H. Langer

Tetrachromatic visual system in the moth Spodoptera exempta (Insecta: Noctuidae)

SummaryThe retina in the compound eye of the African army-worm moth, Spodoptera exempta, contains four different visual pigments (Fig. 4). Their existence was demonstrated by microspectrophotometry and by electrophysiological experiments, using selective colour adaptations. The pigments were localized in different receptor cells of the ommatidia (Fig. 6) by a special electron microscopic technique (Fig. 5). One of the pigments, a rhodopsin with the absorption maximum at about 560 nm, causes this eyes distinct red sensitivity up to more than 700 nm, found electrophysiologically.

Die Struktur des Rhabdoms im „Doppelauge“ des Wasserläufers Gerris lacustris

SummaryThe structure of the rhabdome in the compound eye of Gerris lacustris is investigated electron microscopically. After fixation in osmium tetroxide and embedding in Vestopal the material was cut in transverse and various longitudinal directions. — Main results:1.Within the three parts of the eye differing histologically, there are two types of ommatidia, one of them — with two slight modifications — in the dorsal and lateral, the other in the ventral parts.2.In both types the rhabdome consists of an exterior group of six rhabdomeres in rectangular configuration. Two of them with larger areas of the oval cross section form the smaller sides of the rectangle. The other four, arranged in pairs, build up the longer sides. The axes of the microvilli in the smaller rhabdomeres, are parallel and lie perpendicular to those of the larger rhabdomeres. In the ventral-type ommatidia the four smaller rhabdomeres are shorter, existing only in the distal part of the rhabdome.3.In the dorsal-lateral type the central part of the rhabdome consists of the “outer segments” of two visual cells, the perikarya of which are situated proximal to the rhabdome. Only one of them reaches the proximal end of the four crystal cells and forms a rhabdomere in the distal half of the rhabdome. The “outer segment” of the other central cell extends only up to the middle of the rhabdome and forms two rhabdomeres, lying parallel in the proximal half. The axes of the microvilli in these rhabdomeres are perpendicular to those in the distal rhabdomere of the other central cell. The three rhabdomeres of the two central cells are in such close a conjunction that they together build up one bifurcated pathway for light. The “outer segment” of the proximal central cell is attached to the slim part of the other central cell (without rhabdomere) and embraces it by two lateral lobes forming a tube in the most proximal part. Therefore the distal central cell is thought to have a supporting function for the proximal one.4.In the ventral type the central part of the rhabdome is formed totally by the “outer segment” of the proximal central cell with its two rhabdomeres. In this case, these two reach the proximal end of the crystal cells and have nearly the same length as the two larger rhabdomeres of the ommatidium. So, the volume of the rhabdomeres with one (the dorso-ventral) direction of their microvilli predominates that of the others with cranio-caudal orientated microvilli.5.The ability for analyzing the position of the plane of linear polarized light is postulated for the ommatidia in the dorsal and lateral part of the eye only. On the other hand, ventral-type ommatidia, looking at the surface of the water, evidently are constructed for preferable perception of polarized light with a fixed plane. Light reflected at the surface of water after oblique incident predominantly contains that plane which is normal to the axes of microvilli in the larger rhabdomeres of ventral ommatidia. Therefore their structure is assumed to be differentiated for a screening function against surface-reflected light to get a better view into the water.6.Because of the different structures of their ommatidia, the compound eyes of Gerris prove to be “Doppelaugen” (in the meaning of Bedau, 1911). Most likely their parts have different functions in vision.ZusammenfassungDie Struktur der Facettenaugen von Imagines des Wasserläufers Gerris lacustris wird anhand verschieden orientierter Dünnschnitte elektronenmikroskopisch untersucht:1.In den Augen finden sich zwei Bautypen von Ommatidien, von denen der eine im dorsalen und lateralen Abschnitt, der andere im ventralen Abschnitt vorhanden ist.2.In beiden Typen enthält das offene Rhabdom sechs äußere Rhabdomere, von denen beim ventralen Typ jedoch vier stets wesentlich kürzer sind als die beiden übrigen.3.Beim dorso-lateralen Bautyp wird der Zentralteil des Rhabdoms durch die Rhabdomer-tragenden „Außenglieder“ von zwei Sehzellen gebildet, deren kernhaltige Zellkörper weiter proximal liegen. Nur eine davon (Nr. 7) reicht bis unmittelbar an die Kristallzellen und bildet in der distalen Hälfte des Rhabdoms ein Rhabdomer aus. Das „Außenglied“ der anderen zentralen Sehzelle (Nr. 8) liegt nur in der proximalen Hälfte und hat dort zwei einander gegenüber stehende Rhabdomere gleicher Länge mit zueinander parallel orientierten Tubuli. Dieser Teil der Zelle Nr. 8 umgreift das Übergangsstück der Zelle Nr. 7 zwischen deren Rhabdomer- und deren Kern-tragendem Teil. Das Rhabdomer von Zelle Nr. 7 und die beiden von Zelle Nr. 8 schließen unmittelbar aneinander an, so daß im Längsschnitt das Bild einer Gabelung entsteht.4.Im ventralen Bautyp wird der ganze Zentralteil des Rhabdoms von dem „Außenglied“ der Sehzelle Nr. 8 mit seinen beiden Rhabdomeren gebildet, die vom proximalen Ende der Kristallzellen bis zum basalen Ende des Ommatidiums reichen. In diesem Rhabdom überwiegen die Rhabdomere mit der einen (dorso-ventralen) Tubulusrichtung in ihrer Masse bei weitem.5.Für die Ommatidien des dorso-lateralen Bautyps wird die Fähigkeit zur Analyse der Schwingungsrichtung polarisierten Lichtes postuliert. Die Ommatidien des ventralen Bautyps, die auf das Wasser blicken, sind offenbar für die bevorzugte Wahrnehmung von Licht mit einer bestimmten Polarisation eingerichtet. Dasjenige Licht, das bei schrägem Einfall an der Wasseroberfläche reflektiert wird, weist dagegen bevorzugt Anteile mit der Polarisationsrichtung auf, die senkrecht zur Tubulusrichtung in der Mehrzahl der Rhabdomere im ventralen Augenabschnitt steht. Deshalb wird die Struktur dieser Ommatidien so interpretiert, daß sie eine verringerte Effizienz des an der Wasseroberfläche reflektierten Lichtes bewirkt.6.Aufgrund der unterschiedlichen Struktur ihrer Ommatidien stellen die Facettenaugen von Gerris „Doppelaugen“ (im Sinne von Bedau, 1911) dar, für deren Abschnitte unterschiedliche Funktionen angenommen werden dürfen.

