Scent Glands of Palpatores (Opiliones)

Large prosomal defense glands (syn. scent glands) represent the major exocrine system in harvestmen (Opiliones). Even though a series of studies has been conducted since the 1950ies, knowledge about these glands is still poor, especially with respect to one of the (three) classical sub-orders of Opiliones, the so-called "Palpatores". Scent glands of Palpatores appear to be different from "typical" scent glands of the other two classical suborders, Cyphophthalmi and Laniatores. While scent glands in Cyphophthalmi and Laniatores display the characteristic features of repugnatorial glands of arthropods, scent glands in Palpatores (especially in Dyspnoi) are generally more inconspicuous or even cryptic. Species are reluctant to release secretion or do not release secretion at all. Moreover, in their turn, scent glands within Palpatores are not uniform but highly heterogeneous: apart from distinctly different glandular types between the two "tribes" of the Palpatores, namely the Eupnoi (including the typical daddy longlegs) and the Dyspnoi, preliminary studies also indicate a high degree of heterogeneity of scent gland features within each tribe. Thus, briefly, the scent glands of Palpatores seem to be present in several distinct sub-types with respect to morphological organisation, chemistry and modes of emission of secretions. Since scent glands represent complex synapomorphic characters, the heterogeneity of character states turns them into promising model systems for an in-depth study of phylogenetics and evolution of (possibly paraphyletic) Palpatores. In these terms, scent glands are expected to exhibit novel character sets of phylogenetic importance, possibly useful to trace evolutionary traits, especially by the species- or group-specific chemical profiles of scent gland secretions ("chemosystematics"). Thus, the first (and major) aim of the proposed project is a comparative and comprehensive characterisation of poorly-known scent glands in Palpatores, revealing their different morphological, chemical and functional types and sub-types. In an interdisciplinary approach, these investigations will be done on a representative selection of (Austrian) Palpatores, covering the major palpatorid families and subfamilies. For Dyspnoi, the study is pioneering since the chemistry of dyspnoid scent gland secretions is completely unknown for the time being. For Eupnoi, many chemical types of glandular secretions are expected, far beyond the hitherto known "chemical dichotomy" between naphthoquinone-rich secretions (some Phalangiidae) and ethyl-ketone-rich secretions (some Sclerosomatidae). Secondly, an evaluation of scent gland derived characters - especially chemical characters - as novel and independent data sets for phylogenetic analyses of Palpatores is aimed at.

A pair of large, prosomal, sac-like defensive glands is a characteristic feature of all harvestmen (Opiliones): for these arachnids, being non-poisonous and not able to spin, scent glands are considered one of the most important means for defense against predators, fungi and bacteria. Opilionid scent glands diversified with respect to morphology, function and chemistry, paralleling the radiation of harvestmen into their extant species diversity (6500 species!): by now, many scent gland eco-types and, most interestingly for natural products research, diverse and highly taxon-specific secretions have evolved.In this project, the poorly known scent glands of the so-called Palpatores were investigated: the Palpatores comprise more than 2000 species, including the typical daddy-longlegs but also short-legged, aberrantly-looking harvestmen that show many adaptations to a cryptic life in soil. Traditionally, the Palpatores are divided into Eupnoi and Dyspnoi: both groups, in particular the Dyspnoi, so far represented white spots in opilionid scent gland research. However, these groups exhibit the greatest scent gland-heterogeneity, and represent the key to a consistent and explainable overall-picture of scent gland-evolution in the Opiliones. In this project, 1) we collected model data for hitherto completely unknown dyspnoan scent gland chemistry. Moreover, at least, three scent gland types in Dyspnoi could be classified: a) a solid type which is frequently characterized by hidden glandular orifices (= ozopores), covered by an external atrium as well as by solid secretion boli in the scent gland reservoirs; b) a volatile type which mainly produces short-chain ketones in a naphthoquinone matrix; and c) a semi-volatile type which contains naphthoquinones only. 2) The Eupnoi show variations of the volatile type but produce different, distinctly taxon-specific chemicals: a) a large group of Phalangiidae releases unusual lactones and related compounds; b) a group of Sclerosomatidae produces different phenolics whereas c) another group of sclerosomatids relies on a mix of various acyclic ketones. d) While in most phalangiids, the compounds mentioned above are accompanied by naphthoquinones, the latter are replaced by benzoquinones in the phalangiid subfamily Platybuninae. In total, more than 100 different components, including a number of new natural compounds, could be extracted from palpatorean secretions.Thus, this newly generated chemical data base on palpatorean secretions eventually allows drawing a widely consistent chemosystematic picture of scent gland chemistry in the Opiliones, and thus offers the opportunity for a model reconstruction of character evolution in an exocrine system across a whole animal order.