Damaeidae

[[Family Damaeidae Berlese, 1896]]

Oribatid mites of the family Damaeidae Berlese, 1896 occur mostly in the northern hemisphere, with the majority of known species living in forest soils of temperate, boreal and subarctic zones of palearctic and nearctic regions (Norton 1979a, 1979b).

The present paper represents the first part of a series which aims to revise the European Damaeidae based on the study of extensive material from Northern, Western and Central Europe, the types or topotypes of European authors when available, and recent discoveries on the family. This first part deals with the generic concept of Damaeus sensu lato, together with the detailed morphology of Kunstidamaeus Miko, 2006 including a redescription of its type species. Species of Central and Northern Europe belonging to other genera within Damaeus C. L. Koch, 1836 sensu lato (i.e. including Damaeus s. stricto, Adamaeus Norton, 1977b, Epidamaeus Bulanova-Zachvatkina, 1957, Paradamaeus Bulanova-Zachvatkina, 1957, Spatiodamaeus Bulanova-Zachvatkina, 1957 and Kunstidamaeus Miko, 2006) were revised (see Miko, 2006) and will be redescribed in detail in following articles of this series.

The family Damaeidae is reasonably diverse, being represented by more than 250 described species (Subias 2004). Grandjean (1954a) included the family in Euphérédermes, one of the five sections recognized within Brachypylina, which is characterised namely by the nymphs (less often also by the adults) carrying gastronotic exuviae of previous stages (“scalps”) and by the reduction of the notogastral setae f1, da, dm, dp (the sete da, dm, dp are present only in the larvae). Monophyly of the Euphérédermes was questioned recently (Maraun et al. 2004; Weigmann 2006) and the phylogenetic relations of Damaeidae to other families are not clear.

Damaeid mites are mostly long-legged, middle sized to large forms (rarely smaller than 500 µm, some species exceeding 1 500 µm) with roughly triangular prodorsum, which is separated from usually circular or ovoid notogaster by a deep dorsosejugal furrow. The nymphs and partly also adults, as in other euphéréderm families, carry gastronotic exuviae. Some species are remarkable by carrying a bulk of adherent dirt on the notogaster, serving probably as a kind of camouflage against predators (Luxton 1981).

From the studies of feeding habits of several model species (Schuster 1956; Luxton 1972; Siepel & de Ruiter-Dijkman 1993) it can be assumed that Damaeidae feed predominantly on saprotrophic fungi and readily accept or even prefer green algae. Occasionally, they can consume other materials including higher plant tissues and dead arthropod bodies as well. Their oesophagus is swollen into a voluminous chamber (ingluvies), which seems to be an adaptation for predigestion of fungal material (Schuster 1956; HoebelMaevers 1967; Smrž 1991). Based on the presence of chitinase activity, some damaeid species are expected to be able to digest the cell-wall of the fungal hyphae - "fungivorous grazers", while some others without the chitinase digest probably only the hyphal content - "fungivorous browsers" (Siepel & de Ruiter-Dijkman 1993).

As far as known, Damaeidae are mostly bisexual species with the sex ratio varying during the year, but several species are suspected to reproduce by facultative thelytokous parthenogeny, based on frequent absence of males from the populations (Luxton 1981). Fertilization is indirect, as usual in other oribatid mites, without any remarkable courtship behaviour (Pauly 1952; Norton 1994). Males deposit long-stalked spermatophores on the ground, which are collected by females with the genital atrium. The life history data are available for a few species. In members of Damaeus s. lato, a legless immovable prelarva (visible through a longitudinal split of the egg shell) was found and rests up to 2 weeks prior to the hatching of the larva, whereas in some other species (e.g. in Belba corynopus) the larva hatches directly from the egg (Grandjean 1954c; Luxton 1981). The duration of the life cycle may differ within a given species according to the temperature and other environmental conditions, as much as from 64 to 360 days in Damaeus (Paradamaeus) clavipes (Luxton 1981). In large forms, such as in Damaeus (Adamaeus) onustus, a surprising adult longevity of up to 634 days was observed (Pauly 1956).

