Functional anatomy of scent glands in Paranemastoma quadripunctatum (Opiliones, Dyspnoi, Nemastomatidae)

Abstract

The morphological organization and functional anatomy of prosomal defensive (scent) glands in Paranemastoma quadripunctatum, a representative of the dyspnoid harvestmen, was investigated by means of histological semithin sections, software-based 3D-reconstruction and scanning electron microscopy. Scent glands comprise large, hollow sacs on either side of the prosoma, each of these opening to the outside via one orifice (ozopore) immediately above coxa I. In contrast to the situation known from laniatorean, cyphophthalmid and some eupnoid Opiliones, ozopores are not exposed but hidden in a depression (atrium), formed by a dorsal integumental fold of the carapace and the dorsal parts of coxae I. Glandular sacs are connected to ozopores via a short duct which is equipped with a specific closing mechanism in its distal part: A layer of modified epidermal cells forms a kind of pad-like tissue, surrounding the duct like a valve. Several muscles attached to the anterior parts of the glandular reservoir and to the epithelial pad may be associated with ozopore-opening. The actual mechanism of secretion discharge seems to be highly unusual and may be hypothesized on the basis of corroborating data from behavioral observations, scent gland anatomy and secretion chemistry as follows: Enteric fluid is considered to be directed towards the ozopores via cuticular grooves in the surface of the coxapophyses of legs I. Then, the fluid is sucked into the anterior part of the scent gland reservoirs by the action of dorsal dilator muscles that widen the reservoir and produce a short-term negative pressure. After dilution/solution of the naphthoquinone-rich scent gland contents, a secretion-loaded fluid is thought to be discharged with the help of transversal compressor muscles. This is the first detailed study on the functional anatomy of scent glands and the mechanisms of secretion discharge in the Dyspnoi.

The Scent Glands of the Neotropical Harvestman Discocyrtus pectnifemur: Morphology, Behavior and Chemistry

“…The defensive secretion of D. pectinifemur is a mixture, which is usual for harvestmen, regardless of suborder (Gnaspini and Hara 2007;Jones et al 2009;Pomini et al 2010;Raspotnig et al 2005Raspotnig et al , 2014aRocha et al 2011;Schaider et al 2011). So far, only cryptogeobiids and phalangodids seem to produce a single phenolic compound (Hara et al 2005;Machado and Pomini 2008;Shear et al 2010) as a defensive secretion, although these data could be artifacts of the methods used.…”

“…Therefore, we propose that hydroquinones and their respective benzoquinones should be considered synonymous, if used for phylogenetic purposes. The identification of defensive compounds from the suborders Cyphophthalmi (Jones et al 2009;Raspotnig et al 2005Raspotnig et al , 2012 and Dyspnoi (Raspotnig et al 2010(Raspotnig et al , 2014bSchaider et al 2011) has allowed more precise inferences about the evolution of the defensive secretions in Opiliones (e.g., Raspotnig et al 2014a). As pointed out by these authors, knowing the biosynthetic routes of the defensive secretion is important because it allows a more realistic use of chemical data for phylogenetic purposes (e.g., proposing weight matrices).…”

“…The morphology of the ozopore varies widely among species. Such variation involves not only the actual opening of the gland but also apophyses, folds, channels, and dome-shaped elevations in the surrounding area (Gnaspini and Rodrigues 2011;Schaider et al 2011).…”

On the enigmatic scent glands of dyspnoan harvestmen (Arachnida, Opiliones): first evidence for the production of volatile secretions

“…We assume an analogous situation to be true for species of Carinostoma . The process of secretion emission, however, may clearly differ between these two genera: When Paranemastoma applies its secretion in the course of natural operation, naphthoquinones (and anthraquinones) are liquefied by enteric fluid via a rather intricate mechanism, probably involving the conduction of enteric fluid into scent gland reservoirs and the subsequent discharge of a quinone-loaded mixture (Schaider et al 2011). In Carinostoma , however, a particular liquefaction process of the naphthoquinones by mixing up with enteric fluid has never been observed and may not be necessary either.…”

“…All these compounds again form solids at room temperature and ambient pressure conditions and may be stored in their solid state in the scent gland reservoirs, but are liquefied by mixing up with enteric fluid in the course of the emission process (Schaider et al 2011). …”