G. Struwe

Pigment migration and spectral sensitivity in the compound eye of moths

SummaryThe spectral sensitivity of dark adapted eyes of the moth Manduca sexta (Lepidoptera: Sphingidae) was measured between 350 and 650 nm by determining the relative number of photons necessary to elicit a retinal potential of 50 μV. The spectral sensitivity was determined in eyes with the screening pigment: a) in the extreme distal position, b) in a proximal position, and c) with the pigment removed. Sensitivity maxima were found at about 350 to 370, 450 and 530 nm, irrespective of pigment position.Pigment movement is associated with a change in light attenuation of 2 to 3 log units at all wavelengths between 350 and 650 nm. Only small variations, about 0.6 log units, in screening effect were observed throughout the measured spectrum, and therefore the effect of pigment movement on spectral sensitivity is small.There is a slightly larger decline in sensitivity of eyes with the pigment removed compared to eyes with the pigment in the distal position at wavelengths above 530 nm. This result suggests that the distally located pigments have a slight screening effect.

Spectral sensitivity of the compound eye in butterflies (Heliconius)

SummaryThe spectral sensitivity of the compound eye in three butterfly species (Heliconius erato, H. numata, H, sara) was tested electrophysiologically in the wavelength region 310 to 650 nm. Sensitivity maxima were found at 370 to 390 nm, 450 to 470 nm, and 550 to 570 nm, for all species. The three sensitivity maxima are suggested to be due to different photoreceptor types effecting wave-length discrimination. An interspecies difference in spectral sensitivity was also found. The difference is suggested to be due to the relative number of photoreceptors of each type. In some of the present experiments a small discontinuity in sensitivity was found at 610 or 630 nm. It is probably caused by a selective reflection of these wavelengths from a tapetum.

Spectral sensitivity of single photoreceptors in the compound eye of a tropical butterfly (Heliconius numata)

SummaryThe spectral sensitivity (wavelength region 330 to 650 nm) of single photoreceptor cells in the compound eye of the butterfly Heliconius numata (Lepidoptera) was studied by intracellular recording. The configuration and amplitude of the receptor potential was similar to that reported for other insect photoreceptor cells. Maximal sensitivity was found in one of the following wavelength regions: 390 to 410 nm, 450 to 470 nm and 530 to 550 nm. Side maxima of variable amplitude were found. The results suggest three photoreceptor types with different spectral sensitivity maxima.

Spectral absorption by screening pigment granules in the compound eyes of a moth and a wasp

SummaryThe spectral absorption by single ommin containing pigment granules or clusters of granules from compound eyes was measured spectrophotometrically between 300 and 700 nm. The measurements were made on fresh and fixed slices from compound eyes of Celerio euphorbiae and Vespa spec. In the visible part of the spectrum there is an absorption maximum between 540 and 550 nm, situated nearly 30 nm more towards the red than that of pure ommin in solution. A frequently found side maximum of variable height at about 450 nm is probably caused by oxidized xanthommatin occurring additionally within the granules. The absorption increases from 350 nm towards shorter wavelengths, and gradually declines between 550 and 700 nm.

The physical and morphological properties of the pigment screen in the compound eye of a shrimp (Crustacea)

SummaryThe pigment cells of the compound eye of the shrimps (Crangon crangon andC. allmani) were studied by electron microscopy (SEM and TEM) and microspectrophotometry. The compound eyes of these species contain light-absorbing and -reflecting pigments contained in granules, located in 5 different cells. The light absorbing pigment granules (light screen) are situated in (1) the distal pigment cells, (2) the retinular cells, (3) the basal pigment cells. The reflecting pigment granules are located in (4) the distal, and (5) the proximal reflecting pigment cells. Another innominate cell type investing the ommatidia contains vacuoles without pigment content. The innominate cell type, and the basal absorbing pigment cell (3) listed above, have not earlier been reported for a crustacean species. Measurements of the spectral absorption on sliced and squashed ommatidia show that all components of the light screen have an increased absorption in the wavelength regions 400–450 nm and 530–570 nm, probably due to xanthommatin and ommin. The spectral absorbancy of the reflecting pigment cells were not determined. Similar cells in other species are known to contain pteridines.

Pigment migration and illumination of single photoreceptors in a moth

SummaryThe relation between intensity of illumination of single photoreceptors and position of the screening pigment was studied in the compound eye of the moth Deilephila elpenor. The amplitude of the response to test flashes of white light was measured in 7 dark adapted photoreceptor cells with the screening pigment in the extreme distal and in a proximal position. The experiments demonstrate that the response amplitude of individual photoreceptors varies with the pigment position. The associated variation in screening effect was 1.0 to 1.9 log units.