Ultrastructure of the alimentary canal of the sheep scab mite, Psoroptes ovis (Acari: Psoroptidae).

The mite Psoroptes ovis causes sheep scab disease in flocks throughout much of the world. A serious impediment to the development of novel control measures for this mite is our inability to produce in vitro colonies of the mite. Here, we describe the alimentary canal of the mite with the particular aim of determining what it feeds on, as part of a longer term goal, to develop in vitro culture techniques for P. ovis. The alimentary canal of P. ovis consists of a cuticle-lined foregut and hindgut separated by a microvilli-lined midgut. The foregut is divided into a pre-oral cavity and a muscular pharynx and oesophagus. The midgut consists of three ultrastructurally discrete areas: a stomach with bi-lobed ventriculi, a colon and a post-colon. The stomach is composed of two cell types. The most common cells (Type 1) are either cuboidal or squamous depending on the degree of gut distension and possess short microvilli, a single basally located nucleus, extensive rough endoplasmic reticulum and other components suggesting active secretion. The less common cells (Type 2), possess an extensive apical network of tubules and basally food vacuoles suggesting intracellular digestion. These cells extend into the gut lumen, become free-floating and degenerate. The colon and post-colon are composed of cells similar to Type 1 stomach cells but the post-colon epithelium possesses significantly longer microvilli. Cells from these areas have not been observed to leave the surrounding epithelium and enter the gut lumen, but it is suggested that significant absorption occurs in these two areas. Faecal pellets, often containing a significant number of bacteria, leave the digestive system through the cuticle-lined anal atrium.