Oil glands of oribatid mites (Acari, Oribatida)

"Oil glands" (glandulae lateroabdominales) represent the most striking and - by far - the largest exocrine system in the basic anatomy of oribatid mites and, additionally, have emerged as central paradigms in oribatid systematics. However, despite their obvious importance in oribatid biology and their impact on mite classification, nearly no scientific data on oribatid oil glands are available. In an interdisciplinary effort, we plan to investigate, and characterize this largely unknown gland system amongst the Oribatida in a comprehensive way, including aspects of functional anatomy, chemistry and biological role. To gain more general insights into oil gland features, the study will focus on diverse representatives of all main oribatid groups. Finally, it should be possible to construct a model of the functional anatomy of oil glands of oribatids as well as a model of their significance in oribatid life and survival strategies. Furthermore, following a completely new approach, we will examine chemical characters of oil glands for their suitability as tools for a systematic revision of the oribatid tree. In this way, it would become possible to gain badly needed novel characters for oribatid classification and to reconstruct relationsships amongst oribatids in a strictly phylogenetic sense. This effort should result in a major modification of the traditionally used - but unsatisfactory - oribatid system. The introduction of a phylogenetically based oribatid tree would represent a major breaktrough in the whole field of acarology.

For the first time, comprehensive scientific data were generated on a poorly known, but very large and conspicuous glandular exocrine system of oribatid mites, the so-called oil glands or "opisthonotal glands": these glands characterise the majority of oribatids (except the most primitive taxa) and, in homologous form, also in the Astigmata. In oribatids, these glands represented one of the basic, but hitherto unanswered challenges of Acarology: Briefly, oil glands are paired, hollow, sac-like glands which open to the body surface via a single orifice on either side of the opisthosoma. The glands comprise each a large-scale and undivided reservoir, completely lined by a fine intima, which is surrounded by glandular tissue. The reservoir itself may comprise a single-layered epithelium of non-glandular cells (such as in Collohmannia gigantea) but also may include multi-layered parts with active glandular cells (such as in juveniles of Hermannia convexa). The emission of secretion, however, occurs intermittendy being regulated by flap-like closure mechanisms which underlie neuro-muscular control. According to our results, oribatid oil glands, traditionally often misinterpreted to have osmoregulatory or themoregulatory roles, represent a unique chemical protection system in a wide sense, producing allomones against oribatid predators, alarm pheromones and antifungal agents. In this respect, oil glands emerge as central organs in the life and survival strategies of Oribatida. With regard to oil gland derived pheromones, oil glands represent - as proved for at least two oribatid species yet - the so far only known chemo-communicative system among Oribatida. Oil gland secretions mainly consist of terpenes, aromatics and hydrocarbons, though in distinctive species-specific combinations. In fact, each a distinctive oil gland chemistry characterises monophyletic units among Oribatida on any taxonomic level: from an evolutionary viewpoint, oil gland secretions have undergone evolutionary changes which can be traced from basic oil gland bearing taxa to more derived oribatid groups and also to Astigmata ("chemosystematics"). These chemical data strongly support the hypothesis of astigmatid mites` evolutionary origin in ancient Oribatida. Thus, especially the chemical part of our investigation represents a promising basis for a future chemosystematic survey on Oribatida by a novel pool of phylogenetically important data.