Sensitivity of ocellar interneurons of the honeybee to constant and temporally modulated light

Ocellar adaptations for dim light vision in a nocturnal bee

Growing evidence indicates that insect ocelli are strongly adapted to meet the specific functional requirements in the environment in which that insect lives. We investigated how the ocelli of the nocturnal bee Megalopta genalis are adapted to life in the dim understory of a tropical rainforest. Using a combination of light microscopy and three-dimensional reconstruction, we found that the retinae contain bar-shaped rhabdoms loosely arranged in a radial pattern around multi-layered lenses, and that both lenses and retinae form complex non-spherical shapes reminiscent of those described in other ocelli. Intracellular electrophysiology revealed that the photoreceptors have high absolute sensitivity, but that the threshold location varied widely between 109 and 1011 photons cm–2 s–1. Higher sensitivity and greater visual reliability may be obtained at the expense of temporal resolution: the corner frequencies of dark-adapted ocellar photoreceptors were just 4–11 Hz. Spectral sensitivity profiles consistently peaked at 500 nm. Unlike the ocelli of other flying insects, we did not detect UV-sensitive visual pigments in M. genalis, which may be attributable to a scarcity of UV photons under the rainforest canopy at night. In contrast to earlier predictions based on anatomy, the photoreceptors are not sensitive to the e-vector of polarised light. Megalopta genalis ocellar photoreceptors possess a number of unusual properties, including inherently high response variability and the ability to produce spike-like potentials. These properties bear similarities to photoreceptors in the compound eye of the cockroach, and we suggest that the two insects share physiological characteristics optimised for vision in dim light.

Signal processing in a simple visual system: the locust ocellar system and its synapses

The neurons with the widest axons that carry information into a locust brain belong to L-neurons, the large, second-order neurons of the ocelli. L-neurons play roles in flight control and boosting visual sensitivity. Their morphology is simple, and their axons convey graded potentials from the ocellus with little decrement to the brain, which makes them good subjects in which to study transmission of graded potentials. L-neurons are very sensitive to changes in light, due to an abnormally high gain in the sign inverting synapses they receive from photoreceptors. Adaptation ensures that L-neurons signal contrast in a light signal when average light intensity changes, and that their responses depend on the speed of change in light. Neurons L1-3 make excitatory output synapses with third-order neurons and with L4-5. These synapses transmit tonically, but are unable to convey hyperpolarising signals about large increases in light. Graded rebound spikes enhance depolarising responses. L1-3 also make reciprocal inhibitory synapses with each other and transmission at these decrements so rapidly that it normally requires a presynaptic spike. The resolution with which graded potentials can be transferred has been studied at the inhibitory synapses, and is limited by intrinsic variability in the mechanism that determines neurotransmitter release. Electron microscopy has shown that each excitatory connection made from an L-neuron to a postsynaptic partner consists of thousands of discrete synaptic contacts, in which individual dense-staining bars in the presynaptic neuron are associated with clouds of vesicles. Acetylcholine is likely to be a neurotransmitter released by L-neurons.

Neural organization of ocellar pathways in the cockroach brain

A large number of photoreceptors of insect ocelli converge onto a smaller number of second-order neurons. Second-order neurons exit the ocellus and project into the ocellar tract neuropil of the brain. Here, the anatomy and physiology of ocellar interneurons of the ocellar tract neuropil of the cockroach are described. The total number and gross morphologies of ocellar tract neurons were examined by extracellular cobalt impregnations into the neuropil. Morphology and physiology of individual neurons were examined using intracellular recording and stainings. Each ocellar tract neuropil contains at least 25 interneurons comprising: 1) four second-order neurons, 2) 15 third-order neurons that receive synapses from second-order neurons at the ocellar tract and project into a number of target neuropil areas of the brain, 3) two possible efferent neurons, 4) three third-order or efferent neurons, and 5) one neuron still to be characterized. The projection areas of ocellar third-order neurons include 1) visual, olfactory, and mechanosensory centers; 2) the mushroom body (a higher associative center); 3) the posterior slope, a premotor center from which descending brain neurons originate; and 4) the thoracic motor systems. By comparing the present results to those reported from other insects, I conclude that the cockroach ocellar system has two distinctive features. 1) The ratio of convergence at synapses between photoreceptors and second-order neurons is higher than those reported for other insects so far studied. 2) Ocellar signals are transmitted to various target neuropils by third-order neurons, whereas ocellar systems of all other insects possess pathways in which ocellar signals are transmitted to target neuropils by second-order neurons. The functional significance of these features of the cockroach ocellar system is discussed.

Functional diversity of neural organization in insect ocellar systems

Most insects have simple eyes (ocelli), in addition to compound eyes. Although the ocelli can detect only changes in light intensity averaged over their large visual field, they do play various roles in the behavior of insects, as they have advantages over compound eyes in terms of photic sensitivity and the speed of signal transmission. I summarize here our present knowledge on the neural organization of ocellar systems of a number of insects, and propose that they can be classified into three types: (1) the "cockroach type" ocellar system where signals from the photoreceptors first converge onto a small number of second-order neurons, and then diverge to a large number of third-order neurons which project into a number of target neuropils of the brain; (2) the "bee type" ocellar system where photoreceptor signals are passed to various target neuropils by a large number of second-order neurons; and (3) the "locust type" ocellar system where ocellar signals are transmitted to various target neuropils by both second- and third-order neurons. I propose that: (1) the "cockroach type" ocellar system can be characterized as a sensitive type where higher sensitivity is attained because of the higher ratio of convergence of photoreceptor signals onto second-order neurons; (2) the "bee type" ocellar system is a fast type where a high speed of signal transmission is attained at the possible cost of sensitivity; and (3) the "locust type" ocellar system is an intermediate type where both speed and sensitivity are emphasized. A possible phylogenetic relationship between the three types of ocellar systems is discussed.