Gross morphology, histology, and ultrastructure of the alimentary system of Ricinulei (Arachnida) with emphasis on functional and phylogenetic implications

Abstract

Ricinuleid functional mouthparts are the cucullus, the chelicerae, the pedipalps, and the labrum. These structures are movably jointed to the rest of the prosoma, most likely protruded upon hydrostatic hemolymph pressure and retracted by prosomal muscles. Seta-like protrusions from the labrum and the pedipalpal coxae form a sieve-like filter inside the preoral cavity and the mouth. Although the tip of the labrum can be elevated upon muscle constriction, ingestion of large, solid food particles is unlikely. The mouth has a crescent-shaped cross section. The cuticle-lined, also crescent-shaped pharynx is equipped with a large dilator muscle but lacks antagonistic constrictor muscles. It represents a precerebral sucking pump. The triangular to Y-shaped, cuticle-lined esophagus is equipped with constrictor and dilator muscles. Its posterior part represents a postcerebral sucking pump. Four blind ending diverticula ramify from the anterior prosomal part of the entodermal midgut tube. Two of these diverticula remain inside the prosoma and form few short branches. The other two extend through the pedicel into the opisthosoma and ramify and coil there. A stercoral pocket protrudes ventrally out of the midgut tube. The most distal part of the midgut tube is modified into a contractile rectal gland. Its secretions may have defensive or physiological functions. A short anal atrium is formed by the cuticle-lined ectodermal hindgut which opens at the end of the three-segmented metasoma. The telescoping segments of the metasoma are protruded by hemolymph pressure and retracted by muscles.