Ultrastructure and functional morphology of dermal glands in the freshwater mite Limnochares aquatica (L., 1758) (Acariformes, Limnocharidae)

The structural analysis of secretion in the freshwater mite Limnesia maculata (Acariformes, Limnesiidae) supports the idea of a new form of arthropod silk

The structural characteristics of silk secretion of the freshwater mite Limnesia maculata (O.F. Müller) (Acariformes, Limnesiidae) are described and analyzed for the first time based on light, atomic force and electron-microscopical approaches. The common dermal glands (14 pairs scattered over the body) produce silk mostly during the warm summer season. The process of silk secretion lasts from several hours to several days. The silk may appear like barely recognized clouds of a fine whitish substance. An individual silk thread is an indefinitely long uniform unbranched and non-stretchable tube, hollow or with a vesicular electron-dense residual content. In the silk bundle, threads may be freely interlaced, bent, curved or occasionally broken. The diameter of the tubes is in the range of 0.9-1.5 µm. The width of the tube walls varies greatly from 60 to 300 nm. Chaotically interlaced fine fibrils build the tube walls. On the external surface of the tube wall, these fibrils are loosely organized and frequently rising vertically, whereas on the internal side they are packed more tightly sometimes showing a mesh. The walls may reveal a layered structure or, contrary, are quite thin with through foramens. The revealed organization of silk in the freshwater mites is found to be the simplest among that of other arthropods. We propose a role of the silk in the capture of potential prey in the summer season. Silk in water mites significantly widens the wholesome area for the mites' life and gives them better chances in competition for potential resources.

Organization of dermal glands and characteristic of secretion in the freshwater mite, Limnesia maculata (O.F. Müller, 1776) (Acariformes, Limnesiidae)

The presence of dermal glands is a synapomorphy in the freshwater mites - the large branch of the hyporder Parasitengonina. Dermal glands of the mite Limnesia maculata (O.F. Müller, 1776) (Acariformes, Limnesiidae) were studied using light and electron microscopy (TEM and SEM) for the first time. Two types of dermal glands were recognized in adult mites - 14 pairs of the uniformly organized common dermal glands scattered throughout the entire body volume, and one pair of the characteristic so-called idiosomal dermal glands centrally located and stretched along the ventral body wall, supposedly corresponding to the epimeroglandularia 4. The common dermal glands are composed of a single large alveolus with the basally located columnar epithelium. An intra-alveolar lumen is positioned above the epithelium and packed with a secretion in the form of long curved electron-dense bands. These bands are directed to the excretory opening and released to the outside in form of strongly coiled bands. The cytoplasm of the common dermal glands is filled with short RER cisterns and shows few characteristic Golgi bodies with round dense secretory granules at the distal pole. In contrast, a large elongated sac represents the idiosomal dermal glands having a relatively thin secretory epithelium on the periphery. The cuboidal epithelial cells contain areas with tightly packed RER cisterns and weakly recognized Golgi bodies. The apical cell surface bears irregular short microvilli. The large intra-alveolar lumen always contains an electron-dense secretion with myriads of small lighter particles. This kind of secretion was never seen released through the opening. The excretory opening of the common and the idiosomal dermal glands shows differences in its organization but basically is represented by a cuticular 'bell' narrowing to the opening and showing a particular kind of valves at the inner side of the flaps of the orifice. This article is protected by copyright. All rights reserved.