary neurons provide input from the medulla. Neurons with processes in the dorsal rim of the medulla, a relay station in the polarization vision pathway, project exclusively to the lower unit of the tubercle. Visual input from the lobula to the upper and lower unit originates from topographically distinct strata. The most prominent output target of the tubercle is the lateral accessory lobe in the median protocerebrum. Neurons from the upper unit project widely in the lateral accessory lobe, whereas neurons from the lower unit have focused projections confined to the median olive and to the lateral triangle. The two subunits of the anterior optic tubercle are, therefore, processing stages in two parallel visual pathways from the optic lobe to the median protocerebrum. Pathways via the lower unit of the tubercle appear to be involved in polarization vision.

Animals relying on a celestial compass for spatial orientation may use the position of the sun, the chromatic or intensity gradient of the sky, the polarization pattern of the sky, or a combination of these cues as compass signals. Behavioral experiments in bees and ants, indeed, showed that direct sunlight and sky polarization play a role in sky compass orientation, but the relative importance of these cues are species-specific. Intracellular recordings from polarization-sensitive interneurons in the desert locust and monarch butterfly suggest that inputs from different eye regions, including polarized-light input through the dorsal rim area of the eye and chromatic/intensity gradient input from the main eye, are combined at the level of the medulla to create a robust compass signal. Conflicting input from the polarization and chromatic/intensity channel, resulting from eccentric receptive fields, is eliminated at the level of the anterior optic tubercle and central complex through internal compensation for changing solar elevations, which requires input from a circadian clock. Across several species, the central complex likely serves as an internal sky compass, combining E-vector information with other celestial cues. Descending neurons, likewise, respond both to zenithal polarization and to unpolarized cues in an azimuth-dependent way.

Many insects rely on celestial compass cues such as the polarization pattern of the sky for spatial orientation. In the desert locust, the central complex (CX) houses multiple sets of neurons, sensitive to the oscillation plane of polarized light and thus probably acts as an internal polarization compass. We investigated whether other sky compass cues like direct sunlight or the chromatic gradient of the sky might contribute to this compass. We recorded from polarization-sensitive CX neurons while an unpolarized green or ultraviolet light spot was moved around the head of the animal. All types of neuron that were sensitive to the plane of polarization (-vector) above the animal also responded to the unpolarized light spots in an azimuth-dependent way. The tuning to the unpolarized light spots was independent of wavelength, suggesting that the neurons encode solar azimuth based on direct sunlight and not on the sky chromatic gradient. Two cell types represented the natural 90 deg relationship between solar azimuth and zenithal -vector orientation, providing evidence to suggest that solar azimuth information supports the internal polarization compass. Most neurons showed advances in their tuning to the-vector and the unpolarized light spots dependent on rotation direction, consistent with anticipatory signaling. The amplitude of responses and its variability were dependent on the level of background firing, possibly indicating different internal states. The integration of polarization and solar azimuth information strongly suggests that besides the polarization pattern of the sky, direct sunlight might be an important cue for sky compass navigation in the locust.

Drifting of honeybees depends on the arrangement, the colouring of the hives and on various environmental factors. Bees are able to distinguish between related and non-related individuals, so one aim of this study was to determine whether drifting also depends on relatedness. In addition we wanted to examine whether there are differences in survival of drifted bees and nondrifted bees and to model the component of alien worker bees in a colony's population.We used two non-related strains of bees. The colonies stood in rows. Each of the colonies had two non-related neighbour colonies, except for the colonies on the ends of the rows. From each colony, 200 newly emerged bees were marked individually and 100 of them were reintroduced into their original colony (native bees) and 100 into a non-related neighbouring colony (foreign bees). The marked bees were examined for presence on their 2 nd , 6 th , 9 th , 16 th , 25 th and 34 th day of life. Experiments were done in summer and fall.There was no difference in survival and the amount of drifting of native and foreign bees. We also found no differences in the number of drifting bees for the two non-related strains. The bees never preferred related colonies when drifting. Bees of strain 1 did not show any preferences. In two experiments significantly more bees of strain 2 drifted into colonies of strain 1. Most bees drifted into the neighbouring colony next to the colony they left. In summer significantly more bees that had drifted until their 9 th day of life, survived until the 25 th day than bees that did not drift until that day. In fall we did not find this difference. This was true for bees of both strains.Our data support the results of Jay (1965), who found that more bees drift from a centre colony to the end colonies of a row than vice versa.With a model we calculated the component of alien worker bees living in a colony's population. On average, 22 ± 3% alien worker bees were calculated for edge and 42 ± 6 % for inner hives in a row in our first experiment (summer). In a second experiment (fall) the components were 13 ± 1 % for the edge colonies and 39 ± 4% for the inner colonies in the row. Drifting seems not to be influenced by relatedness and did not shorten the lifetime of the bees. This indicates a great tolerance against drifted bees and a high intermix of an apiary's population when no precautions to reduce drifting are taken.Insectes soc. 45 (1998) 151 -167