Gary D. Bernard

THE VISUAL SYSTEM OF INSECTS

Publisher Summary This chapter focuses on the structural organization of compound eyes, which are the principal photoreceptors of adult insects. They are composed of structural units called ommatidia. Compound eyes can be divided into two groups: photopic eyes and scotopic eyes. The former are characteristic of diurnal insects active in bright light, while the latter are found in nocturnal or crepuscular species and have short, fat rhabdoms that are separated from the crystalline cones by a relatively large distance. The chapter further illustrates examples of insect ommatidia. Many insects, particularly males, have eyes divided into two regions that are characterized by markedly different sizes and pigmentation of ommatidia. In some dragonflies, the dorsal facets are nearly twice the diameter of the ventral.

Functional similarities between polarization vision and color vision.

Abstract Consideration o the similarities between color vision (CV) and polarization vision (PV) leads to improved theoretical understanding of PV and to suggestions for new kinds of behavioral and electrophysiological experiments involving partially polarized light. We propose that angle and degree of polarization as parameters for a PV system are analogous to dominant wavelength and purity as parameters for a CV system. Although three linearly independent receptors must cooperate to determine polarizational angle independent of both degree and intensity, a PV system based on only two receptors is not polarization-blind but is characterized by neutral points and confusion states of polarization if the eye fixates.

Red-Absorbing Visual Pigment of Butterflies

Noninvasive photochemical and physiological experiments with intact butterflies of 17 species showed that nine species have a rhodopsin absorbing maximally at 610 nanometers, contained in retinular cells that are maximally sensitive at 610 nanometers. This is the longest-wavelength visual pigment known for an invertebrate. Eight species of butterflies lack the 610-nanometers rhodopsin. All species possess a rhodopsin absorbing maximally in the green region of the spectrum.

A proposed mechanism for multiplication of neural signals

The probability of the joint occurrence of two statistically independent events is the product of the probabilities of the individual events. This fact is used to show that a neuron which detects coincident arrivals of spikes from two input neurons can function as a multiplier, i.e. its average output spike frequency is proportional to the product of the average input spike frequencies. The theoretical analysis is checked in two ways: (a) Computer simulations confirm the derived expressions for the output frequency and show that increasing the jitter in the input spike trains improves the operation of the multiplier by making the output spike train more regular (b) Experimentally recorded spike trains are used to demonstrate that the type and amount of jitter present in real spike trains is adequate for satisfactory operation of the proposed scheme for multiplication. The operating characteristics of the proposed multiplier make it an attractive candidate for the multiplicative mechanism that is involved in the optomotor response of insects.

Averaging over the foveal receptor aperture curtails aliasing.

We show that the entrance aperture of the foveal cone is about 80% of the 3 micron center-to-center spacing. Reduction in contrast caused by averaging of illumination over this aperture prompts us to predict detection with aliasing between the Nyquist limit of 60 c/deg (where contrast reduction is 63%) and 150 c/deg (where contrast is zero) when the eyes optics are bypassed by impressing interference fringes directly on the fovea.

The effect of motion on visual acuity of the compound eye: A theoretical analysis

Abstract Spatial resolution of moving objects is examined at the level of the photoreceptor axons on a theoretical basis using a linear description of the receptor potential for low object contrasts, and computer simulation with a nonlinear model for high object contrasts. It is shown that: (1) the dynamic properties of transduction by the photoreceptors can cause visual acuity to be significantly affected by motion within the domain of biologically meaningful velocities; (2) eyes with better static visual acuity need faster photoreceptor dynamics if their visual acuity is to be retained up to a reasonable velocity. This requirement appears to be met by the compound eyes of several different species, and in a given eye as it changes its state of adaptation.

