Modification of spectral sensitivities by screening pigments in the compound eyes of twilight-active fireflies (Coleoptera: Lampyridae)

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Abstract
ERG S(λ) were determined in darkadapted intact preparations of 6 North American firefly species (Photinus collustrans, marginellus, pyralis, macdermotti, scintillans and Bicellonycha wickershamorum) which restrict their flashing activity to twilight hours. The curves possess narrow (1/2 bandwidth=50–60 nm) peaks in the yellow (560–580 nm) and a shoulder in the violet (370–420 nm), with amarked attenuation (1.4–2.2 log units) of sensitivity in the green (480–530 nm) region of the spectrum (Fig. 1). Two additional species (Photuris potomaca and frontalis) which initiate flashing at twilight and continue on late into the night (twi-night) possess broad sensitivity maxima around 560 nm (Fig. 3). Selective adaptation experiments isolated near-UV and yellow inP. scintillans (Fig. 2). In the dorsal frontal region of the compound eyes inP. frontalis, high sensitivity existed only in the short wavelength region (near-UV and blue) with a maximum in the blue (λ max 435 nm) (Fig. 4). The in situ MSP absorption spectrum of the screening pigments was determined in preparations of firefly retina. a) Two kinds of dark brown granules were found in the clear zone region. These granules absorb all across the spectrum with a gradual increase in optical density in the shorter wavelength region inP. pyralis (Fig. 5). b) Besides dark granules, pink-to-red colored screening pigments were present in the vicinity of the rhabdoms. The absorption spectra of these pigments determined in five species were narrow (1/2 bandwidth=50–80 nm) with species-specific differences in their peak absorption in the green at 525 nm, 510 nm, 512 nm and 517 nm inP. scintillans, macdermotti, collustrans and pyralis, respectively (Fig. 6). A similar pigment was found inP. marginellus with aλ max at 512 nm (Fig. 7). In all cases, transmission increased both at long and short wavelengths, but more sharply in the long wavelength region (Figs. 6 and 7). Hence each twilight-restricted species has its own unique colored screening pigment. A yellow pigment whose absorption spectrum differed from those found in genusPhotinus was found in twi-night activePhoturis potomaca (λ max 461 nm) and night-activeP. versicolor (λ max 456 nm). The transmission of thePhoturis pigment increased sharply only in the long wavelength region (Fig. 8). In the twilight-restricted species, pink-to-red screening pigments modify dramatically the long green wavelength part of S(λ) functions. The calculated effect of the absorption of these screening pigments (O.D.=1.6 to 2.2; ¯X=1.8, n=4) on a theoretical S(λ) curve represented by a green (P550) rhodopsin, match the shape of the experimentally obtained dark-adapted ERG S(λ) in all cases (Figs. 9, 10). These screening pigments (Figs. 6, 7, 8) then would cause attenuation of sensitivity selectively in the green in twilight-restricted fireflies (Fig. 1) with a concomitant shifting of the peak of the sensitivity in the yellow as well as a narrowing of the visual spectral sensitivity. The pink-to-red colored screening pigments presumably would enhance color and/or brightness contrast in the mesopic range of ambient illumination. The presence of the colored screens attenuates absolute sensitivity from 5–25% among different twilight-active species as compared to a night-activePhoturis lucicrescens (Fig. 11).