| abstract | A new histiostomatid species, Tensiostoma veliaphilum n. gen. n. sp., is described from West Malaysia and Thailand. The male holotype is deposited in Senckenberg-Musem, Frankfurt am Main, Germany. All instars are described and figured using light and electron microscopical techniques. The mites live at the water-air boundary in water-filled bamboo internodes. The ventral parts of the body and the legs are submerged, while the dorsal parts of the mite project above the water surface. This particular position of the body is vital for feeding and locomotion. It is achieved by a specific arrangement of hydrofuge structures of the cuticle (for example, setae, parts of gnathosoma) leading to a stable position on the water surface, which prevents sinking. The mites make use of the surface tension for locomotion by clinging to the water surface with legs I and II from below and swinging the legs backwards, thereby pulling themselves forward. The mites collect their food from the uppermost water layer. A detailed description of the gnathosoma is given, and for the first time SEM pictures of internal structures of a histiostomatid gnathosoma are presented. The traditional view that parts of the histiostomatid palps perform periodic movements for sweeping food particles towards the buccal cavity is refuted, since in T. veliaphilum and several other histiotomatids we only observed movements of the chelicerae. Based on the lack of birefringence, both of the long palpal chaetae of histiostomatids are regarded as solenidia. A legless inert deutonymph may be produced instead of the motile phoretic deutonymph. This is the first record of inert deutonymphs in the Histiostomatidae. The inert deutonymph bears two long appendages rostrally that serve as sheaths for the long palp solenidia of the tritonymph during its formation within the deutonymph. The motile deutonymph leaves the uppermost water layer immediately after hatching and walks about randomly on the water surface. For phoresy exclusively broad-shouldered water striders Lathriovelia rickmersi and L. capitata are used which share the habitat with the mites. The occurrence of one of the deutonymphal forms is obligatory in the life cycle. |