Functional morphology and ultrastructure of the mouth apparatus in the freshwater mite larvae Limnochares aquatica (L., 1758) (Acariformes, Limnocharidae).

The mouth apparatus in larvae of the freshwater mite Limnochares aquatica (L., 1758) (Acariformes, Limnocharidae), belonging to the lower Hydrachnidia, was studied with light microscopy, scanning (SEM) and transmission (TEM) electron microscopy, as well as with confocal laser scanning microscopy (CLSM) to reveal its main morphological and functional characteristics. The gnathosoma, functioning as a mouth organ takes a prognathous position, and inclines to the body axial line at the angle of around 30°. The gnathosoma is composed of segments of the two pairs of anterior appendages and their derivatives - two-segmented chelicerae lying above the infracapitulum - the compound medial portion of the palpal coxae, and five-segmented palps, articulated with the infracapitulum laterally. The palp tibia bears the bifid palpal claw facing downward nearly at right angle. The basal cheliceral segments are free, i.e., are not covered with the integumental fold posteriorly, and do not fuse with each other. Apically, they articulate with the relatively short movable digits composed of the basal portion (lever) and the distal portion (the cheliceral blades) curved upward by their ends. The flexible fixed digits protrude forward from the distal parts of the basal cheliceral segments. The ventral wall of the gnathosoma (mentum) transforms into a very specific papillary area (velum). A papillary area with long slim papillae is formed of a light cuticle and likely acts as a sucker but does not have contractile elements. The hypostome possesses the well developed lateral lips (galea) tightly embracing the distal portions of the chelicerae and hiding the movable digits. The roof of the infracapitulum (the cervix, or epistome) lying beneath the chelicerae, shows a compound structure. Anteriorly it connects with the inner hypostomal walls forming a particular pharyngeal ring framing the narrow mouth opening. We saw no evidence of a labrum. A characteristic valve projects forward from the dorsal surface of the cervix. A crescent-shaped pharynx runs along the bottom of the infracapitulum and is attached but not fused with the cuticle of the papillary area. The dorsal pharyngeal dilators originate on the cervix, sigmoid pieces and the lateral walls of the infracapitulum at their junction with the dorsal walls of the basal cheliceral segments. A pair of sigmoid pieces having small cavities inside serves for termination of the cheliceral elevators originated on the posterior portions of the dorsal walls of the basal cheliceral segments. The organizational pattern of the gnathosoma in larvae of L. aquatica is rather specific and is quite different from those of other aquatic or terrestrial parasitengonin larvae.

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 the 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.

Stylostome formation by parasitic larvae of Allothrombium fuliginosum (Trombidiformes: Trombidiidae): morphology of feeding tubes and factors affecting their size.

The morphology and formation of stylostomes (feeding tubes) in hosts' body during the parasitic phase of Allothrombium fuliginosum (Hermann) larvae were studied for the first time with light microscopy (LM) and transmission electron microscopy (TEM). The stylostomes were observed in three aphids species-Acyrthosiphon pisum (Harris), Elatobium abietinum (Walker), and Macrosiphum rosae (L.)-parasitized by mites under laboratory conditions. They consisted of 2-6 main branches, preliminarily unbranched, then producing secondary and sometimes also tertiary branches as finally formed structures. Their walls were uniformly electron-dense, without any longitudinal and transverse stratifications and showed rather irregular outlines. Distally, the stylostome branches revealed transparent pores and cavities in their walls, connecting the stylostome canal with surrounding haemocoelic space. The total length of stylostomes at the end of the parasitic phase was on average 16× greater than that recorded in the youngest stylostomes. No differences in the overall shape of feeding tubes between host species were stated. The stylostomes formed in different host species did not differ significantly, except their total length, which attained the highest value in tissues of Ac. pisum.

Ultrastructure of the intestinal system in unfed larvae of Limnesia maculata (O.F. Müller, 1776) (Acariformes, Limnesiidae).

