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Lamina monopolar cells (LMCs) are the first-order visual interneurons of insects and crustacea, primarily involved in achromatic vision. Here, we investigated morphological and electrophysiological properties of LMCs in the butterfly Papilio xuthus. Using intracellular recording coupled with dye injection, we found two types of LMCs. Cells with roundish terminals near the distal surface of the medulla demonstrating no or small depolarizing spikes were classified as L1/2. Cells with elongated terminals deep in the medulla that showed prominent spiking were classified as L3/4. The majority of LMCs of both types had broad spectral sensitivities, peaking between 480 and 570 nm. Depending on the experimental conditions, spikes varied from small to action potential-like events, with their amplitudes and rates decreasing as stimulus brightness increased. When the eye was stimulated with naturalistic contrast-modulated time series, spikes were reliably triggered by high-contrast components of the stimulus. Spike-triggered average functions showed that spikes emphasize rapid membrane depolarizations. Our results suggest that spikes are mediated by voltage-activated Na+ channels, which are mainly inactivated at rest. Strong local minima in the coherence functions of spiking LMCs indicate that the depolarizing conductance contributes to the amplification of graded responses even when detectable spikes are not evoked. We propose that the information transfer strategies of spiking LMCs change with light intensity. In dim light, both graded voltage signals and large spikes are used together without mutual interference, as a result of separate transmission bandwidths. In bright light, signals are non-linearly amplified by the depolarizing conductance in the absence of detectable spikes.

The ommatidia in the ventral two-thirds of the compound eye of male Pieris rapae crucivora are not uniform. Each ommatidium contains nine photoreceptor cells. Four cells (R1-4) form the distal two-thirds of the rhabdom, four cells (R5-8) approximately occupy the proximal one-third of the rhabdom, and the ninth cell (R9) takes up a minor basal part of the rhabdom. The R5-8 photoreceptor cells contain clusters of reddish pigment adjacent to the rhabdom. From the position of the pigment clusters, three types of ommatidia can be identified: the trapezoidal (type I), square (type II), and rectangular type (type III). Microspectrophotometry with an epi-illumination microscope has revealed that the reflectance spectra of type I and type III ommatidia peak at 635 nm and those of type II ommatidia peak at 675 nm. The bandwith of the reflectance spectra is 40-50 nm. Type II ommatidia strongly fluoresce under ultra-violet and violet epi-illumination. The three types of ommatidia are randomly distributed. The ommatidial heterogeneity is presumably crucial for color discrimination.

The ommatidial characteristics of the eyes of many insects are non-uniform (Stavenga, 1992). The retinal heterogeneity has been investigated in the satyrid Bicyclus anynana by in vivo microspectrophotometry of the eye shine from individual ommatidia. The ommatidial reflectances fall into two classes. The reflectance of the first, yellow-green (YG) class has a main band peaking at 570 nm (width ca 50 nm) and a subsidiary band in the violet; the second, red (R) class has a broad band from 580 to 680 nm. All ommatidia in the dorsal part of the eye belong to the YG-class. The ommatidia of the ventral part of the eye belong to either the YG- or the R-class. The density of the R-class is highest in the downward-looking area. The eye shine is maximal in the dark-adapted eye, but it rapidly decreases upon illumination with bright light, due to migration of pigment granules towards the light guiding rhabdom. The time constant of this pupil mechanism is in the order of a few seconds. The reflectance also exhibits distinct spectral changes due to changes in the content of the visual pigments in the rhabdom. This phenomenon allows studying the decay and renewal of visual pigment in vivo.