The pursuit response of the housefly and its interaction with the optomotor response

SummaryPursuit responses that are probably involved in chasing behavior can be evoked and quantitatively measured in male houseflies under conditions of tethered flight (Figs. 2, 3, 5). Pursuit responses of females are significantly different from those of males (Table 1).Characteristics of the pursuit response are compared with those of the optomotor response to show that they are mediated by different neural subsystems that are in parallel. A slow system mediates the optomotor response, while a much faster system mediates the pursuit response (Table 1).The interaction between the pursuit response and the optomotor response is one of switching. The optomotor stimulus, when presented alone, evokes the optomotor response. When the pursuit stimulus is superposed, the fly switches from the optomotor system to the pursuit system, and ignores the optomotor stimulus. When the pursuit stimulus is removed, the animal switches back to the optomotor system (Fig. 8).

SPECTRAL SENSITIVITIES OF RETINULAR CELLS MEASURED IN INTACT, LIVING FLIES BY AN OPTICAL METHOD

SummaryThe spectral sensitivity of the peripheral retinular cells R1–6 in nine species of intact flies was determined using non-invasive, optical measurements of the increase in reflectance that accompanies the pupillary response. Our technique is to chronically illuminate a localized region of the eye with a long wavelength beam, adjusted to bring pupillary scattering above threshold, then, after stabilization, to stimulate with monochromatic flashes. A criterion increase in scattering is achieved at each wavelength by adjusting flash intensity. Univariance of the pupillary response is demonstrated by Fig. 3.Action spectra measured with this optical method are essentially the same as the published spectral sensitivity functions measured with intracellular electrophysiological methods (Fig. 4 forCalliphora, Fig. 5 forDrosophila, Fig. 7 forEristalis, and Fig. 8 forMusca). This holds for both the long wavelength peak and the high sensitivity in the UV as was consistently found in all investigated fly species.Spectral sensitivity functions for R1–6 of hover flies (family Syrphidae) are quite different in different regions of the same eye. There can also be substantial differences between the two sexes of the same species. The ventral pole of the eye of femaleAllograpta (Fig. 10) contains receptors with a major peak at 450 nm, similar to those ofEristalis. However, the dorsal pole of the same eye contains receptors with a major peak at 495 nm, similar to those ofCalliphom. Both dorsal and ventral regions of the maleToxomerus eye, and the ventral region of the female eye, contain only the 450 nm type of R1-6 (see Fig. 12). However, the dorsal region of the female eye also contains another spectral type of receptor that is maximally sensitive at long wavelength. Eyes of both sexes ofAllograpta (Figs. 10 and 11) contain a mixture of spectral types of receptors R1-6.

Evidence for visual function of corneal interference filters

Corneal interference filters are more than just a device to influence the external appearance of the head by creating coloured eye patterns. They are also optical components of the compound eye. Measurements of reflectance spectra from corneal filters on single facets of living flies from four families are presented. They show that in localized regions of an eye that contains a mixture of filter colours, the reflectance characteristics often ‘fit’ in the sense that one type of filter is maximally reflecting at a wavelength where another type is minimally reflecting. In long-legged flies of the genus Condylostylus, which have alternating rows of two filter types, despite large variability in wavelength for maximal reflectance within a species the ‘fit’ of the two reflectance characteristics is preserved. Intersections between facets are often highly reflecting at a wavelength where the facet surface has low reflectance. They may serve to reduce glare caused by sources outside of the ommatidial visual field. These findings together with strong correlations between habitat and eye pattern support the hypothesis that corneal interference filters optically enhance contrast for coloured objects in a background of dissimilar colour, but do not exclude the possibility that they also could be part of a colour-vision system.

Separation and identification of geometric isomers of 3-hydroxyretinoids and occurrence in the eyes of insects

The 9-cis, 11-cis, 13-cis and all-trans isomers of 3-hydroxyretinal oxime, and the 11-cis, 13-cis and all-trans isomers of 3-hydroxyretinol have been resolved by high pressure liquid chromatography (HPLC) and identified by their spectral properties. The antimony chloride reaction product of 3-hydroxyretinol is spectrally indistinguishable from that of retinol. Heads of insects from 8 genera of Diptera and Lepidoptera were found to contain 3-hydroxyretinoids. The most abundant isomers of 3-hydroxyretinal are 11-cis and all-trans.