Labhart T, Meyer E P, Schenker L
Zoologisches Institut, Universität, Zürich, Switzerland.
Cell Tissue Res. 1992 Jun;268(3):419-29. doi: 10.1007/BF00319148.
The superposition eye of the cockchafer, Melolontha melolontha, exhibits the typical features of many nocturnal and crepuscular scarabaeid beetles: the dioptric apparatus of each ommatidium consists of a thick corneal lens with a strong inner convexity attached to a crystalline cone, that is surrounded by two primary and 9-11 secondary pigment cells. The clear zone contains the unpigmented extensions of the secondary pigment cells, which surround the cell bodies of seven retinula (receptor) cells per ommatidium and a retinular tract formed by them. The seven-lobed fused rhabdoms are composed by the rhabdomeres of the receptor cells 1-7. The rhabdoms are optically separated from each other by a tracheal sheath around the retinulae. The orientation of the microvilli diverges in a fan-like fashion within each rhabdomere. The proximally situated retinula cell 8 does not form a rhabdomere. This standard form of ommatidium stands in contrast to another type of ommatidium found in the dorsal rim area of the eye. The dorsal rim ommatidia are characterized by the following anatomical specializations: (1) The corneal lenses are not clear but contain light-scattering, bubble-like inclusions. (2) The rhabdom length is increased approximately by a factor of two. (3) The rhabdoms have unlobed shapes. (4) Within each rhabdomere the microvilli are parallel to each other. The microvilli of receptor 1 are oriented 90 degrees to those of receptors 2-7. (5) The tracheal sheaths around the retinulae are missing. These findings indicate that the photoreceptors of the dorsal rim area are strongly polarization sensitive and have large visual fields. In the dorsal rim ommatidia of other insects, functionally similar anatomical specializations have been found. In these species, the dorsal rim area of the eye was demonstrated to be the eye region that is responsible for the detection of polarized light. We suggest that the dorsal rim area of the cockchafer eye subserves the same function and that the beetles use the polarization pattern of the sky for orientation during their migrations.
五月鳃金龟（Melolontha melolontha）的重叠眼具有许多夜行性和晨昏性金龟子甲虫的典型特征：每个小眼的屈光装置由一个厚角膜透镜组成，该透镜具有强烈的内凸面，附着在一个晶锥上，晶锥被两个初级色素细胞和9 - 11个次级色素细胞包围。透明区包含次级色素细胞的无色素延伸部分，它们围绕着每个小眼的七个视杆（受体）细胞的细胞体以及由它们形成的视杆束。七叶融合视杆由受体细胞1 - 7的视杆节组成。视杆通过围绕视杆束的气管鞘在光学上彼此分离。每个视杆节内的微绒毛以扇形方式发散。位于近端的视杆细胞8不形成视杆节。这种标准形式的小眼与在眼睛背缘区域发现的另一种小眼类型形成对比。背缘小眼具有以下解剖学特化特征：（1）角膜透镜不透明，含有散射光的气泡状内含物。（2）视杆长度大约增加了两倍。（3）视杆呈无叶状。（4）在每个视杆节内，微绒毛彼此平行。受体1的微绒毛与受体2 - 7的微绒毛成90度角。（5）围绕视杆束的气管鞘缺失。这些发现表明，背缘区域的光感受器对偏振光高度敏感且具有大视野。在其他昆虫的背缘小眼中，也发现了功能上类似的解剖学特化特征。在这些物种中，眼睛的背缘区域被证明是负责检测偏振光的眼区。我们认为五月鳃金龟眼睛的背缘区域具有相同的功能，并且这种甲虫在迁徙过程中利用天空的偏振模式进行定向。
