| The majority of investigated eyes describe specific variations of known eye types. But the eyes of two different insects, the compound-lens eyes of Strepsiptera and the scanning eyes of dytiscid diving beetle larvae, do not follow known design principles. Most adult insects possess a pair of large compound eyes, often occupying significant portion of their head. Compound eyes are typically composed of hundreds to thousands of ommatidia, each containing 8-10 photoreceptors. For the most part the receptors within each ommatidium act as a single sampling unit, averaging light intensities within all of them. Males of the insect order Strepsiptera are different: their eyes are composed of a smaller number of relatively large units (eyelets), each with an extended retina with often more than one hundred photoreceptors. In the strepsipteran species, Xenos peckii, each eye has about 50 eyelets. By using a behavioral paradigm based on the optomotor response, I have provided evidence that the eyelets in Xenos peckii eyes are image forming units. Each eyelet could sample up to 13 points, as opposed to one sampling point in an ommatidium. This unusual design has already inspired engineers to apply it into artificial optical solutions, such as a compact infra-red camera. Like strepsipteran eyes, the principal eyes of the Sunburst Diving Beetle (Thermonectus marmoratus) larvae are among the most bizarre in the animal kingdom. There are three different larval instars, all of which bear six eyes (stemmata) on each side of their head. The two frontal pairs, known as the principal eyes, are used to scan potential prey prior to capture. The principal eyes form long tubes, have bifocal lenses and are characterized by at least two one-dimensional retinas at their ends: a deep distal retina closer to the lens, and a proximal retina that lies directly underneath. The distal retina expresses long-wavelength opsin (TmLW) mRNA, whereas the proximal retina expresses ultraviolet opsin (TmUV II) mRNA. In contrast to third instars, the proximal retina of first instars shows a weak expression of the TmUV I mRNA limited only to its dorsal half. Third instars lack expression of TmUV I mRNA in their proximal retina. By using intracellular recordings from photoreceptor cells in third instars, I have shown that the distal retina has maximum sensitivity in green (LW), approximately 520-540 nm with an addition of a smaller peak in ultra-violet (UV), around 340-360 nm. The proximal retina is UV-sensitive with peak absorbance at 374 nm. This arrangement, to my knowledge, is the first example of a tiered system with the LW-sensitive cells distal to the UV-sensitive cells. Perhaps this unusual spectral arrangement creates a novel contrast enhancement mechanism. It is still unknown if these animals are capable of color and polarization sensitivity, and both of these visual modalities, including monochromatic vision, can be affected by the strange placement of the distal and proximal retina. Additional optical, physiological and behavioral studies will be necessary to answer these questions. |