The intestinal system of unfed fresh-water mite larvae Limnesia maculata (O.F. Müller, 1776) (Acariformes, Limnesiidae) has been studied with transmission electron microscopy. The intestinal system is composed of the foregut, including the pharynx and the esophagus, the sac-like blind midgut and the excretory organ. The pharynx begins with the mouth covered by the labrum. The pharynx runs along the bottom of the gnathosoma and shows ventral dilators. No valves are expressed between the pharynx and the esophagus. The esophagus possesses strongly plicate walls and, before entering the midgut, passes through the brain. The sac-like midgut does not reveal a well-pronounced lumen, developed epithelium and separate lobes. It consists of the two cell types of endoderm origin mixed in the midgut volume. The first type - the vacuolated cells - does not possess Golgi bodies and lysosomal apparatus and shows electron-lucent vacuoles with a granular inclusion inside. These cells apparently do not take part in digestion of the embryonic yolk. The second type - the non-vacuolated cells - shows both, a well developed Golgi complex and large heterolysosomes, and obviously digests the embryonic yolk. Consequently, they may be attributed as specialized vitellophages. Nevertheless, both cell types may take part in formation of the definitive midgut epithelium. The sac-like thin-walled excretory organ is strongly dilated and contains the embryonic wastes in the form of electron-dense globules and birefringent particles. No muscle envelope surrounds the excretory organ. The embryonic wastes together with wastes accumulated during feeding may be evacuated from the organ only after completion of feeding. The excretory canal on this developmental stage is not connected with the excretory organ. It opens to the outside with a simple slit-like excretory pore. Before feeding, larvae have to pass the process of the post-molt development before their midgut would be ready to receive nutrients.

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

This study is the first attempt to describe the ultrastructure and functional morphology of the dermal glands in Limnochares aquatica (L., 1758). The dermal glands were studied using light-optical, SEM and TEM microscopy methods during different stages of their activity. In contrast to the vast majority of other fresh water mites, dermal glands of the studied species are originally multiplied and scattered freely over the mite body surface. The opening of the glands is saddle-like, formed of several tight cuticular folds and oriented freely to the long axis of the mite body. Either a small cuticular spine or, rarely, a slim sensitive seta is placed on one pole of the opening. On the inside, the central gland portion is provided with a complex cuticular helicoid armature. The glands are composed of prismatic cells situated around the intra-alveolar lumen, variously present, and look like a fig-fruit with the basal surface facing the body cavity. The glands are provided with extremely numerous microtubules, frequently arranged in bundles, and totally devoid of synthetic apparatus such as RER cisterns and Golgi bodies. Three states of the gland morphology depending on their functional activity may be recognized: (i) glands without secretion with highly folded cell walls and numerous microtubules within the cytoplasm, (ii) glands with an electron-dense granular secretion in the expanded vacuoles and (iii) glands with the secretion totally extruded presenting giant empty vacuoles bordered with slim cytoplasmic strips on the periphery. Summer specimens usually show the first gland state, whereas winter specimens, conversely, more often demonstrate the second and the third states. This situation may depend on some factors like changes of the seasonal temperature, pH, or oxygenation of the ambient water. On the assumption of the morphological characters, dermal glands may be classified not as secretory but as a special additional excretory organ system of the body cavity. Despite the glands lack cambial cells, restoration of functions after releasing of 'secretion' looks possible. Organization of dermal glands is discussed in comparison to other water mites studied.

Comparative ultrastructure of coxal glands in unfed larvae of Leptotrombidium orientale (Schluger, 1948) (Trombiculidae) and Hydryphantes ruber (de Geer, 1778) (Hydryphantidae).

Coxal glands of unfed larvae Leptotrombidium orientale (Schluger, 1948) (Trombiculidae), a terrestrial mite parasitizing vertebrates, and Hydryphantes ruber (de Geer, 1778) (Hydryphantidae), a water mite parasitizing insects were studied using transmission electron microscopy. In both species, the coxal glands are represented by a paired tubular organ extending on the sides of the brain from the mouthparts to the frontal midgut wall and are formed of the cells arranged around the central lumen. As in other Parasitengona, the coxal glands are devoid of a proximal sacculus. The excretory duct, joining with ducts of the prosomal salivary glands constitutes the common podocephalic duct, opening into the subcheliceral space. The coxal glands of L. orientale are composed of a distal tubule with a basal labyrinth, an intermediate segment without labyrinth, and a proximal tubule bearing tight microvilli on the apical cell surface and coiled around the intermediate segment. The coxal glands of H. ruber mainly consist of the uniformly organized proximal tubule with apical microvilli of the cells lacking the basal labyrinth. This tubule shows several loops running backward and forward in a vertical plane on the side of the brain. In contrast to L. orientale, larvae of H. ruber reveal a terminal cuticular sac/bladder for accumulation of secreted fluids. Organization of the coxal glands depends on the ecological conditions of mites. Larvae of terrestrial L. orientale possess distal tubule functioning in re-absorption of ions and water. Conversely, water mite larvae H. ruber need to evacuate of the water excess, so the filtrating proximal tubule is prominent.

