Biology of the Arthropod Cuticle

The visual world offers a variety of information that enables animals to orient themselves to their surroundings. One specific feature of the visual information is the wavelength difference of light emittedfrom light sources or reflectedfrom objects. Under natural conditions, a general difference in the spectral composition of light is based upon the fact that light coming directly from light sources, e.g., the sun, sky, and moon, is characterized by its relatively high content of short wavelength ( 450nm) (Figs. 1). The physical basis of these differences is the preferential absorption by pigment molecules, which—in contrast to atoms and small molecules—have resonances above 300 nm and shift the bulk of the reflected light into the green and orange region of the spectrum. In water, the wavelength range is compressed in the long-wavelength part of the spectrum due to water’s high absorption of red and infrared light (Fig. 1). But in water, too, direct light is relatively richer in shortwave light. Neglecting the finer differences in the spectral content of light, this general distinction between direct and reflected light could be the cue that allows an animal to differentiate such basically different habitats as open spaces, shadowed areas, spaces rich in food, hiding places, etc.

Spectral Sensitivity and Color Vision in Invertebrates

In nearly thirty intervening years we have not overcome the exasperation so poetically expressed by Gordon Walls, for in achieving the conditions of illumination under which our own cones “see” color, the important somethings of the cones we observe are inevitably destroyed. It would no doubt have been of great satisfaction to Walls, himself an engineer, to know that the technical revolution, which has steadily progressed since the wedding of the physical sciences with biology, would so soon extend man’s senses to make possible the detection of the invisible, and thus, in a fashion perhaps even more satisfactory than detection directly by the experimenter’s own eye, enable us to observe the invisible somethings in nearly any of the “completely colorless structures” he spent his life examining. It is an arresting coincidence that it was the very “living cones” of frog and goldfish mentioned in Walls’ rhetoric whose “bland innocence” was first denied through discovery of their contained pigments by microspectrophotometry.

Microspectrophotometry of Photoreceptors

“I come to the discussion of a remarkable optical phenomenon the unravelling of which has caused me a good deal of worry.” In this way Sigmund Exner (1891, p. 162) begins the chapter on pseudopupils in his invaluable monograph “Die Physiologie der facettirten Augen von Krebsen und Insecten”. His worry has proved worthwhile because his recognition that pseudopupils are optical revelations of the interior of compound eyes has been of longlasting and immense importance. The aims of this chapter are to show that pseudopupil phenomena provide a direct and deep insight, both figuratively and literally, into compound-eye structure and function, and also to indicate possible paths for future research.

Pseudopupils of Compound Eyes

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.

The visual system of insects

Spectral sensitivity of the lateral eyes of the isopodPorcellio scaber (wood louse) and the decapodsCallinectes sapidus (blue crab),Palaemonetes paludosus (Everglades prawn),Orconectes virilis, andO. immunis (crayfish) have been measured between 300 and 660 nm by determining the reciprocal number of photons required to evoke a constant size retinal action potential.

Comparative studies of crustacean spectral sensitivity

1.

Receptor potentials have been recorded intracellularly from single retinular cells in the anterior and dorsal quadrants of the compound eye of the crayfish Procambarus (Fig. 1) stimulated with equal quantum flashes of linearly polarized monochromatic light. Comparisons between two orthogonal stimulus e-vectors respectively parallel and perpendicular to the microvilli of each receptor cell's rhabdomere were made sequentially in about one minute's running time at 20 nm intervals between 400 and 740 nm (Fig. 2).




2.

Of the 91 cells studied 17 responded maximally in the violet (av. λmax= 440 nm) whereas the other 74 cells were most responsive in the yellow-orange (av.λmax=594 nm) (Table 1). For the latter group the λmax of individual cells ranged widely from 538 to 634 nm (Fig. 4). Violet sensitive cells were found only in the anterior quadrant of the eye.




3.

For 29 cells spectral sensitivity curves were plotted from the spectral efficiency curves using response-energy functions determined at λmax or spectral efficiency curves taken at two or more stimulus energy levels (Figs. 5B, 6B, 7). When the sensitivity curves are normalized the vertical and horizontal e-vector responses are closely similar indicating that dichroism of the visual pigment is undoubtedly responsible for the observed differential sensitivity (Figs. 5C, 6C).




