Some Observations on External Anatomy of Oribatid Mites by the Scanning Electron Microscope

Morphology is the basis of zoological classification and interpretations of functional anatomy. The structural details of any animal are usually observed to determine its identity and to separate it from other organisms closely or remotely related in a classificatory scheme. Identity of the animal is fundamental to all other aspects of investigation. Subsequent to identification, one is led to define in depth the morphological and functional aspects of the organism under consideration. Observations of oribatid mites are no different from studies of other animals, even though the small size of these mites necessitates the use of a light microscope to elucidate structural details and to compare diagnostic parts. Chaetotaxy, the minute structure of the integument, and the fine details of small external organs in oribatid mites are important features that need to be accurately represented in drawings or photographs for use in classification as well as interpretations of functional anatomy. Such anatomical features observed with the light microscope are usually drawn by the observer rather than photographed because the optical sectioning and focusing in light microscopy enables the observer to view only one particular plane at a time. Oribatid mites are quite rotund and photomicrographs thus show a limited plane in clear detail. Depth is perceived in alternating levels of this optical sectioning as the observer focuses the instrument, so the drawing becomes the observer's interpretation of what he viewed through the microscope. These interpretations are subject to errors resulting from artifacts, mounting techniques, etc. Even when drawn with a camera lucida or a microprojector, the drawing still is really an approximation of the organism as interpreted by the observer and graphically represented. Photomicrographs of oribatid mites taken through the light microscope are limited by the same planar difficulties and optical sectioning as is found with drawings (Fig. 1A). The depth of field of the compound microscope is flattened by the optical lens system and photomicrographs thus obtained do not demonstrate the spatial depths and relationships one observes with a stereoscopic microscope or by optical sectioning in the focusing of the compound microscope. The scanning electron microscope (SEM) is a new scientific tool that enables the investigator to reduce interpretive errors, to see external features more exactly, and to determine spatial relationships and details in threedimensional micrographs with greater clarity than heretofore possible. Details of the morphology of many mites are more demonstrable through the use of this instrument because the whole organism can be placed on a specimen holder, viewed in the SEM, and magnified to extreme size for the study of details. The micrographs accompanying this article indicate new features previously not reported in the literature or not seen in oribatid mites with such clarity of detail. Specimens used in this study were attached to specimen stubs for use in a Cambridge Stereoscan electron microscope Mark II and a Jeolco JSM-U3. The specimens were cemented to the upper surface of the specimen stubs with Duco cement, fingernail polish, or silver paint, and coated with gold in a vacuum evaporator.