Development of damaeid taxonomy

Given the large body size of many species, Damaeidae have been well known since the beginning of oribatid studies, and many classical species relate to this family (Koch 1836; 1841; Michael 1888; Nicolet 1855; Hermann 1804; Oudemans 1900). The first comprehensive study of the family was published at the beginning of the last century (Kulczynski 1902). Subsequently, a broad range of species was described by German (Willmann 1930, 1931, 1932, 1936, 1951, 1953, 1954; Seilnick 1926; Strenzke 1950; Märkel & Meyer 1960) and other European authors (Mihelcic 1954, 1955, 1957, 1963, 1964; Kunst 1957, 1961; Perez-Inigo 1966; van der Hammen & Strenzke 1953). The first detailed morphological studies and a basis for the systematics of the group were published by Grandjean (1936, 1954a, 1954b, 1960). New taxonomical arrangements and descriptions of many new genera and species from the former Soviet Union were proposed by Bulanova-Zachvatkina (1957a, 1957b, 1962, 1965, 1967). Although some of these concepts were later questioned (Norton 1977b, 1978a, 1978b, 1979b, 1979c; Norton & Ryabinin 1994; Behan-Pelletier & Norton 1983, 1985; Wang & Norton 1989), they are still used as a basis for the recent taxonomy of the family (Balogh & Balogh 1992; Perez-Inigo 1997). Still, the taxonomy of the family is not stabilised, many relationships are unclear and the majority of authors have agreed on the need for a taxonomical revision.

The family concept of Damaeidae (under the junior synonym Belbidae) as a monophyletic group was proposed by Grandjean (1954a). It is well characterized by several autapomorphies (Norton 1979b), namely by the presence of horn-like cornicle (k) on the nymphal notogaster (by which the gastronotic exuviae are attached); notogastral setae of rows c, l and h arranged in two more or less parallel longitudinal rows; rutella with a pair of distal globular hyaline expansions and cheliceral seta chb with fringe of barbs in distal third becoming shorter toward the tip. Expanded funnel-like bothridial rim is shared with Hungarobelbidae (Miko & Travé 1996) - originally placed within Damaeidae, now belonging to Ameroidea Grandjean, 1954 (see Chen et al. 2004). Based on Norton’s (1979b) argumentation, we reject the splitting of Damaeidae into three families, as proposed by Bulanova-Zachvatkina (1967) and consider Belbidae and Belbodamaeidae in her sense to be artificial groups within Damaeidae.

Recently, Subías (2004, 2007) proposed extensive generic recombinations within the family, but unfortunately he neither explained the reasons, nor published any modified diagnoses. His work also lacks references to previous studies. In our view, this way of publishing taxonomic judgements may lead to confusion and destabilisation of the group systematics and should therefore be avoided (see also the opinion of Bayartogtokh and Norton 2007).

Generic concepts in Damaeidae are based mainly on the following characters: presence, arrangement and shape of setae on legs; development of different tubercles and apophyses on the prodorsum and coxisternum; presence of spines (spinae adnatae) on the anterior border of the notogaster; presence of the propodolateral apophysis; and development of cerotegument and cuticle. Other, more widely used characters include body size and colour; length and shape of prodorsal and notogastral setae, sensillus etc. In the present work we use the terminology and general approach of Grandjean (cit. see above) as modified by Norton (1977a), Behan-Pelletier and Norton (1983, 1985) and Miko and Travé (1996).

The morphological features of Damaeus sensu lato were discussed by several European and American authors (Grandjean 1960; Bulanova Zachvatkina date or cit. see above; Norton 1977b; Behan-Pelletier & Norton cit. see above; Bernini 1970, 1980; Bernini & Arcidiacono 1979; Cancela da Fonseca & Bahou 1970; Luxton 1989; Wang & Norton 1989), as well as by some of the authors describing new species from Asia (e.g. Bayartogtokh 2000a, 2000b, 2001; Fujikawa & Fujita 1985; Enami & Fujikawa 1989; Enami et al.1994).

Clear separation of genera within Damaeus sensu lato remains a problem, despite efforts to define natural species-groups with new morphological details (Norton 1978a, 1979a, 1979b; Norton & Ryabinin 1994; Wang & Norton 1989; Behan-Pelletier & Norton 1983, 1985), mostly because at the generic level (sensu Bulanova-Zachvatkina) and sometimes also at the species level, traits seem distributed in patterns that are closer to a mosaic rather than to hierarchical distributions.

Basically, two approaches have been used for Damaeus: recognizing Damaeus sensu lato (accepting a broadly defined genus with a broad range of subgenera) or splitting the genus into several smaller genera and subgenera. In the extreme form, the second approach may lead to the establishment of a new genus for every single combination of major characters states, resulting in many minor, closely related and probably artificial genera. On the other hand, some of these "narrow" genera include numerous species, and are broadly accepted. For example, Epidamaeus, originally proposed as a subgenus of Damaeus by Bulanova-Zachvatkina (1957), is currently usually accepted as the most species-rich genus of the family (see e.g. Behan-Pelletier & Norton 1983, 1985; Perez Inigo 1997; Balogh & Balogh 1992) with two clearly defined subgenera (Epidamaeus s. stricto and Akrodamaeus Norton, 1978a), but see Subías (2004). Also Spatiodamaeus, first established as a subgenus of Damaeus, is broadly accepted as a separate genus (see e.g. Schatz, 1983; Balogh & Balogh, 1992; Subías 2004, 2007), mostly because of quite uniform look of species with shared character states.