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.

Sophisticated digestive systems in early arthropods

“…The homonomous series of digestive glands in Megadictyon, Jianshanopodia and Pambdelurion has no exact equivalent in modern arthropods. Many arachnids possess comparable numbers of digestive glands, but these structures vary greatly in morphology even within a single individual 37,38 . Crustaceans rarely have more than a single pair of digestive glands, which can be extremely developed and occupy much of the trunk (for example, the hepatopancreas of decapods 33 ).…”

The harvestman tarsus and tarsal flexor system with notes on appendicular sensory structures in laniatores

“…The addition of molecular data for combined analyses of arachnid relationships has often yielded similar results (Wheeler and Hayashi, 1998;Giribet et al, 2002) although fossil evidence only partially supports these hypotheses (Dunlop, 2010). Alternative hypotheses are offered by studies focused on current sister-group relationships (Alberti and Peretti, 2002;Pepato et al, 2010;Talarico and Michalik, 2010;Talarico et al, 2011). Recently, the number of ultrastructural studies has been rapidly increasing for arachnid groups with disputed sistergroup relationships such as Solifugae (Alberti and Peretti, 2002;Klann et al, 2008Klann et al, , 2009Klann and Alberti, 2010) and Ricinulei (Talarico and Michalik, 2010;Talarico et al, 2011).…”

First Characterization of the Behavioral Repertory in a Ricinuleid: Cryptocellus narino Platnick & Paz 1979 (Arachnida, Ricinulei, Ricinoididae)

“…They exhibit unique morphological modifications such as the hood-like cucullus covering the mouthparts dorsally as well as the sexually dimorphic third pair of legs in males which is modified for sperm transfer (e.g., Platnick 2002). Most studies about this group, including the most recent publications, have mainly focused on its taxonomy (e.g., Botero-Trujillo 2014; Valdez-Mondragón and Francke 2011; Tourinho and Saturnino 2010;Tourinho et al 2014), ultrastructure (e.g., Talarico et al 2006Talarico et al , 2008Talarico et al , 2011, biogeography (e.g., Murienne et al 2013) and ecology (e.g., Barreiros et al 2005), while other aspects remain unknown. Indeed, the behavior of ricinuleids relies mostly on occasional observations without detailed analyses of the behavioral patterns.…”

“…This position enables the male to insert one of its copulatory organs (modified metatarsus and proximal two tarsomeres of the third legs) for the purpose of sperm transfer into the genital opening on the ventral side of the female. Descriptions about the defensive behaviors of ricinuleids include the display of thanatosis and the production of anal secretions by a rectal gland (Pollock 1967;Legg 1977;Talarico et al 2011). …”