The arrangement of colour receptors in a fused rhabdom of an insect

SummaryMicrospectrophotometric measurements on the retina ofDeilephila elpenor were performed using fresh (cut by razor blade) as well as frozen (cryostate microtome) sections. Difference spectra from various parts of the rhabdom clearly showed that in the distal part of the rhabdom the UV pigment is predominant, while in the medial part only green pigment was found. From the difference spectra, wave-length and relative height of the absorption maxima of rhodopsins and metarhodopsins were computed and were found to be in close agreement with other results reported in the literature. These findings confirm the work of Welsch (1977) who concluded from his electron microscopic analysis that the two distal receptor cells are UV sensitive, while the six median receptor cells are green sensitive. There is also good evidence to assume that the ninth, proximal receptor is violet sensitive.

The Photopigments in an Insect Retina

Colour vision is not an exclusive property of vertebrates. Also insects can discriminate wavelengths. The best known example is the honeybee, as shown by training experiments (1) and electrophysiological recordings (2,3). The peripheral wavelength discrimination is accomplished by at least three receptor types. The spectral sensitivity of the receptors fairly well agrees with resonance spectra for rhodopsins (3), and bee heads contain retinol and retinal (4). These results suggest that the visual pigments in insects are rhodopsins, i. e. they consist of retinal bound to a protein.

Photoregeneration of visual pigments in a moth

SummaryThe spectral absorbance by the visual pigments in the compound eye of the mothDeilephila elpenor was determined by microphotometry. Two visual pigments and their photoproducts were demonstrated. The photoproducts are thermostable and are reconverted to the visual pigments by light. The concentrations of the visual pigments and the photoproducts at each wavelength are determined by their absorbance coefficients at this wavelength. P 525: The experimental recordings (difference spectra and spectral absorbance changes after exposure to monochromatic lights) were completely reproduced by calculations using nomograms for vertebrate rhodopsin. The identity between experimental recordings and calculations show: One visual pigment absorbs maximally at 525 nm (P 525). The resonance spectrum of the visual pigment is identical to that for a vertebrate rhodopsin (λmax at 525 nm). The photoproduct of this pigment absorbs maximally at 480 nm (M 480). It is similar to the acid metarhodopsin in cephalopods. The relative absorbance of P 525 to that of M 480 is 1∶1.75. The quantum efficiency for photoconversion of P 525 to M 480 is nearly equal to that for reconversion of M 480 to P 525. Wavelengths exceeding about 570 nm are absorbed only by P 525, i. e. P 525 is completely converted to M 480. Shorter wavelengths are absorbed both by P 525 and M 480. At these wavelengths a photoequilibrium between the two pigments is formed. Maximal concentration of P 525 is obtained at about 450 nm. P 350: A second visual pigment absorbs maximally at about 350 nm (P 350), and its photoproduct at 450 to 460 nm. In the region of spectral overlap a photoequilibrium between the two pigments is formed.The visual pigment and the photoproduct are similar to those in the neuropteran insectAscalaphus.