Comparative stylostome ultrastructure of Hirsutiella zachvatkini (Trombiculidae) and Trombidium holosericeum (Trombidiidae) larvae.

Stylostomes (feeding tubes) of Hirsutiella zachvatkini (Schluger) (Trombiculidae), feeding on bank voles [Myodes glareolus (Schreber)], and of Trombidium holosericeum (L.) (Trombidiidae), feeding on larvae of Stenodemini sp. (Heteroptera, Miridae), were studied by TEM methods and on semi-thin sections. The stylostome of H. zachvatkini is a homogeneous structure of low electron density and without strict margins. It extends within the concave host epidermis, undergoing hyperplasia and hyperkeratosis. TEM does not reveal any obvious stratification in the stylostome walls. The cheliceral movable digits are moved apart by 5-6 µm and tightly applied/adhered to the stylostome substance. A local area beneath the open end of the stylostome canal is not empty but contains a nearly homogeneous substrate, which can pass into the central stylostome canal. The latter is mostly free of contents. In contrast to H. zachvatkini, larvae of T. holosericeum form a root-like stylostome chaotically branching within the clear space underneath the host cuticle free of tissue elements. Tubules of the distal stylostome branches become progressively thinner and disappear blindly. As in H. zachvatkini, the stylostome walls of T. holosericeum are devoid of stratification but show moderate to high electron density. The cheliceral movable digits are moved apart by the same distance, as in H. zachvatkini, and tightly applied to the stylostome substance. The lumen of the central canal is either electron lucent, in the distal portions, or filled with a fine granular or homogeneous substrate of low electron density in the proximal portions forming a type of ampoule. This study shows that Trombiculidae and Trombidiidae share similar initial stages of stylostome formation but the resultant stylostome of each family is distinctly different.

Mouthparts in Leptotrombidium larvae (Acariformes: Trombiculidae).

Mouthparts of Leptotrombidium larvae (Acariformes: Trombiculidae), potential vectors of tsutsugamushi disease agents, were studied in detail using light microscopy, scanning electron microscopy, and transmission electron microscopy. The mouthparts incorporated within the pseudotagma gnathosoma are composed of the infracapitulum ventrally and the chelicerae dorsally. The ventral wall of the infracapitulum is formed by a wide mentum posteriorly and a narrowed malapophysis anteriorly. The malapophysis firmly envelops the distal cheliceral portions by its lateral walls. The lateral lips of the malapophysis are flexible structures hiding the cheliceral blades in inactive condition and turning back forming a type of temporary sucker closely applied to the host skin during feeding. The roof of the infracapitulum is formed by a weakly sclerotized labrum anteriorly and a cervix with the capitular apodemes extending posteriorly. The labral muscles are lacking. The capitular apodemes serve as origin for pharyngeal dilators running to the dorsal wall of the pharynx fused with the bottom of the infracapitulum. The basal cheliceral segments are separated from each other besides the very posterior portions where they are movably joined by the inner walls. The sigmoid pieces serve for insertion of the cheliceral elevators originating at the posterior portions of the basal segments. The movable digits reveal the solid basal sclerite and the cheliceral blade curved upward with a tricuspid cap on its tip. Dendrites of nerve cells run along the digits to their tips. The ganglia are placed within the basal segments just behind the movable digits. The chelicerae also reveal well developed flexible fixed digits overhanging the basal portions of the blades. The gnathosoma possesses several sets of extrinsic muscles originating at the scutum and at the soft cuticle behind it. Laterally, the gnathosoma bears five-segmented palps with a trifurcate palpal claw.