4.

For 51 yellow-orange cells where e-vector comparisons can be made more than half (57%) were more responsive to vertical e-vector (Table 2) corresponding very closely with the estimated percentage of retinular cells with microvilli parallel to the body's dorso-ventral axis (57.2%). In contrast five of the seven violet cells available for this comparison gave stronger responses to horizontal e-vector suggesting they may predominantly be the one asymmetrical cell in each ommatidium. Nevertheless both color discriminating types were found to be present in both e-vector channels.




5.

For the 29 cells for which spectral sensitivity curves can be plotted the average sensitivity ratio for the two polarization planes is 3.1 with a range from 1.2 to 11.9 at λmax. Since dichroic absorption ratios directly measured in crayfish have previously been shown to be about 2, the origin of greater spectral sensitivity ratios in individual retinular cells most likely must depend on other functions than photon absorption by a single rhabdomere.

E-Vector and wavelength discrimination by retinular cells of the crayfish Procambarus

Microspectrophotometric measurements of isolated crayfish rhabdoms illuminated transversely show that their photosensitive absorption exhibits a dichroic ratio of 2 in situ. The major absorption axis matches the axial direction of the closely parallel microvilli comprising the receptor organelle. Since these microvilli are regularly oriented transversely in about 24 layers, with the axes of the microvilli at 90° in alternate layers, transverse illumination of a properly oriented rhabdom displays alternate dichroic and isotropic bands. Because all the microvilli from any one cell share the same orientation, the layers of microvilli constitute two sets of orthogonal polarization analyzers when illuminated along the normal visual axis. Furthermore, since the dichroic ratio is 2 and transverse absorption in isotropic bands is the same as that in the minor absorbing axis of dichroic bands, the simplest explanation of the analyzer action is that the absorbing dipoles of the chromophores, as in rod and cone outer segments, lie parallel to the membrane surface but are otherwise randomly oriented. The rhabdom's functional dichroism thus arises from its specific fine structural geometry.

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Dichroism of Photosensitive Pigment in Rhabdoms of the Crayfish Orconectes

1. The spectral sensitivity of the photoreceptors of a white-eye mutant of the housefly Musca domestica has been measured to 250 nm. in the mid-ultraviolet. Maximum sensitivity is at 340-350 nm., as in the wild-type eye, and decreases at shorter wavelengths with a distinct shoulder at 280 nm. 2. Microspectrophotometric measurements of individual corneal facets show little absorption at wavelengths longer than 300 nm. but a sharp band (peak density about 0.4) at 277 nm. Adjustment of the spectral sensitivity curve for the filtering effect of the cornea makes the 280 nm. shoulder more prominent, suggesting the presence of energy transfer from the protein component of the visual pigment to the chromophore. 3. The short-wavelength limit of the housefly9s visible spectrum is determined by the availability of ultraviolet light and is about 300 nm. in nature. The long-wavelength limit is set by the falling absorption of the visual pigment in the red.

The sensitivity of housefly photoreceptors in the mid-ultraviolet and the limits of the visible spectrum.

Microspectrophotometric measurements of individual dark-adapted rhabdoms of the prawn Palaemonetes vulgaris reveal the presence of two light-sensitive pigments. A pigment with maximum absorbancy at 555 nanometers is converted by light to a long-lived intermediate with wavelength of maximum absorbancy at 496 nanometers. A second pigment with wavelength of maximum absorbancy at 496 nanometers bleaches in the light, seemingly without forming detectable products at wavelengths longer than 375 nanometers. Both pigments occur in each layer of microvilli.

Hector R. Fernandez

Papers

Comparative studies of crustacean spectral sensitivity

E-Vector and wavelength discrimination by retinular cells of the crayfish Procambarus

Dichroism of Photosensitive Pigment in Rhabdoms of the Crayfish Orconectes

The sensitivity of housefly photoreceptors in the mid-ultraviolet and the limits of the visible spectrum.

Microspectrophotometry of photoreceptor organelles from eyes of the prawn Palaemonetes.