Localization of visual pigments within rhabdoms of the compound eye of Spodoptera exempta (Insecta, Noctuidae)

SummaryIn the noctuid moth Spodoptera exempta, the distribution of visual pigments within the fused rhabdoms of the compound eyes was investigated by electron microscopy. Each ommatidium regularly contains eight receptor cells belonging to three morphological types: one distal, six medial, and one basal cell (Meinecke 1981); four different visual pigments — absorption maxima at approximately 355, 465, 515, and 560 nm — are known to occur within the eye (Langer et al. 1979). The compound eyes were illuminated in situ by use of monochromatic light of different wavelengths. This illumination produced a wide scale of structural changes in the microvilli of the rhabdomeres of individual cells. Preparation of eyes by freeze-substitution revealed the structural changes in the rhabdomeres to be effects of light occurring in vivo.The degree of structural changes may be considerably different in rhabdomeres within the same ommatidium; it was found to depend on the wavelength and the duration of illumination, the intensity received by the ommatidia as well as the spectral sensitivity of the receptor cells. Therefore, it was possible to estimate the spectral sensitivities of the morphological types of receptor cells. Generally, all medial cells are green receptors and all basal cells red receptors; distal cells are blue receptors in about two-thirds of the ommatidia, while in the remaining third of them distal cells are sensitive to ultraviolet light.

Spectral absorption by screening pigment granules in the compound eye of butterflies (Heliconius)

SummaryThe spectral absorption by single granules, clusters and masses of granules of the screening pigment in the compound eye of the butterfly genusHeliconius was studied by microspectrophotometry. Most of the pigment granules were found to have an almost constant absorption in the wavelength region 300 to 700 nm. Other granules showed a maximal absorption either at about 450 or 560 nm. The maximum at 450 nm is suggested to be caused by xanthommatin and that at 560 nm by ommines. The pigment screen inHeliconius is concluded to be a neutral grey filter.

Functional morphology of the ommatidia in the compound eye of the moth, Antheraea polyphemus (Insecta, Saturniidae)

SummaryThe fine structure of the superposition eye of the Saturniid moth Antheraea polyphemus Cramer was investigated by electron microscopy. Each of the approximately 10000 ommatidia consists of the same structural components, but regarding the arrangement of the ommatidia and the rhabdom structure therein, two regions of the eye have to be distinguished. In a small dorsal rim area, the ommatidia are characterized by rectangularly shaped rhabdoms containing parallel microvilli arranged in groups that are oriented perpendicular to each other. In all other ommatidia, the proximal parts of the rhabdoms show radially arranged microvilli, whereas the distal parts may reveal different patterns, frequently with microvilli in two directions or sometimes even in one direction. Moreover, the microvilli of all distal cells are arranged in parallel to meridians of the eyes. By virtue of these structural features the eyes should enable this moth not only discrimination of the plane of polarized light but also skylight-orientation via the polarization pattern, depending on moon position. The receptor cells exhibit only small alterations during daylight within the natural diurnal cycle. However, under illumination with different monochromatic lights of physiological intensity, receptor cells can be unbalanced: Changes in ultrastructure of the rhabdomeres and the cytoplasm of such cells are evident. The effects are different in the daytime and at night. These findings are discussed in relation to the breakdown and regeneration of microvilli and the influence of the diurnal cycle. They are compared with results on photoreceptor membrane turnover in eyes of other arthropod species.

Identification and localization of visual pigments in the retina of the moth, Antheraea polyphemus (Insecta, Saturniidae)

SummaryIn the compound eye of the moth Antheraea polyphemus, three types of visual pigments were found in extracts from the retina and by microspectrophotometry in situ. The absorption maxima of the receptor pigment P and the metarhodopsin M are at (1) P 520–530 nm, M 480–490 nm; (2) P 460–480 nm, M 530–540 nm; (3) P 330–340 nm, M 460–470 nm. Their localization was investigated by electron microscopy on eyes illuminated with different monochromatic lights. Within the tiered rhabdom, constituted of the rhabdomeres of nine visual cells, the basal cell contains a blue-and the six medial cells have a greenabsorbing pigment. The two distal cells of most ommatidia also have the blue pigment; only in the dorsal region of the eye, these cells contain a UV-absorbing pigment, which constitutes a portion of only ∼ 5% of the visual pigment content within the entire retina. The functional significance of this distribution is discussed.

Visual pigment, dark adaptation and rhodopsin renewal in the eye of Pontoporeia affinis (Crustacea, Amphipoda)

The benthic amphipod Pontoporeia affinis lives in the Baltic sea and in northern European lakes in an environment where very little light is available for vision. The eyes, consisting of 40–50 ommatidia, are correspondingly modified. Microspectrophotometric recordings on isolated eyes show the presence of at least two kinds of screening pigments in the ommatidia with maxima at 540–580 nm and 460–500 nm. Difference spectra obtained from the rhabdoms after exposure to red and blue light, respectively, give evidence of a single rhodopsin with its maximum at 548 nm and a 500-nm metarhodopsin. In ERG recordings sensitivity in the dark-adapted state, after saturating exposures to blue and to red light, stabilizes at levels determined by the rhodopsin concentration. No change is observed during 10–14 h after the beginning of dark adaptation. However, using animals pre-exposed with a strong red light and then kept in darkness, it is found that after a delay of 20–40 h sensitivity of the dark-adapted eye begins to increase and finally, after 5–6 days reaches a level corresponding to 100% rhodopsin. Thus, a slow renewal of rhodopsin appears to occur in darkness, where a photoisomerization of metarhodopsin is excluded.