Spectral and ocellar inputs to honeybee motion-sensitive descending neurons
Optomotor reflexes have been observed in many insects and in some cases the neural pathways that mediate these reflexes have been identified physiologically and anatomically. In honeybees Kaiser (1975) established that the spectral sensitivity of optomotor responses in bees almost exactly matched that of the green photoreceptors, suggesting an exclusive input from green photoreceptors. However, physiological studies showed that the motion detectors in the optic lobes have a secondary response peak in the UV region of the spectrum suggesting that there may be more than one type of photoreceptor involved in the optomotor response. Thus in this thesis, I investigate the neural basis of motion and spectral wavelength processing in motion-sensitive descending neurons, which are on the optomotor response pathway, to reveal the neural contributions from other spectral receptor types. In this study, intracellular recording techniques were utilised. The stimuli consisted of a wide-field LED (light emitting diode) display in which green (peak 530 nm) and short-wavelength (peak 380 nm) LEDs were mounted in pairs across a wide visual area. Six types of motion-sensitive descending neurons were recorded and anatomically identified, including two pitch-sensitive neurons (Locth3, DNII2), two roll-sensitive neurons (DNIV2 and DNIV3) and two yaw-sensitive neurons (DNVII1 and DNVII2). The results show that for the vertical sensitive (pitch and roll) neurons, the cells have equal-sized excitatory responses to motion when using short-wavelength or green motion stimulation. However, for the horizontal sensitive (yaw-sensitive) neurons excitatory responses only occurred for the green stimulus in the preferred direction. The short-wavelength stimulus induced clear inhibitory responses for all tested motion directions. The results suggest that besides green photoreceptors, the motion-sensitive descending neurons also receive inputs from the short-wavelength photoreceptors, but only for motion detectors tuned for vertical motion. Honeybees, like most flying insects, have three ocelli (simple eyes) located on the top of the head, in addition to the compound eyes. However, the exact function of the bee ocelli and the information computation between the ocelli, compound eyes and central brain remain unclear. In this thesis, I investigate the ocellar properties morphologically, anatomically and physiologically. Semi-thin sections and focal length measurements were performed on both median and lateral ocelli, a 3-dimensional reconstruction model of the honeybee ocellar lenses and retinas was developed to understand the visual fields of the ocelli. Intracellular electrophysiology experiments were carried out on descending neurons to understand the information processing between the ocelli and compound eyes. Cell responses to different stimuli were recorded with and without the ocelli covered. It is shown that the ocellar input provides a faster response to motion stimuli than with compound eye stimulation alone, and also increases the amplitude of responses to flashed stimuli. In the case of the DNII2 neuron, it is also shown that the ocelli provide a directional contribution to the responses.
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